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Few questions regarding reverse polarity protection..
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kaputtpanzer



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PostPosted: Fri Aug 02, 2019 5:09 am    Post subject:  Few questions regarding reverse polarity protection.. Reply with quote  Mark this post and the followings unread

Hellooo,

the last days I was searching for a way to protect my circuits and synthesizers from reverse polarity and shorts. I think in the past I already did some damage to my semimodular synths, because I power several of my diy and commercial synths with one 12v power supply and a dc splitter cable. And I think what happend is that I accidentally connected ground/0v with the positive supply voltage, so there was a short. This happend a few times. I use those dc jacks with a metalring around the hole. In my circuits that metalring/connector is connected to ground, so when I plug it in and I touch the ring with the tip of the dc barrel connector and a midi cable or a audio/cv cable is connecting both synths, then I short ground to the positive supply. I even saw some little sparks the other day!

Now I am looking for a way to secure all of my circuits once for all. I guess there must be a nice way and I hope somebody here can help me. So what I always do, is I use voltage regulators (7805/7809) and I put a schottky diode in series, right before the input of the regulator. But I think this method is kind of useless when I accidentally connect the positive supply to ground, isn't it? So I thought it could help when I put the diode in parallel, but then I should use a polyfuse or something. And now I found another protection, which is using a mosfet. I read about it and simulated the circuit, it looked ppromisingso far.
But can I be sure that this circuit will protect my synthesizer against this positive-supply-connected-with-ground-problem? If so, I will start using it for all my diy circuits and designs.

Btw I am not sure but it could be that I fried my Behringer Neutron once, because of a similar accident. What definitely already happened, was that I burned some arduino-based circuit, because of no reverse voltage protection.

For better understanding, I added some screenshots of the simulation of the n channel and p channel mosfet protection.

So, what do you think is the best way to protect my synths? Very Happy


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p channel mosfet protection
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kaputtpanzer



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PostPosted: Thu Aug 15, 2019 6:28 am    Post subject: Reply with quote  Mark this post and the followings unread

I see, everybody is to lazy to answer Laughing

The last days I really thought alot about different protection methods.
In the past I built a few of those circuits that looks like the one in the attached picture. I am not planing to put these circuit boards into any enclosure and I wonder if this is a good idea at all. My main concern is that there could be a risk of electrostatic discharges, that could damage my diy synths and also the connected commercial synths, when I accidently touch some of the components.

What do you think, could this be a real issue with those little pc mount circuits? Do they need any kind of shielding/enclosure? I wonder if its possible that an ESD event destroy ics or transistors, when the current is being reduced by resistors or other components? For an example when a discharge enters a eurorack modul through a mono jack cable, it is most likely that the first component it will reach is a resistor or a capacitor. This alone should prevent the semiconductors from damage, even when there are no clamping diodes. Am I wrong?

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PHOBoS



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PostPosted: Thu Aug 15, 2019 7:04 am    Post subject: Re: Few questions regarding reverse polarity protection.. Reply with quote  Mark this post and the followings unread

hmm I thought I did reply, apparently I only thought about doing it Embarassed

Quote:
And I think what happend is that I accidentally connected ground/0v with the positive supply voltage, so there was a short. This happend a few times. I use those dc jacks with a metalring around the hole. In my circuits that metalring/connector is connected to ground, so when I plug it in and I touch the ring with the tip of the dc barrel connector and a midi cable or a audio/cv cable is connecting both synths, then I short ground to the positive supply. I even saw some little sparks the other day!

Although shorts are never good, I don't think this would actually cause any damage (at least not with a single supply) except to the power supply itself.
The circuits just wouldn't get any power. Well, I guess it would be possible if you have something else connected to it. For example if you have a circuit (A)
that is powered separately and you have an output of it connected to the input of another circuit (B). If circuit B isn't powered (because you are shorting it out)
then the signal coming from the other circuit (A) might cause some damage. (this is why you should only connect circuits together after they are powered).

With these kinds of circuits electrostatic discharge is usually not really a concern once components are connected in a circuit, as most of the time there is
a path to GND, often a resistor/pot directly connected to the input. However, clamping diodes could prevent any damage with the example I mentioned before
where a circuit is connected but not powered and also just over-voltage in general. For example when you have a circuit powered by 5 Volt and you connect an
input to a circuit that can produce a higher output voltage. Also outputs should have protection against shorts.

