DC Source Impedance measurement

hugeli60 · 12th February 2004, 05:49 PM

I have a challenege for you analog guru's out there.

Let's say I have a DC source with some internal impedance (kinda like a battery). This impedance is complex is nature with more than one equiv. C, L and R' inside. Given that this impedance will be on the order of milliohms at any particular frequency:

Does anyone out there know of a simple circuit that could accruately measure the impedance of this source. Keep in mind this is AC impedance. What I am looking for are say curves from 1hz - 10kHz for example.

This is a difficult measurement because the magnitude of the impedance is so small!

azira · 12th February 2004, 06:32 PM

Hmm... a couple of quick 'duh' questions:

1) If your source is DC, why are you interested in your AC impedances?

2) If your source impedance is known to be on the order of milliohms, then what does the value matter. As long as your input impedance is 1-ohm or more, it'll be significantly larger than your source impedance and your attenuation won't really matter.

Nelson Pass · 12th February 2004, 06:35 PM

If you have a quality setup like an Audio Precision, this would
not be a problem, as you can drive the DC source with an
AC signal through a big cap and precisely measure the AC
voltages which appear across the DC source. Even milliohms
would not be particularly daunting. You can insert a known
impedance in series with the AP output, or use its internal
25, 50, or 600 ohm source solely.

If you don't have such a refined measurement apparatus, drive
the DC source through a cap and resistor, but with a big fat
power amplifier, and use a less sensitive, but wide band, AC
voltameter.

sreten · 12th February 2004, 06:43 PM

Add a reasonable value series resistor and a DC blocking capacitor.

Drive the circuit it with an AC source and measure the AC voltage across the DC source.

This will give its impedance, assuming the R effects a current source.

sreten.

hugeli60 · 12th February 2004, 07:59 PM

That is the way I do it now. The problem is that I am trying to condense the circuit were a large cap is not needed. I am trying to fit all of this cicuitry into oa very small package. The big cap right now hatkes up all of the space. I am looking for an all op-amp solution.

Thanks!

janneman · 12th February 2004, 08:06 PM

Are you building a super smart auto-correcting zero-impedance power supply?? Admit it!!

Jan Didden

hugeli60 · 12th February 2004, 09:01 PM

No, I am actually measuring AC impedance output imp. of a DC source. For most practical situations this information is meaningless.

rherber1 · 13th February 2004, 03:07 AM

To measure the output impedance of a generator (ac or dc) you can use the load substitution method and Ohms law.

See http://www.mitedu.freeserve.co.uk/Theory/inzoz.htm

12th February 2004, 05:49 PM	#1
hugeli60 diyAudio Member Join Date: Jan 2002 Location: Williamsport, PA	DC Source Impedance measurement I have a challenege for you analog guru's out there. Let's say I have a DC source with some internal impedance (kinda like a battery). This impedance is complex is nature with more than one equiv. C, L and R' inside. Given that this impedance will be on the order of milliohms at any particular frequency: Does anyone out there know of a simple circuit that could accruately measure the impedance of this source. Keep in mind this is AC impedance. What I am looking for are say curves from 1hz - 10kHz for example. This is a difficult measurement because the magnitude of the impedance is so small!

13th February 2004, 03:07 AM	#8
rherber1 diyAudio Member Join Date: Jan 2004 Location: Perth	Measuring output impedance To measure the output impedance of a generator (ac or dc) you can use the load substitution method and Ohms law. See http://www.mitedu.freeserve.co.uk/Theory/inzoz.htm

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12th February 2004, 06:32 PM	#2
azira diyAudio Member Join Date: Dec 2003 Location: Near Seattle	Hmm... a couple of quick 'duh' questions: 1) If your source is DC, why are you interested in your AC impedances? 2) If your source impedance is known to be on the order of milliohms, then what does the value matter. As long as your input impedance is 1-ohm or more, it'll be significantly larger than your source impedance and your attenuation won't really matter.

12th February 2004, 06:35 PM	#3
Nelson Pass The one and only Join Date: Mar 2001	If you have a quality setup like an Audio Precision, this would not be a problem, as you can drive the DC source with an AC signal through a big cap and precisely measure the AC voltages which appear across the DC source. Even milliohms would not be particularly daunting. You can insert a known impedance in series with the AP output, or use its internal 25, 50, or 600 ohm source solely. If you don't have such a refined measurement apparatus, drive the DC source through a cap and resistor, but with a big fat power amplifier, and use a less sensitive, but wide band, AC voltameter.

12th February 2004, 06:43 PM	#4
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	Add a reasonable value series resistor and a DC blocking capacitor. Drive the circuit it with an AC source and measure the AC voltage across the DC source. This will give its impedance, assuming the R effects a current source. sreten.

12th February 2004, 07:59 PM	#5
hugeli60 diyAudio Member Join Date: Jan 2002 Location: Williamsport, PA	That is the way I do it now. The problem is that I am trying to condense the circuit were a large cap is not needed. I am trying to fit all of this cicuitry into oa very small package. The big cap right now hatkes up all of the space. I am looking for an all op-amp solution. Thanks!

12th February 2004, 08:06 PM	#6
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	Are you building a super smart auto-correcting zero-impedance power supply?? Admit it!! Jan Didden

12th February 2004, 09:01 PM	#7
hugeli60 diyAudio Member Join Date: Jan 2002 Location: Williamsport, PA	No, I am actually measuring AC impedance output imp. of a DC source. For most practical situations this information is meaningless.

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