# How to measure high DC voltage?

Discussion in 'General Electronics Chat' started by Dexter8181, Jan 21, 2009.

1. ### Dexter8181 Thread Starter New Member

Jan 21, 2009
4
0
Hi all,
I'm wanting to monitor DC bus voltage in a variable frequency drive for a 5hp motor. This drive is 480VAC input so I'm assuming the max voltage is 480*2^.5 which is 678VPP. So, let's design for 700VDC. That is also assuming that the rectifiers in the VFD's are referenced to the other phases. I plan to measure the DC Voltage with a voltmeter sometime soon to really get some accurate data. That is just some background information. Now to my question.

What is a good way to accurately monitor changes on 700VDC with a decent error?
I know I could use a voltage divider, but I'm afraid of error produced by voltage drop in the DC bus by adding a voltage dividing load. And, I also don't want to waste alot of power.

2. ### leftyretro Active Member

Nov 25, 2008
394
2
Depending on the voltmeter used you may be able to read it directly. Or else use a simple voltage divider and measure the scaled down voltage. The divider resistors can be sized such that only a milliamp or so of current would be drawn from the DC buss, certainly not enough to pull down the DC buss for a 5hp motor. First thing first is to specify the voltage meter you will be using as it's input impedence is the critical spec to designing a accurate voltage divider string.

Lefty

Jan 18, 2008
5,699
907
4. ### italo New Member

Nov 20, 2005
205
1
480 vac can be measured with practicaly any voltmeter. God know when you got AC you want to measure peak dc. DC means rectifiers capacitor then worry about % divider. Anything you add make a simple measurement complicated. JUST MEASURE ac with a divider or directly.

5. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
513
Don't forget that the 'DC' you will be looking at from the rectifiers will not read the same as true DC on a meter.

Italo's comment about a capacitive peak detector is one way, if you can find 1000 volt caps.

Remember also that meter probes for voltages above 500 volts or so should include anti flashback guards etc. Normal probes can be unsafe.