Of course with these open circuits there is also the possibility of a cable or something else dropping on it and shorting something out.

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JovianPyx



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PostPosted: Thu Aug 15, 2019 9:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, I wouldn't say we're too lazy to write about this.

In fact, there is a wide range of opinions about this. Some say "don't bother, just know what you're doing and be careful", while others try every which way with diodes and whatnot to prevent a disaster.

In my opinion, a polarized plug that is impossible to plug in backwards is a good method and then no diodes or other added components are necessary. The EuroRack PSU connector looks good for this.

As for shorts, the DMM is your best friend. If you measure a module's rail connectors to ground for ohms, you'll immediately see that as say 5 ohms or less - that's either a low resistance shunt or a short. Fix that and check again with a DMM repeating until it's good. You can even calculate the current drawn by the shunt/short with Ohm's Law I=E/R. You can see that 15 volts into 5 ohms would try to draw 3 amperes. Most modules won't need that kind of current, so that's a signal something is wrong.

If making your own boards, be vigilant about how things are wired up. I would never install diodes to correct for a bad etch. Also remember that diodes will "eat" about 0.6 volts. It's also good to remember that the diodes are there forever even tho you plug it in once in most cases and if the circuit is correctly wired to the power connector, the diodes aren't doing anything but eating that .6v.

Same with making your own cables. Make them correctly and then check the wiring of the cable with a DMM. If it's correct, it will work and no active protection is necessary.

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kaputtpanzer



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PostPosted: Fri Aug 16, 2019 4:54 am    Post subject: Re: Few questions regarding reverse polarity protection.. Reply with quote  Mark this post and the followings unread

Hey Phobos,

thank you very much for your answer. Your example is very good, but I wonder if the mosfet protection could help in exactly the case you describe. What I don't understand here, is why the signal from output A can cause damage to B when it's powered off or did you mean that it will just cause damage in the case that B is shorted out (only that option would makes sense to me Surprised )?

I guess in the case that a cable will fall on the pcb and causing a short, it is most likely that a component on that board will get fried and not one in a connected synth. Well, I could live with replacing an ic or so then. But probably I will just make some pcb panels for those circuits, this will also look fine I guess Cool
To protect all the outputs with diodes would be a little bit of an overkill for these little boards, I think. But all the inputs are protected with schottky diodes already.

PHOBoS wrote:
hmm I thought I did reply, apparently I only thought about doing it Embarassed

Quote:
And I think what happend is that I accidentally connected ground/0v with the positive supply voltage, so there was a short. This happend a few times. I use those dc jacks with a metalring around the hole. In my circuits that metalring/connector is connected to ground, so when I plug it in and I touch the ring with the tip of the dc barrel connector and a midi cable or a audio/cv cable is connecting both synths, then I short ground to the positive supply. I even saw some little sparks the other day!

Although shorts are never good, I don't think this would actually cause any damage (at least not with a single supply) except to the power supply itself.
The circuits just wouldn't get any power. Well, I guess it would be possible if you have something else connected to it. For example if you have a circuit (A)
that is powered separately and you have an output of it connected to the input of another circuit (B). If circuit B isn't powered (because you are shorting it out)
then the signal coming from the other circuit (A) might cause some damage. (this is why you should only connect circuits together after they are powered).

With these kinds of circuits electrostatic discharge is usually not really a concern once components are connected in a circuit, as most of the time there is
a path to GND, often a resistor/pot directly connected to the input. However, clamping diodes could prevent any damage with the example I mentioned before
where a circuit is connected but not powered and also just over-voltage in general. For example when you have a circuit powered by 5 Volt and you connect an
input to a circuit that can produce a higher output voltage. Also outputs should have protection against shorts.

Of course with these open circuits there is also the possibility of a cable or something else dropping on it and shorting something out.
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PostPosted: Fri Aug 16, 2019 5:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey Jovian,

also thanks for your answer! Yes after I built something, I measure quite often the resistance between the power rails to make sure there is no short. I saw the sparks on the dc connector because these baralel connectors are often haven't an isolated tip (but a few have a plastic isolator there), when they touch a grounded metal enclosure for example this will cause a short, in case there is another circuit powered with the same psu and if they are also connected with a cable between some inputs and outputs.
Like I said, this happend to me quite a few times. I guess I will just do it like Phobos said and connect all the cables after I connected the power connectors. Somehow I didn't knew that Embarassed
It is a true that there will be voltage drop when using diodes, that is why I often use schottky diodes with a lower forward voltage.

