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funkyfarm
Joined: Jan 21, 2007 Posts: 583 Location: France
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Posted: Fri Apr 18, 2008 11:52 am Post subject:
10ohm/22ohm as ferride beads and small voltage drop |
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Hi,
on every module I build, I put 22ohm resistors in place of these unobtainable ferride beads.
Many of them has burnt because of wrong wiring and short-circuits ; the resistor burn with blue smoke but that saves the other ic on the board (ad633 safe, SADxxx safe and on)
So, this solution is fine for me, but I notice a small voltage drop behind these 22R : I put +15v to it and only get 14.86 or 14.67 available for the board. (i guess more ic on board leads to a greater drop)
Is this also happens with beads ?... bead resistance can be considered as insignificant ?
Until now i put 10ohm R for +/- 12V (with its associated 10uF electrolytic cap) and 22R/22uF for +/-15v.
This RC network works as a LPF ? I remember ken stone or else associating 10R and 22uF... |
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lanxe
Joined: Feb 24, 2005 Posts: 102 Location: Columbus, OH - Now Carson City NV
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funkyfarm
Joined: Jan 21, 2007 Posts: 583 Location: France
Audio files: 3
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Posted: Fri Apr 18, 2008 1:41 pm Post subject:
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thany you, in fact someone will give me some for synthesizers (the good ones)
that's why i ask i guess |
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Inventor
Stream Operator
Joined: Oct 13, 2007 Posts: 6221 Location: near Austin, Tx, USA
Audio files: 267
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Posted: Fri Apr 18, 2008 5:35 pm Post subject:
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I don't know too much about ferrite beads, just that they provide a small impedance to AC signals and not to DC signals. They are made in several ways, all of which consist of placing soft magnetic material in near proximity to an insulated wire, and often looping the wire.
If you ever saw one of those "tootsie-roll" thingies that old computer monitors have, that's a big ferrite bead used to take the edge off of the video signals, I'm fairly sure if I remember correctly.
Your problem with the smoke coming out of the resistors is that they are providing a DC impedance, causing a voltage drop and power dissipation at DC. I suppose you could make your own ferrite beads by getting any soft magnetic material (not a magnet - that's a hard magnetic material) such as a transformer core or an inductor core, and wrap a few turns of wire around it. That might do the job, though I never tried it. _________________ "Let's make noise for peace." - Kijjaz |
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blue hell
Site Admin
Joined: Apr 03, 2004 Posts: 24079 Location: The Netherlands, Enschede
Audio files: 278
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Posted: Fri Apr 18, 2008 6:24 pm Post subject:
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Inventor wrote: | Your problem with the smoke coming out of the resistors |
Not a problem a feature (for this particular application). The resistors release the smoke and hopefully not the circuit that has the error causing smoke to be released.
The ferrite beads will not offer this protection, so during experimentation I'd suggest to use the resistors, and when all is fine use the ferrites.
The advantage of the ferrites is that they will not cause a noticeable DC voltage drop but they do provide a higher AC impedance, helping to isolate the circuit from the outer world with respect to (fast) disturbances (e.g LFO clicks and oscillator bleed through). _________________ Jan
also .. could someone please turn down the thermostat a bit.
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funkyfarm
Joined: Jan 21, 2007 Posts: 583 Location: France
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Posted: Fri Apr 18, 2008 6:48 pm Post subject:
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Blue Hell wrote: | Inventor wrote: | Your problem with the smoke coming out of the resistors |
Not a problem a feature (for this particular application) |
for sure ! this is what was meant.
Blue Hell, you're like a star in the gloom of the night
Did your name came from the fact you've seen many of these 22R dying ?
