
Home  Forums  Rules  Articles  The diyAudio Store  Gallery  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
9th August 2012, 06:16 PM  #1 
diyAudio Member
Join Date: Mar 2012

Unregulated power supply output impedance
I suppose it is a simple question, but I haven't been able to find the answer.
When designing an unregulated power supply, how do we determine if the output impedance is low enough? Less than half the input impedance of the total load? And do we include the transformer's secondary resistance in the determination of output impedance? Thanks. 
9th August 2012, 06:28 PM  #2 
diyAudio Member
Join Date: Oct 2008
Location: Vancouver

Is it a Ac to Dc supply using a bridge rectifier and caps? If so, the output impedance is the same as the cap impedance, until the charge pulse happens, then the transformer impedance becomes part of the equation but it gets complicated. The cap impedance is a good approximation. And the output impedance should be closer to 1/10 (idealy as low as possible) of the load.

9th August 2012, 06:46 PM  #3 
diyAudio Member
Join Date: Mar 2012

Yes I meant an AC to DC supply, basically Transformer > Rectifier > Passive filters (RC/LC) kind of power supply.
I suppose for mid to high frequency, the cap's ESR is a good approximation. But for low frequencies where the current draw is beyond what the caps can store, the output impedance became the DC resistance of all components in series? 
9th August 2012, 07:11 PM  #4  
diyAudio Member
Join Date: Sep 2006

Quote:
Basically, it will obey the rules of sampled systems, ie. the transformer's impedance and upstream impedances will be multiplied by the ratio of the sampling period to the aperture time, but with some quirks: the average aperture time isn't constant and depends on the load, and the sampling itself is not purely binary, it is in fact a multiplication by a complex function, also varying with the load. And because it is a discrete time system, funny things will happen when the stimulus frequency is a multiple or submultiple of the sampling frequency. Unless you are familiar with Ztransform and similar tools, your best bet is to use very large filter caps...
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪ 

9th August 2012, 07:17 PM  #5  
Banned
Join Date: Aug 2012

Quote:
For DC derived from 60Hz with full wave rectifier is 15.10 power 3/C. C is the Recervoir value in Earad. 

9th August 2012, 07:23 PM  #6 
diyAudio Member
Join Date: Mar 2012

Meaning, the 1:10 impedance ratio can not be practically achieved? And, how large is "very large"? I suppose there is some kind of rules of thumb with respect to average current draw?

9th August 2012, 07:33 PM  #7 
diyAudio Member
Join Date: Oct 2011

why do you care about the output impedance of the Unregulated PSU?
Trying to calculate efficiency, losses, or some sort of distortion? Thanks.
__________________
This is not reality... 
9th August 2012, 07:44 PM  #8 
diyAudio Member
Join Date: May 2007

Are you asking about AC impedance or DC impedance? AC impedance is roughly given by the last cap in the supply. DC impedance (i.e. how much does it droop under load) is given by a messy combination of transformer secondary effective resistance (multiplied by charging pulse duty cycle) and storage capacitor value.
Effective secondary resistance R's = Rs + Rp x (Vs/Vp)^2 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). You need to know/measure/guess the charging duty cycle, otherwise use a factor of 5. Then add on any DC resistance from RC smoothing or choke resistance. So Rpsu = 5 R's + 0.005/C + Rdc This is only a rough estimate, but it will tell you whether you are close or way off the mark. 
9th August 2012, 07:47 PM  #9 
diyAudio Member
Join Date: Mar 2012


9th August 2012, 07:56 PM  #10  
diyAudio Member
Join Date: Mar 2012

Quote:


Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Unregulated Power Supply  maxseeley  Vendor's Bazaar  1  18th October 2011 06:41 PM 
Speaker impedance vs output power  kubeek  MultiWay  10  2nd May 2011 08:25 PM 
Measuring Output Impedance of Power Supply  korneluk  Tubes / Valves  21  27th December 2010 09:58 PM 
unregulated wallmart power supply cap size  ash_dac  Power Supplies  11  17th August 2005 11:39 PM 
ZV4 Impedance to Power Supply  Michael Rothacher  Pass Labs  1  13th March 2003 10:47 AM 
New To Site?  Need Help? 