Btw I like and use the Eurorack format, but I found those little boards nicer somehow. They are a bit like Eric Archers spacer rockers, just with more cables Laughing

JovianPyx wrote:
Well, I wouldn't say we're too lazy to write about this.

In fact, there is a wide range of opinions about this. Some say "don't bother, just know what you're doing and be careful", while others try every which way with diodes and whatnot to prevent a disaster.

In my opinion, a polarized plug that is impossible to plug in backwards is a good method and then no diodes or other added components are necessary. The EuroRack PSU connector looks good for this.

As for shorts, the DMM is your best friend. If you measure a module's rail connectors to ground for ohms, you'll immediately see that as say 5 ohms or less - that's either a low resistance shunt or a short. Fix that and check again with a DMM repeating until it's good. You can even calculate the current drawn by the shunt/short with Ohm's Law I=E/R. You can see that 15 volts into 5 ohms would try to draw 3 amperes. Most modules won't need that kind of current, so that's a signal something is wrong.

If making your own boards, be vigilant about how things are wired up. I would never install diodes to correct for a bad etch. Also remember that diodes will "eat" about 0.6 volts. It's also good to remember that the diodes are there forever even tho you plug it in once in most cases and if the circuit is correctly wired to the power connector, the diodes aren't doing anything but eating that .6v.

Same with making your own cables. Make them correctly and then check the wiring of the cable with a DMM. If it's correct, it will work and no active protection is necessary.
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PostPosted: Fri Aug 16, 2019 7:07 am    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:
In my opinion, a polarized plug that is impossible to plug in backwards is a good method and then no diodes or other added components are necessary. The EuroRack PSU connector looks good for this.


Wrong, Scott -- a lot of Eurorack modules (including the Doepfer range) don't use box headers for their power connector, and you are able to plug power into modules backwards.

It's a recurring bugbear for new users.
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PHOBoS



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PostPosted: Fri Aug 16, 2019 8:34 am    Post subject: Re: Few questions regarding reverse polarity protection.. Reply with quote  Mark this post and the followings unread

kaputtpanzer wrote:
What I don't understand here, is why the signal from output A can cause damage to B when it's powered off or did you mean that it will just cause damage in the case that B is shorted out (only that option would makes sense to me Surprised )?

It depends a lot on the circuits, but it can be bad to inject a voltage into a circuit that isn't powered. Something I didn't realize is that you are actually
shorting out the power rails of the circuit itself too, not just the power when you plug it in. I think for a lot of circuits this probably won't do any damage,
although the power supply itself won't be happy with it. But still, injecting a voltage into an input could also potentially damage something if the power
rails are shorted.

kaputtpanzer wrote:
I guess I will just do it like Phobos said and connect all the cables after I connected the power connectors. Somehow I didn't knew that Embarassed

To be honest I have different synths that are patched together and have seperate supplies and haven't had any problems (yet) when powering them up
one after the other. Sometimes I don't even power all of them while they are still connected. so do as I say don't do as I do Laughing It's just a precaution.

For the outputs a resistor in series would give you enough protection. When using those connectors you first touch the gnd connection when you plug
a cable in. If the other side of that cable is connected to an output you are shorting it to GND.
Midi connections shouldn't be able to cause any problems btw, as midi inputs should be opticallly isolated. At least for the DIN connectors, no idea if
that is still the case with USB.

I have to check the circuits you suggested again, as I didn't realize that you were shorting the rails.


oh and I forgot to say: great looking PCB! I love it when the components are neatly lined up.

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JovianPyx



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PostPosted: Fri Aug 16, 2019 9:24 am    Post subject: Reply with quote  Mark this post and the followings unread

@AlanP

Oy - I wasn't aware of that. I'm working with Tim Goslin who does Eurorack modules. For our project, he sent me what he said was a Eurorack PSU and it along with a module board for a DAC has connectors that cannot be plugged in backwards.

To use a reversible connectore is, IMO, not good, and well, a bit lame. Power connectors should, IMO, NEVER be reversible. I, personally would never do reversible connectors for power.