Last time, I didn't see any smoke, but the resistor was so hot that it was emitting white (not red) light ?! for three seconds i've thought I've put a white LED instead (true ! i've said to myself : "huh, hah, a (bicolor) white LED must shines red or green, what's this mess ) |
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funkyfarm
Joined: Jan 21, 2007 Posts: 583 Location: France
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Posted: Fri Apr 18, 2008 7:03 pm Post subject:
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On my oakley 1o3 VCO (lot of stuff on-board) i put 15.01v and get 14.26v on the other side on the resistor.
should I better boost the voltage line to 15.5v or even 16v in order to get something closer to 15v available for the board ? |
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blue hell
Site Admin
Joined: Apr 03, 2004 Posts: 24079 Location: The Netherlands, Enschede
Audio files: 278
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Posted: Fri Apr 18, 2008 7:11 pm Post subject:
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funkyfarm wrote: | Blue Hell, you're like a star in the gloom of the night
Did your name came from the fact you've seen many of these 22R dying ?
Last time, I didn't see any smoke, but the resistor was so hot that it was emitting white (not red) light ?! for three seconds i've thought I've put a white LED insteads (true ! i've said to myself : "huh, hah, my (bicolor) white LED must shine red or green, what's this mess ) |
No, the name is just a reminder that hell is on earth, just like heaven is, or like there is no darkness without light
Yes electronics can do funny stuff at times, like green leds shining orange .. for a short while ... I never saw white hot resistors though. I do remember things flying up to the ceiling when making a little oops with the measuring probe
Bed time indeed, I hear birds _________________ Jan
also .. could someone please turn down the thermostat a bit.
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blue hell
Site Admin
Joined: Apr 03, 2004 Posts: 24079 Location: The Netherlands, Enschede
Audio files: 278
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Posted: Fri Apr 18, 2008 7:20 pm Post subject:
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funkyfarm wrote: | On my oakley 1o3 VCO (lot of stuff on-board) i put 15.01v and get 14.26v on the other side on the resistor.
should I better boost the voltage line to 15.5v or even 16v in order to get something closer to 15v available for the board ? |
When the modules are tested and being good there is no need for the resistors anymore for protection of the circuit, they still do provide some isolation for clicks & stuff.
The modules should run ok on 14 V as well, and when they do not you could remove the resistors, I would not alter the supply voltage itself. Or you could use ferrites instead of the resistors, not getting the DC drop and still providing for isolation.
I'd not worry too much about the voltage being a bit low, I'd worry about modules influencing each other more, in general that is. _________________ Jan
also .. could someone please turn down the thermostat a bit.
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funkyfarm
Joined: Jan 21, 2007 Posts: 583 Location: France
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Posted: Fri Apr 18, 2008 7:52 pm Post subject:
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Quote: | Yes electronics can do funny stuff at times, like green leds shining orange .. for a short while ... |
Quote: | I never saw white hot resistors though |
it was in the middle of the night, around 3:24 am, birds were starting singing, so I can not swear it's a good description but for sure it was not red or orange...
thank you for all. |
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bugbrand
Joined: Nov 27, 2005 Posts: 846 Location: Bristol, UK
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Posted: Mon Nov 24, 2008 3:00 am Post subject:
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Hi, I'm just dragging this thread back up again after some observations / discoveries...
I've been using the arrangement of 10ohm resistors on all module power inlets for the last couple of years - in that time it has certainly saved the ass of many chips by acting like a fuse. I'd not located a source for ferrite beads before (though have now, i think)
However....
Well, I've been working on a Weevil module PCB with three VC squarewave oscs that are ring-modulated together (uses 4046s - especially as they have internal XORs). After some prototyping I designed a PCB for test-fabrication -- I was quite proud to fit everything in so compactly and when I got the boards back the initial tests made me go WOW. I then noticed that there was a load of interaction between the oscs and my heart sank -- basically, if you listened to one osc at audio rate, while another one was at LFO rate, the pitch of the audio-osc would rise up and down each time the LFO osc changed state. I pulled my hair out for a while, but couldn't find a fix and put it down to bad layout techniques - thinking I'd better redesign and focus more on keeping power lines separate for dirty and clean supplies. (still very much learning on this front)
Yesterday, though, I had a problem with another design where I was getting a little supply droop -- I thought to jumper the input resistors and, bingo, the problem vanished. Click-click --- I tried the same method on the Weevil board and, again, the problems vanished! Happiness & relief, but still a little confusion....
