Simple PSU & choke question

[Ant]

I have two simple PSU's, to which I'd like to add chokes - Is there a specific mH or current value I should be looking for? The Transformers are 15vac & 9vac, with around 100mA draw on each TR. Would it be okay to use a wound resistor, say 0.25w 1R with 20 turns of annealed transformer wire? Any recommendations?

Thanks,
Ant

 

[PigletsDad]

There are two different effects you can try for.

A small choke has little effect at the input, but is worth adding between two caps - your 1R with 20 turns of wire will eat up some RF, but not make much difference in the audio band. However, for a low current application, a resistor is even more effective; try say 10Ohms (which will drop about 1V).

With the circuit as drawn, 9V AC will give about 13VDC raw, allowing for 1V of drop in the resistor will give 12ish V to feed to a regulator or whatever. 15VAC will give 21V raw, maybe 20V after a resistor.

A big choke can be added at the input. You want something like 100mH (or more) rated for 100mA operation - this will be be expensive; it is about the size of a transformer, but has to be specially made with an air gap in the magnetic path, so the iron doesn't saturate. Be aware that it will reduce the voltage coming out of the supply, as you get the average voltage of the rectified wave, not the peak value. In this case, you will get about 9VDC and 15VDC respectively.

 

[Ant]

Thanks PigletsDad, very informative.

I was hoping a simple small choke would be beneficial without dropping the voltage with a resistor, TeddyRegs follow the PSU's and are set to give 5v & 12v so dropping the Vin by 1v will drop the TR's output below the IC's tolerance (if that makes any sense?).

Will I benefit from a small choke say 100mH or is it not necessary?

Thanks,
Ant

 

[PigletsDad]

100mH is a big choke! at 100Hz (full wave mains ripple) it will have an impedance of about 60Ohms (2pi F L).

If you are short of voltage headroom, it will pull the voltage down by way more than 1V if you use it as an input choke. So the only way to use it would be to use two capacitors, with choke between, in a CLC arrangement. I think you also need to worry about its DC resistance, which will give extra voltage drop.

A choke will be effective, but the TRs are good by themselves, so why complicate matters.

 

[Barrymagrec]

100mH is a big choke! at 100Hz (full wave mains ripple) it will have an impedance of about 60Ohms (2pi F L).

If you are short of voltage headroom, it will pull the voltage down by way more than 1V if you use it as an input choke. So the only way to use it would be to use two capacitors, with choke between, in a CLC arrangement. I think you also need to worry about its DC resistance, which will give extra voltage drop.

A choke will be effective, but the TRs are good by themselves, so why complicate matters.

62.8 ohms, I think.

Sorry - misread your 60 ohms as 600 - mea culpa.

 

[Ant]

100mH is a big choke! at 100Hz (full wave mains ripple) it will have an impedance of about 60Ohms (2pi F L).

If you are short of voltage headroom, it will pull the voltage down by way more than 1V if you use it as an input choke. So the only way to use it would be to use two capacitors, with choke between, in a CLC arrangement. I think you also need to worry about its DC resistance, which will give extra voltage drop.

A choke will be effective, but the TRs are good by themselves, so why complicate matters.

Thanks again.

Complicate matters? Honestly - not a clue, just going on SQ's post on another thread. I was thinking I was missing out on something that may be beneficial?

Anyhow, thanks for your time - I'll leave it as you say.

Ta,
Ant