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PHOBoS



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PostPosted: Fri Aug 16, 2019 9:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
In my opinion, a polarized plug that is impossible to plug in backwards is a good method and then no diodes or other added components are necessary. The EuroRack PSU connector looks good for this.

yep a proper polarized connector and no diodes would be ideal, but as alan mentioned that's not really the case with eurorack so
I'd always add some polarity protection to modules. You can indeed use the boxed header connectors, at least if you design/solder
modules youself, however they can be a PITA to unplug, especially from a PSU inside a case. They also take up a bit more space on
the PCB so my designs don't use them Embarassed (but they do have polarity protection).

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ixtern



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PostPosted: Fri Aug 16, 2019 2:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

My way to protect against reverse polarity and shorts is like below: resetable fuses and diodes.


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PostPosted: Fri Aug 16, 2019 5:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

In-line diodes are better, but then you do have to live with 11.3V and -11.3V.
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kaputtpanzer



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PostPosted: Sat Aug 17, 2019 2:23 pm    Post subject: Re: Few questions regarding reverse polarity protection.. Reply with quote  Mark this post and the followings unread

I think I never built a circuit with no resistor in series with the output. But I learned that people do this sometimes. For example, directly connecting a jack socket to the output of an op amp Shocked I think people are afraid that their signals get attenuated too much. I mostly use a 10k resistor at the output, sometimes a higher value and sometimes a lower. Often I remove the dc offset with a highpass filter. I find that somewhat important when building single supply op amp circuits.

Thanks Phobos, oh yes I also really like to have the components neatly lined up and when everything is placed quite close together.

BTW thanks to everybody for your help and tips so far. I ordered a few TO220 mosfet transistors and also a few polyfuses and I am going to make some experiments the next weeks Cool

PHOBoS wrote:
kaputtpanzer wrote:
What I don't understand here, is why the signal from output A can cause damage to B when it's powered off or did you mean that it will just cause damage in the case that B is shorted out (only that option would makes sense to me Surprised )?

It depends a lot on the circuits, but it can be bad to inject a voltage into a circuit that isn't powered. Something I didn't realize is that you are actually
shorting out the power rails of the circuit itself too, not just the power when you plug it in. I think for a lot of circuits this probably won't do any damage,
although the power supply itself won't be happy with it. But still, injecting a voltage into an input could also potentially damage something if the power
rails are shorted.

kaputtpanzer wrote:
I guess I will just do it like Phobos said and connect all the cables after I connected the power connectors. Somehow I didn't knew that Embarassed

To be honest I have different synths that are patched together and have seperate supplies and haven't had any problems (yet) when powering them up
one after the other. Sometimes I don't even power all of them while they are still connected. so do as I say don't do as I do Laughing It's just a precaution.

For the outputs a resistor in series would give you enough protection. When using those connectors you first touch the gnd connection when you plug
a cable in. If the other side of that cable is connected to an output you are shorting it to GND.
Midi connections shouldn't be able to cause any problems btw, as midi inputs should be opticallly isolated. At least for the DIN connectors, no idea if
that is still the case with USB.

I have to check the circuits you suggested again, as I didn't realize that you were shorting the rails.


oh and I forgot to say: great looking PCB! I love it when the components are neatly lined up.
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PostPosted: Sat Aug 17, 2019 2:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

ixtern wrote:
My way to protect against reverse polarity and shorts is like below: resetable fuses and diodes.


I also built this protection circuit quite a few times, but sometimes I just used a 10R or 22R resistor instead of a fuse. One time I had this variant on the breadboard and I was accidentally shorting something out and then the resistor burned a little hole in my breadboard Rolling Eyes looks like I am the master of short circuits!
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PostPosted: Sat Aug 17, 2019 7:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

The 10R needs a significant watt rating if it is to withstand a direct short and not burn out. A 10R in series with a 12 volt powered circuit will need to dissipate 14.4 watts of power in that case or it can easily burn out.

So that method needs a 10R at 15 watt in order to stay a 10R.

Anything less than 14.4 watts rating can easily self destruct.

Of course, a properly operating circuit will have some resistance as well and that will cause voltage sharing between the 10R and the rest of the circuit which will cause the 10R to dissipate less, but this protection method assumes that the circuit bit could have a zero ohm short. In that case, the 10R takes the entire brunt of the voltage and it will dissipate the 14.4 watts.