Would the 10ohm resistors be limiting the current perhaps? I know that it IS good to have them there in early stages to avoid chip-blow-outs, but I'm more wary of them now.
I then had a look on Rapid again and found what look like very suitable ferrite beads -- I'd totally overlooked these before..
My understanding is that they have negligible resistance for DC supply, but they provide a little resistance (enough to reduce noise) at higher freqs -- the datasheet suggests that they probably don't have much effect on anything under 500kHz?! _________________ http://www.bugbrand.co.uk
http://www.bugbrand.blogspot.com |
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
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Posted: Mon Nov 24, 2008 5:17 am Post subject:
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bugbrand wrote: | Would the 10ohm resistors be limiting the current perhaps? I know that it IS good to have them there in early stages to avoid chip-blow-outs, but I'm more wary of them now. |
reminds me of the problems i had with my GMvoice motherboards. since 300mA where running through the 22R protection resistors they got VERY hot, and i saw a critical voltage drop. i was thinking it was a problem of the onboard regulators. blue hell put me in the right direction... _________________
cheers,
matthias
____________
Big Boss at fonitronik
Tech Buddy at Random*Source |
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frijitz
Joined: May 04, 2007 Posts: 1734 Location: NM USA
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Posted: Mon Nov 24, 2008 8:28 am Post subject:
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Good discussion! I've always been unhappy about how people automatically put in the series resistors. What these resistors do is turn your nice hard regulated supply into a soft supply -- IOW the voltage supplied to the circuit depends on how much current it draws. You turn a regulated suppply into an unregulated one.
The combination of the resistor and capacitor does provide lowpass filtering, but if you look at the numbers this filtering usually only goes down to the audio range. Slowly varying signals -- LFOs EGs, etc. -- go through unchanged. So the voltage supplied to the circuit also varies with frequency!
There certainly are situations where the RC decoupling is helpful, but there are also many situations where it is harmful. It is best IMO to use the ferrite beads and a fair amount of capacitance to kill HF garbage and to make sure your power distribution wiring is done carefully to avoid ground loops.
Ian |
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doctorvague
Joined: Mar 14, 2007 Posts: 281 Location: new mexico
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Posted: Mon Nov 24, 2008 10:22 am Post subject:
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Thanks all for all the good info. I've wondered about this for a while now.
Cheers
Phil |
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bugbrand
Joined: Nov 27, 2005 Posts: 846 Location: Bristol, UK
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Posted: Mon Nov 24, 2008 1:09 pm Post subject:
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Thanks for your comments Ian -- it is so useful and interesting when a heavyweight like yourself brings their experience to such a subject.
Righty, I'm going to order 500 ferrite beads...
(though I'll probably still make use of resistors in some initial testing) _________________ http://www.bugbrand.co.uk
http://www.bugbrand.blogspot.com |
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frijitz
Joined: May 04, 2007 Posts: 1734 Location: NM USA
Audio files: 54
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Posted: Mon Nov 24, 2008 3:45 pm Post subject:
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bugbrand wrote: | Thanks for your comments ... |
You're welcome. There are many better experts at the decoupling business around here, I'm sure. And the RC decoupling method has its place.
I've spent the last several days on a problem with my TZ VCO having to do with decoupling. The core oscillator has a method of syncing itself with the modulation source. A fairly subtle and clever method at that, as I eventually figured out. To get rid of the coupling causing the sync I ended up decoupling the comparator chips using zener diodes in series with the individual chips. The RC network approach made matters worse, because the voltage drop across the R's was about .1 V.
Always interesting new things to figure out.
Ian |
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nicolas3141
Joined: May 25, 2007 Posts: 185 Location: Christchurch, New Zealand
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Posted: Tue Nov 25, 2008 2:25 am Post subject:
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I think the trick is that you need decoupling - whether capacitors or 10R resistors or ferrite beads or 7812s or zener diodes or whatever - per functional unit. This is not necessarily per module or per PCB, but may need to be per chip or close to it. So for example if you have two VCOs on a single PCB and getting unwanted interaction, then maybe you need to design the PCB so that the two VCOs use separate chips and then decouple the two power feeds separately. |
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jhaible
Joined: May 25, 2007 Posts: 2014 Location: Germany
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Posted: Tue Nov 25, 2008 2:46 am Post subject:
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frijitz wrote: | Good discussion! I've always been unhappy about how people automatically put in the series resistors. What these resistors do is turn your nice hard regulated supply into a soft supply -- IOW the voltage supplied to the circuit depends on how much current it draws. You turn a regulated suppply into an unregulated one.
