Bench dc power supply bridging capacitor values.

Thread Starter

Tony Elliott

Joined May 8, 2015
144
Hi,

I have a bench power supply and wish to power a 12v circuit, I used the below schematic for a 9 v circuit and it was fine with the bench power supply and 100uf capacitors. What capacitors would I use with a 12 voltage supply? is there a way to calculate different supply voltages?
 

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#12

Joined Nov 30, 2010
18,076
The amount of the capacitance is not related to voltage. It is related to current. A battery is already a fairly large capacitor, just not very good at high frequencies. If you are using fast pulses of current, you need to place the capacitors close to the load and calculate them to supply the proper current for something less than a millisecond.
 

dl324

Joined Mar 30, 2015
8,918
What capacitors would I use with a 12 voltage supply?
As #12 has already said, it's current that matters. The caps filter noise and bolster the power supply for current spikes.

I have a bunch of 470uF and 680uF caps, so that's what I tend to use. For high frequency decoupling, you need to use 0.1uF or 0.01uF ceramic caps in parallel with an electrolytic.
 

dl324

Joined Mar 30, 2015
8,918
don't know about how much current this circuit uses, but I would like to learn how to fix these kind of challenges.
Power usage by that type of circuit is generally a don't care when using a bench supply. Current draw will be no more than 10's of mA and the schematic already includes high frequency decoupling caps.
 

#12

Joined Nov 30, 2010
18,076
The answers are in the schematic. 0.1 uf capacitors are connected to the amplifier chips.
If you want to know the DC currents, look at the datasheets for the amplifier chips. If you want to know the current of the fast pulses, look to the load resistors in the schematic.
It looks like everything in there is 1.2 milliamps or less per circuit.
A capacitor that will only lose 10% of the supply voltage at 2 KHz while feeding 1.2 milliamps would be 0.0005 uf.
dV = I x time/C
A 0.1 uf cap is already 200 times as large as you need.
The trick is in using a small ceramic capacitor because they are fast responders, up to the megahertz range.
When you are in low frequencies, the impedance of the wiring is so small that the battery can deliver the current quickly enough from several inches away.
 

Thread Starter

Tony Elliott

Joined May 8, 2015
144
The answers are in the schematic. 0.1 uf capacitors are connected to the amplifier chips.
If you want to know the DC currents, look at the datasheets for the amplifier chips. If you want to know the current of the fast pulses, look to the load resistors in the schematic.
It looks like everything in there is 1.2 milliamps or less per circuit.
A capacitor that will only lose 10% of the supply voltage at 2 KHz while feeding 1.2 milliamps would be 0.0005 uf.
dV = I x time/C
A 0.1 uf cap is already 200 times as large as you need.
The trick is in using a small ceramic capacitor because they are fast responders, up to the megahertz range.
When you are in low frequencies, the impedance of the wiring is so small that the battery can deliver the current quickly enough from several inches away.
I am reading Forest Minns electronic book and others so I am still on learning curve. On this circuit I am using a dc bench power supply. the attached schematic is going into this triangle converter http://www.theremin.us/Circuit_Library/converter.htm what capacitors would you recommend please?
 

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ISB123

Joined May 21, 2014
1,236
You don't need any additional capacitors,included circuits already have them and your bench supply already has built in filtering.
 

#12

Joined Nov 30, 2010
18,076
what capacitors would you recommend please?
The capacitors that are in the schematic.
The only change I would make is to move C3 so it is physically connected to the 555 timer chip at pin 8 to pin 1 and place a 0.1 uf ceramic in parallel with C3.

C4 is already displayed as being connected to IC2A
 

Thread Starter

Tony Elliott

Joined May 8, 2015
144
Here are the results, the amplitude is very stable on high frequencies 20 Hz down to 420 Hz, but as you can see on the 4 images the triangle wave completely distorts below 420 Hz. (this is the design of the circuit)

Are there any changes to the triangle converter circuit I could make to get full frequency (1000 Hz down to 1 Hz approx) range or should I scrap this and try a different tri converter circuit altogether?

http://www.theremin.us/Circuit_Library/converter.htm
 

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#12

Joined Nov 30, 2010
18,076
This is a completely different question.
1) "What to do about the capacitors that afford high frequency stability."
2) "What to do about using a design that isn't intended to work at low frequencies."

Maybe time for a new Thread...maybe not. Wait and see if anybody follows a power supply title and knows about Theremins.
 

Thread Starter

Tony Elliott

Joined May 8, 2015
144
This is a completely different question.
1) "What to do about the capacitors that afford high frequency stability."
2) "What to do about using a design that isn't intended to work at low frequencies."

Maybe time for a new Thread...maybe not. Wait and see if anybody follows a power supply title and knows about Theremins.
Yes ok good point, lets see. :)
 
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Thread Starter

Tony Elliott

Joined May 8, 2015
144
I think I've got it! From the square wave audio out into the frequency to voltage circuit into the 556 exponential VCO J1 input with the Triangle and Sine wave outputs. therefore I have the original square wave circuit (for square wave), the frequency is converted into voltage to control the triangle circuit or the sine.
 

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