15 volts is even worse, a 10R protector needs to be able to dissipate 22.5 watts (wow!) in order to remain good in case of a dead short (not entirely uncommon in the world of DIY electronics). This is calculated with Ohm's Law as P = E^2 / R (Power = E squared over R). Ohm's Law is your friend. Smile Cool

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PostPosted: Sun Aug 18, 2019 3:19 am    Post subject: Reply with quote  Mark this post and the followings unread

hehe yeah I know, the 10R and a big capacitor formed a low pass filter at the dc input to get rid of some noise and ripple. When I will use this method again I will throw in a ptc or something, for sure!

But it also depends on the wallwart, if you use a 200ma psu then it is only (12 V * 0.2 A =) 2.4 watts Razz Isn't it? But even this is a pretty high value and too much for the 0.6 W resistors I use.

And it will also not be very healthy for the power supply.

JovianPyx wrote:
The 10R needs a significant watt rating if it is to withstand a direct short and not burn out. A 10R in series with a 12 volt powered circuit will need to dissipate 14.4 watts of power in that case or it can easily burn out.

So that method needs a 10R at 15 watt in order to stay a 10R.

Anything less than 14.4 watts rating can easily self destruct.

Of course, a properly operating circuit will have some resistance as well and that will cause voltage sharing between the 10R and the rest of the circuit which will cause the 10R to dissipate less, but this protection method assumes that the circuit bit could have a zero ohm short. In that case, the 10R takes the entire brunt of the voltage and it will dissipate the 14.4 watts.

15 volts is even worse, a 10R protector needs to be able to dissipate 22.5 watts (wow!) in order to remain good in case of a dead short (not entirely uncommon in the world of DIY electronics). This is calculated with Ohm's Law as P = E^2 / R (Power = E squared over R). Ohm's Law is your friend. Smile Cool
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PostPosted: Sun Aug 18, 2019 4:42 am    Post subject: Reply with quote  Mark this post and the followings unread

10 ohm resistors burning out is a feature, and I am not even kidding. Of course I don't mean that hey should
self-immolate under normal conditions, but besides acting as a low pass filter they also act as a fuse.

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PostPosted: Sun Aug 18, 2019 5:55 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
10 ohm resistors burning out is a feature, and I am not even kidding. Of course I don't mean that hey should
self-immolate under normal conditions, but besides acting as a low pass filter they also act as a fuse.


That is true, however I think a real fuse would blow faster which might be a measure that saves a diode or other part. I've never been one to use the RC filter method to reduce noise that is being caused by something else that needs to be fixed. I'd much rather eliminate the source of the noise than to try to kill it like that because in reality, if that noise is there, some of it will get through the filter anyway. Seems a bit brute force where more careful construction could do a better job. Things like transformer magnetic fields transferring mains AC noise into the DC power - much better to have put the transformer farther away. I realize that some folks construct a modular in a piecemeal manner, but even then, proper planning of such things can eliminate noise at the source rather than using a shotgun to blast a mosquito. Just my opinion.

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PostPosted: Thu Aug 22, 2019 10:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Regarding your implementation of resetable fuses, if you use them on or before every module i guess you could get away with it, but when you use them on the main power rails bus board you might get some voltage drops caused by the resistance of the fuses itself and well your VCO's wouldn't like that and would drift.

Don't know the specs of the fuses you'Re going to use. But i now from my own experience, as i have build in fuses to protect the + and - 15V rails to protect my PSU and devices... After all i ended up replacing them with big much overrated ones (slow 4A upwards) that do have literally no effect in protection but also don't mess the rails voltages up. I no rely on my PSU's self protecting abilities. But for testing a module and using small fast types 100mA or 250mA they can be nice... Perhaps i would build for such testing purposes if you don'T have a separate desktop PSU a small adapter that can be sticked in between PSU and the PCB that needs to be tested first.

...And as pointed out by Scott already, i would use some connectors with reverse polarity protection that only fit into each other in one direction.

Depending on the amount of modules to power and size of your rack, you might also want to consider to not use those regular busboards as pointed out in several threads by Graham Hinton over at Muffs.
Because the cables are a bit thin and do introduce too much resistance and many modules would share a common way at length on the same small rails, which could create crosstalk, no proper gnd arriving at the modules because it's a few ohms above the gnd of the PSU, etc.