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Right.
And I would never use RC filtering on whole boards that contain a mix of audio processing circuits and CV processing circuits.
RC filtering makes the supply soft, but rather noise-free.
With discrete audio circuits, where a supply rail is often used as reference point from one stage to the next, it's still the best thing you can do.
Sometimes it helps with opamp-based, ground-referenced audio circuits, too.
Regulation, OTOH, makes the supply stiff, but also a tad noisy. (Depending on the regulator) That's good for VCOs, for instance.
I often run the +/-15V to the CV processing circuits on my PCBs directly, and use RC filtering for the audio parts.
Every reference voltage in the CV processing circuits (if it's derived from the supply rails and doesn't has its own reference diode) gets it's own RC filtering that isn't shared with anything else that could alter the voltage drop by drawing current.
JH. _________________ "I tell you the truth, if anyone says to this mountain, 'Go, throw yourself into the sea,' and does not doubt in his heart but believes that what he says will happen, it will be done for him. Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours." (Mk 11,23f) |
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funkyfarm
Joined: Jan 21, 2007 Posts: 583 Location: France
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Posted: Tue Nov 25, 2008 5:05 am Post subject:
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jhaible wrote: | And I would never use RC filtering on whole boards that contain a mix of audio processing circuits and CV processing circuits.
I often run the +/-15V to the CV processing circuits on my PCBs directly, and use RC filtering for the audio parts.
JH. |
Damn ! i've never heard anything like that !
(guess it worthed waiting six months ) |
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frijitz
Joined: May 04, 2007 Posts: 1734 Location: NM USA
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Posted: Tue Nov 25, 2008 8:11 am Post subject:
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jhaible wrote: | And I would never use RC filtering on whole boards that contain a mix of audio processing circuits and CV processing circuits. |
Agreed!
Quote: | I often run the +/-15V to the CV processing circuits on my PCBs directly, and use RC filtering for the audio parts. |
That's a sensible overall approach. But how do you handle cases where the separation isn't this simple? What was killing me this week was an opamp comparator in the audio waveshaper of a VCO. I had to give it its own decoupling, and not merely caps at the PS pins. Zeners in series with the PS lines (and caps) reduced cross talk by about 10x.
Ian |
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jhaible
Joined: May 25, 2007 Posts: 2014 Location: Germany
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Posted: Tue Nov 25, 2008 10:41 am Post subject:
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frijitz wrote: | But how do you handle cases where the separation isn't this simple? What was killing me this week was an opamp comparator in the audio waveshaper of a VCO. I had to give it its own decoupling, and not merely caps at the PS pins. Zeners in series with the PS lines (and caps) reduced cross talk by about 10x.
Ian |
Ian - well, you have a solution already!
(One I wouldn't have thought of, btw. I've only used zeners a shunts.)
My standard solution for critical cases would be a series resistor and TL431 programmable zener shunt with 10uF in parallel to the regulator.
JH. _________________ "I tell you the truth, if anyone says to this mountain, 'Go, throw yourself into the sea,' and does not doubt in his heart but believes that what he says will happen, it will be done for him. Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours." (Mk 11,23f) |
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numbertalk
Joined: May 05, 2008 Posts: 992 Location: Austin, TX
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Posted: Wed Nov 26, 2008 8:03 am Post subject:
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I have a question about shield beads. How do you determine which beads to use - i.e. what resistance at what frequency? Paul S from Synthtech told me he uses 600Ohm but the biggest I could find at Mouser was 133Ohm. Also the ones I've seen seem to be at 100MHz. Is there a problem with using on at a lower ohm-age like that? And how do you decide what to use - does it depend on the module and how much current it draws or more on your power supply voltage?
