Help needed - reservoir capacitor after reservoir capacitor

wwenze

Member
2008-03-07 12:46 pm
I have a PSU board with 1,000uF of capacitance after the regulator. I plan to buy a DAC board to use with it, but majority of DAC boards come with its own regulated power supply on-board.

I'll not use the on-board regulators of course, but what should I do with where the DAC board's reservoir caps (total capacitance a few hundred uF)are supposed to be?

Considering that typical impedance of electrolytics are in the order of 0.1 to 0.01 ohms the 5cm wire shouldn't add any significant impedance, am I right to say that? If that's the case, will using those few-hundred uF capacitors on the DAC board be useful or detrimental? (since the impedance of the single smaller capacitor is likely to be higher than the pair of 470uF on the PSU board)

Should I replace it with ceramic or plastic film?
 

wwenze

Member
2008-03-07 12:46 pm
Another idea... if I make the LM317 on the PSU board output 9V which the DAC board with its regulators require and leave the regulator there, will I get better or worse performance this way?

I'll be using the PSU board to power some op-amps somewhere anyway (via another pair of 317/337 also on the board), the transformer I'll be using has a 9.5VAC for the 5V I don't think letting the unheatsinked LM340 on the DAC board drop the full 8.4V is a good idea.
 
The fact that many people do something does not mean it is necessarily a good idea. It should not be necessary to add lots of capacitance after a good regulator, just a little to deal with HF noise. Too much capacitance can cause instability by upsetting the negative feedback loop within the regulator. It also creates a failure mode, if the downstream caps discharge backward through the regulator on switch-off.
 
You need the local bypass. It is the inductance of the leads from power supply to load that is troublesome, not the resistance. The bypass caps close to the load mitigate this. They don't need to be big caps, especially for small loads.

I totally agree with CRC filtering. I'd add enough R to get maybe 100 mV drop from power supply to load. This can dramatically reduce high frequency noise. It also slightly reduces load regulation, but for many loads the trade-off is worth it. If you use CRC filtering then bigger local bypass caps are better.