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ixtern



Joined: Jun 25, 2018
Posts: 132
Location: Poland

PostPosted: Fri Aug 23, 2019 12:50 am    Post subject: Reply with quote  Mark this post and the followings unread

wackelpeter wrote:
Regarding your implementation of resetable fuses, if you use them on or before every module i guess you could get away with it, but when you use them on the main power rails bus board you might get some voltage drops caused by the resistance of the fuses itself and well your VCO's wouldn't like that and would drift.
...

Typical resistances for some resetable fuses are (datasheet):
0.4A/72V: 0.71 Ohm,
0.5A/72V: 0.64 Ohm,
0.5A/60V: 0.425 Ohm,
0.5A/30V: 0.68 Ohm
1.1A/72V: 0.2 Ohm
1.1A/16V: 0.075 Ohm
4A/60V: 0.026 Ohm.

1. Resetable fuses should be used on every module.
2. VCO which pitch depends on the supply voltage is a very bad design and should be avoided or modded (in such a way I've modded TH VCO Maximus, for example).
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wackelpeter



Joined: May 05, 2013
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Location: germany
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PostPosted: Fri Aug 23, 2019 2:32 am    Post subject: Reply with quote  Mark this post and the followings unread

ixtern wrote:

2. VCO which pitch depends on the supply voltage is a very bad design and should be avoided or modded (in such a way I've modded TH VCO Maximus, for example).


ehmmm... tell that Thomas Henry or any other VCO designer, all the different VCO'S i've build so far TH555VCO, 4046VCO, 258clone, ASM1 VCO rely on a stable voltage on their input within a few mV, otherwise the VCO pitch isn'T stable and you can't beef that simply up by adding bigger electrolytes on each of their power rails... a 10mV drop is on one of the rails is pretty much prone to cause some instability... Depending on what you have in your rack and how much is the difference of the current drawn at max and min this can cause bigger or smaller drops within operation, depending on your supply design and how much headroom you have calculated... and well, of course i'm not speaking about a permanent drop from 15 to 14,85V or so, i'm refering to a shifting voltage on the VCO's rails while running, for example going up and down between 14,85V and 14,95V or something like that...

I had run several of these VCO's in racks supplied by the MFOS wallwart PSU, with a 1A rating and drawing on the positive side something around 350mA... even with some cheesy 2x3300uF on each rail, before the regulator the output was slightly drifting after the regulators (somewhere in the 3-5mA region) that alone wasn't noticable but as said with a medium fast fuse of 1A before the module "bus" this translated into bigger differences that were noticeable...

So yes, adding them on each module would prevent that, but i think actually it's a bit of a overkill, at least for me and my personal needs.
I'll stick to the resistor/cap variant where i mostly exchange the resistors for ferrite beads afterwards.

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ixtern



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PostPosted: Fri Aug 23, 2019 2:56 am    Post subject: Reply with quote  Mark this post and the followings unread

wackelpeter wrote:
ixtern wrote:

2. VCO which pitch depends on the supply voltage is a very bad design and should be avoided or modded (in such a way I've modded TH VCO Maximus, for example).


ehmmm... tell that Thomas Henry or any other VCO designer, all the different VCO'S i've build so far TH555VCO, 4046VCO, 258clone, ASM1 VCO rely on a stable voltage on their input within a few mV, otherwise the VCO pitch isn'T stable and you can't beef that simply up by adding bigger electrolytes on each of their power rails... a 10mV drop is on one of the rails is pretty much prone to cause some instability...

Thomas Henry designs were aimed to be rather simple and cheap, not to be the best. ASM-1 VCO had high-quality parts (like MAT-02, CA3140) but bad-quality design (no ref voltages, pitch pit connected directly to the supply lines), Buchla 258 also. Maybe anyway no so "bad quality" as a "cost cutting" designs as good stable voltage references were not cheap at those times.
Most of the modern analog VCO designs have separate reference voltages for pitch-sensitive parts of circuit.

BTW. For my unmodded TH VCO Maximus main source of instability was ... LED on the MIDI2CV module. Blinking LED causes a few mV change of the supply line what in turn caused small but audible pitch change (when freq coarse pot was not on the minimum position).
So in the times with no blinking LEDs, stability of the VCOs even without of dedicated reference voltages could be better Smile
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