And does anyone know where to get larger ohm beads in the U.S.?
Thanks. |
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/mr
Joined: Aug 05, 2007 Posts: 223 Location: Elektron City, Sweden
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Posted: Thu Nov 27, 2008 5:21 pm Post subject:
Re: 10ohm/22ohm as ferride beads and small voltage drop |
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funkyfarm wrote: | So, this solution is fine for me, but I notice a small voltage drop behind these 22R : I put +15v to it and only get 14.86 or 14.67 available for the board. (i guess more ic on board leads to a greater drop) |
The module draws a current I, and the resistance R gives a voltage drop U=R*I according to Ohm's law. It's that simple! 22R on a module that draws 50 mA will lower your 15 V to 14 V. It's that serious...
Some modules will get problems out of this, some others won't.
funkyfarm wrote: | Until now i put 10ohm R for +/- 12V (with its associated 10uF electrolytic cap) and 22R/22uF for +/-15v.
This RC network works as a LPF ? I remember ken stone or else associating 10R and 22uF... |
It's an LPF, filtering away HF disturbances above some hundreds Hz, as already said above. The drawback is that it's also lowering your supply voltage and makes it unregulated - it varies with current draw.
It would be great if this RC LPF could go several orders of magnitude lower and eat also the 50 Hz mains hum, but that would need impractically big caps and/or bigger resistors resulting in even more voltage drop.
So beads seem better for a tested and working module... personally I don't know enough of them, so I'm also eager to hear more about their frequencies and impedances and what to choose why.
I guess it's often a good idea to build a module with a resistor, and then change it to a bead (why not just bypass it, soldering the bead on top of the R?). A thought for PCB constructors: why not make holes for both a resistor and a bead in parallel, and instruct the builder to put the bead there at the end, after she's tested the rest?
funkyfarm wrote: | On my oakley 1o3 VCO (lot of stuff on-board) i put 15.01v and get 14.26v on the other side on the resistor. |
Never use supply resistors on VCOs! Their stability [normally] relies on a stable supply voltage, which is actually used both for the frequency potentiometers, the offset trimmer, the comparator threshold, and the exponential converter! This makes the VCO as sensible to the supply as to the CV-ins! Scary, huh?
And RC filtering with a VCO on the same board as a blinking LED, another VCO, or something else with varying current draw - congratulations, they will modulate each other through the power line.
Actually, with RC filtering even a single VCO can end up untunable because its current draw depends a little bit on what is connected to its jacks...
funkyfarm wrote: | should I better boost the voltage line to 15.5v or even 16v in order to get something closer to 15v available for the board ? |
No! Definitely a dangerous idea that doesn't reduce the problem. |
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frijitz
Joined: May 04, 2007 Posts: 1734 Location: NM USA
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Posted: Thu Nov 27, 2008 8:54 pm Post subject:
Re: 10ohm/22ohm as ferride beads and small voltage drop |
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/mr wrote: | Never use supply resistors on VCOs! Their stability [normally] relies on a stable supply voltage, which is actually used both for the frequency potentiometers, the offset trimmer, the comparator threshold, and the exponential converter! This makes the VCO as sensible to the supply as to the CV-ins! Scary, huh? |
Not to disagree about the series resistors, but don't forget that most serious modern designs use stable reference voltages for these critical bits.
Ian |
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/mr
Joined: Aug 05, 2007 Posts: 223 Location: Elektron City, Sweden
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Posted: Fri Nov 28, 2008 5:05 am Post subject:
Re: 10ohm/22ohm as ferride beads and small voltage drop |
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frijitz wrote: | /mr wrote: | Never use supply resistors on VCOs! Their stability [normally] relies on a stable supply voltage, which is actually used both for the frequency potentiometers, the offset trimmer, the comparator threshold, and the exponential converter! |
Not to disagree about the series resistors, but don't forget that most serious modern designs use stable reference voltages for these critical bits. |
True, I was meaning to mention that possibility as well but forgot in the middle of the night. The VCO in question is unknown to me, so maybe I should have dropped my excited pitch a bit... |
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