Unregulated power supply output impedance

[Navyblue]

Originally Posted by DF96

Are you asking about AC impedance or DC impedance?

Which one of these do I have to meet the 1:10 source to load ratio? Both?

Originally Posted by DF96

For 50Hz supply and fullwave rectification the reservoir caps get recharged every 10ms. They then droop under load by roughly I x 0.01/C, but the average DC droop will be half this so effective DC resistance due to the reservoir cap is 0.005/C (e.g. 5 ohms for 1000uF).

Sorry I don't quite under stand this part. SO do I get 10 ohms for 2000uF?

Thanks again.

[godfrey]

No, 2000uF => 2.5 Ohms, 10000uF => 0.5 Ohms etc
It's a nice rule of thumb.

[Navyblue]

Thanks.

For the size power supply cap, here it talks about a rule of thumb of about 1000uF per 1A of current output. Is that a good number?

Solid State Power Amplifier Supply Part 2

[Tony]

Originally Posted by Navyblue

Thanks.

For the size power supply cap, here it talks about a rule of thumb of about 1000uF per 1A of current output. Is that a good number?

Solid State Power Amplifier Supply Part 2

yes.....i have lots of caps so i use them on my tube amps as welll....

[jan.didden]

Originally Posted by Navyblue

Which one of these do I have to meet the 1:10 source to load ratio? Both?



Sorry I don't quite under stand this part. SO do I get 10 ohms for 2000uF?

Thanks again.

I think we should realize that although we talk about a DC power supply, the load from an audio device is an AC load.
The Zout of a simple rectifier + cap is the impedance of the cap at the freq of the load signal. The formula has been given before and it is of course dependent on the freq of the signal you are amplifying. Meaning the Zout is highest at low freqs. So you probably want to size the cap for say 1/10 of an 8 ohms load at 20Hz. Just plug the numbers in the formula C=1/(2.pi.f.(0.8 ohms)).
Don't worry about DC Zout - it's pretty irrelevant for audio in this context.

jan didden

[Navyblue]

Originally Posted by janneman

Don't worry about DC Zout - it's pretty irrelevant for audio in this context.

Thanks Jan.

So, we can completely ignore it? In my case, my transformer's secondary DC resistance is 220 ohms, and my DC load is under 1k ohms. If I plug the numbers into the formula I can't even hit 1:1 ratio let alone 1:10.

[Tony]

^what your figure means is that you will have bigger voltage drop with your load connected....

as long as you have your operating points achieved regardless you should be fine.....

it is not just secondary dc resistance of your traffo.....the primary dc resistance has an effect also.....

the kind of dc resistances you have is indicative of tube type psu's...

what are you building anyway? post some schematics and we will be able to help you for sure....

[Navyblue]

Ok, this is my last iteration of the circuit, I intend to make some changes (no parts yet) but the general idea remains the same.



So this has a high PSU Zout and a low DC load. Please let me know everything that is wrong with it.

[Tony]

aha....just as i suspected.....

what are the dc resistances of your chokes?

110v at 30mA is a 3.7k load

your total load is 70mA......this is in series with your chokes so you have to consider that also...

you want to know what your raw secondary ac is? right?

[Navyblue]

Originally Posted by Tony

aha....just as i suspected.....

what are the dc resistances of your chokes?

110v at 30mA is a 3.7k load

your total load is 70mA......this is in series with your chokes so you have to consider that also...

you want to know what your raw secondary ac is? right?

Thanks again.

The power transformer is rated at 230V, when loaded it is around 200V. Secondary windings DC resistance is 220 ohms per winding.

Chokes' DC resistance is 90 ohms for both.

The chain of 390 ohms resistors, it draws about another 70mA.

If you wonder why the resistor chain is wasting so much current, I did this to cut down the supply voltage. If you wonder why I didn't cut voltage the usual way (putting them in series with the chokes), I observed a noticeable improvement in bass when comparing between the 2. I thought it interesting, and this made me look into the effect of Zout of PSU. But this also keep the load DC resistance low, so I plan to get rid of it.

Of course in theory I should just dump the power transformer, replace it with one with appropriate rating, and do without the resistors. but that cost money.

So total current draw is around 140mA, at 110V, it would be about 786 ohms overall.