# impedance problem

Discussion in 'General Electronics Chat' started by ingram010, Oct 31, 2012.

1. ### ingram010 Thread Starter New Member

May 2, 2007
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Hi I need to read the voltage from a power supply, the datasheet for the power supply states that any digital measuring device needs an input impedance of at least 10M Ohms, unfortunately the Digital measuring device I have states that the maximum input impedance is 1M Ohm. What options do I have? Is there a circuit that can increase the input impedance of my measuring device?

Regards

John

2. ### t06afre AAC Fanatic!

May 11, 2009
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What kind of power supply is this? I think you can go ahead and try your current digital measuring device. If the measuring range fits the power supply output.

3. ### ingram010 Thread Starter New Member

May 2, 2007
6
0

Here is the extract from the data sheet, it is a 1KV Glassman MK series.

Voltage monitor. J1-4
A 0-10v signal, positive with respect to common, and in direct proportion to output voltage, is available at this pin. a 10k Ohm limiting impedance protects the internal circuitry so that a digital voltmeter with greater than 10 Megohms input impedance should be used to monitor this output. It is also acceptable to use a 1 mA DC full scale instrument (i.e analog meter) for monitor purposes.

I am not using a multimeter to monitor the voltage I am using a labjack u3-HV.

I have a very tight budget so I am stuck with what I have already got.

Regards

John

4. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,227
Yes I see now. In your case it will be a question of accuracy. You may be able to calculate the error by using the voltage divider formula here
http://en.wikipedia.org/wiki/Voltage_divider
And since all factors are known in this case. You can also calculate the actual voltage by using the measured voltage. Is your supply a 1 KV supply? So the voltage you use is (Vout/100)

Last edited: Oct 31, 2012
5. ### ingram010 Thread Starter New Member

May 2, 2007
6
0

I was concerned that I might damage the labjack, but I see now its only the accuracy that will be affected.

Kindest regards

John

6. ### JMac3108 Active Member

Aug 16, 2010
349
67
You could use an op-amp buffer between the output of the supply and your meter to measure the power supply.

7. ### crutschow Expert

Mar 14, 2008
20,491
5,801
To measure 1kV you could make a 10:1 voltage divider consisting of a 9M ohm resistor in series with the 1M ohm input resistance of your meter. Make sure the 9M ohm resistor is rated for 1kV.

8. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,227
I had to Google Labjack. Is is an USB AD unit (among other things). Remember as I said. You have not a single unknown factor here. So by doing some calculations you will not loose any accuracy. I should be quite easy since I guess you are using some program language to process the data from the Labjack