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dadinfinitum
Joined: Dec 16, 2019
Posts: 41
Location: Maryland, US
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 Posted: Tue Feb 02, 2021 6:42 am Post subject:
Adjusting values for different voltages |
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So, I'm aware the best answer for this is likely "experiment!" but I was wondering if there's a rule of thumb for changing values on resistors, pots, and caps when dealing with different voltages. For instance, a schematic uses a +12V single supply, but I want to use a +5V single supply.
Thanks! |
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dk
Joined: Feb 12, 2019
Posts: 115
Location: Europe
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| Posted: Wed Feb 03, 2021 7:32 am Post subject:
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With digital circuits like you'll find here, in many cases it won't make much of a difference... especially where pull up/pull down resistor are involved, etc. They shouldn't be too big or too small (ie anything between 10-100k is definitely safe).
With LED's and their current limiting resistors, the best is to put one on breadboard and test out different values to get the brightness you want, then you can use that same value everywhere.
For circuits not digital in nature (where values are most likely going to matter more), it depends. If we're talking summing resistors around an op-amp, you can probably just leave them as-is without it effecting anything. If it's a place where a certain amount of current needs to be present, you can apply ohm's law and aim to get roughly the same current ( I = V/R ).
Do you have a specific circuit in mind?
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dadinfinitum
Joined: Dec 16, 2019
Posts: 41
Location: Maryland, US
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| Posted: Wed Feb 03, 2021 9:10 am Post subject:
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| dk wrote: | | Do you have a specific circuit in mind? |
The two I'm looking at currently are both from PHOBoS: his PWLFO and AD/AR. |
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dk
Joined: Feb 12, 2019
Posts: 115
Location: Europe
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| Posted: Wed Feb 03, 2021 11:01 am Post subject:
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Both of those will probably work without needing to adjust anything. For the AD/AR, you won't need the 10V zener and you might need to adjust the 51k resistor attached to pin 5, but I would try it there first as-is and see what happens (it primarily effects the upper threshold while in AD mode).
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dadinfinitum
Joined: Dec 16, 2019
Posts: 41
Location: Maryland, US
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| Posted: Wed Feb 03, 2021 11:05 am Post subject:
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Thanks for all the feedback!
I guess while I'm here, super quick question, is the AD/AR switch DPDT? Which is which mode? Super newbie question! Haha. |
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PHOBoS
Joined: Jan 14, 2010
Posts: 5825
Location: Moon Base
Audio files: 709
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| Posted: Wed Feb 03, 2021 11:06 am Post subject:
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| dadinfinitum wrote: | | The two I'm looking at currently are both from PHOBoS: his PWLFO and AD/AR. |
The AD/AR would need some adjustments which you can figure out if you know how it works. I probably explained in the Xplorer thread or
for another similar AD/AR but here's a recap: when the 555 get's triggered with a high-to-low voltage on the trigger input (pin2) the output
will be high and start charging the 10uF capacitor through the attack pot. Once the voltage over the capacitor which is connected to pin6
reaches the treshold level the output will be low and discharge the capacitor through the decay pot. At least in AD mode.
In AR mode the output will not be low as long as the input is still high even when the voltage is above the treshold level and that's a slight problem.
Since the output will stay high it will keep charging the capacitor so the output voltage would get higher than in AD mode. I added 2 fixes for this.
The first one is the 10V zenerdiode which will just limit the voltage and prevent the capacitor from getting charged further.
The other one is the 51K* resistor. Normally the upper treshold level is set with the internal resistors of the 555 to 2/3 of the supply voltage.
2/3 of 12V is 8V but I wanted it to be just under 10V for AD mode (if it would be higher it would never reach it because of the zenerdiode).
By adding the extra resistor, which is parallel to one of the internal resistors, it will raise the treshold voltage. Important to note here is that the
circuit is designed for the CMOS verion of the 555 (i.e. 7555) which uses higher internal resistor values than the regular 555, so for for a
regular 555 it would have to be much lower than 51K.
Now for a lower supply voltage you will need to make some changes:
1. the zenerdiode obviously has to be rated lower than 5V.
2. the treshold voltage will be lower and thus it will take less time to charge the capacitor to that voltage, so for the same timing its value would have
to be increased. however this is a bit of a personal preference and maybe you want shorter or longer times so you can play around with different values.
3. the value of the 51K resistor will likely have to be adjusted. Since in this case it is about the relation between the external and internal resistor values
the supply voltage doesn't actually matter. But depending on what you want the output voltage to be you'll have to try what works.
You could also leave the diode and extra resistor out if you don't really care about the output voltage, it will still work fine.
The PWLFO is actually confusing me a bit. My first hunch would be that it works as is but at a higher frequency. The bottom part is just the PW section
and that should work fine. The top part is the actual oscillator and its amplitude will likely be affected which would change the frequency, but I don't know
by how much. It should work though.
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Last edited by PHOBoS on Wed Feb 03, 2021 11:11 am; edited 2 times in total |
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PHOBoS
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dadinfinitum
Joined: Dec 16, 2019
Posts: 41
Location: Maryland, US
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| Posted: Wed Feb 03, 2021 11:35 am Post subject:
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| Thank you very much for the explanation, extremely helpful. I'll play with the values on a breadboard. |
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