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# Measuring Voltage

## Measuring Voltage

(OP)
If I have a voltmeter that measures voltages up to 300 Volts, how can I use that same voltmeter to measure a signal between 300 volts and 600 volts? Specifically, this is for measuring the no load secondary voltage of a transformer.

Thanks

### RE: Measuring Voltage

You can't.

Then, there are several ways out. It depends on what power your transformer is. If it is a small (few hundred VA) unit, you can use a voltage divider hooked up from a couple of resistors. Use a low resistance to avoid capacitive influence and resistive loading by the voltmeter. Make sure that you understand what you are doing. Especially when it comes to power dissipation in the voltage divider and flash-over voltage of the resistors, which accidently (no pun) happens to be around 250 V.

If you are in doubt regarding the resistor's tolerance, you can easily check that by switching resistors. If you get the same reading - then your resistors are close enough.

If your voltmeter is an analogue one with typically 1000 ohms/volt, then you can simply double the range by adding 300 kohms between hot terminal and transformer terminal. If the consumption is something else, use that number.

If the transformer is a power unit, then just don't do it. The risks are too big. Arc-flash and all that.

Don't even measure the output of such a transformer with anything without a fuse (like in a fused measurement lead) or on the downstream side of a low-amp fuse.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

### RE: Measuring Voltage

(OP)
Thank you Skogsgurra for the reply. Right now I am thinking of inserting a low voltage potential transformer between the device under test and the voltmeter (which in this case is a Digital Multimeter). The transformers I would be testing range between 25VA and 7.5KVA. I would like to have it set up so that it would work for all those transformers.

### RE: Measuring Voltage

At those power levels, you will not risk much. At least not in the way of arc-flash.

A transformer - that's somewhat out-of-the-box. But surely doable.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

### RE: Measuring Voltage

There are High Voltage probes designed for this precise purpose.

Google some keywords such as: high voltage probe DVOM

The advantage is that they're off-the-shelf and (presumably) safety approved.

They seem to about $100-$200 price range.

You wouldn't want one with too high a division ratio as you'd lose resolution. But 10-to-1 would be nice.

I presume it would put a slight load on the source, due to the resistive divider.

### RE: Measuring Voltage

While I agree with the previous posts, I will suggest:
If you will need to spend any money, it will be better spent buying a meter intended and certified for 600 Volt use.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: Measuring Voltage

Exactly, Bill!

But I got the feeling that there were no money to spend. If someone uses a DMM that cannot handle more than 300 V, then there are no money to spend at all.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

### RE: Measuring Voltage

(OP)
Hey everyone. Thank you for the replies. I was advised to use a 300V DMM and a 300V multiplexer and to build a circuit between the device under test (which can have measurements up to 600V) and the multiplexer. This option was presented as the cheaper option. There is a 600V DMM available but I am not able to get a 600V multiplexer from the same company. In the meantime I will search for other possible multiplexers to use and concurrently look into the methods listed above.

### RE: Measuring Voltage

My meter is Cat 3 rated and can handle many functions... doesn't seem to have one for Homework though.

### RE: Measuring Voltage

If you have an oscilloscope probe with a 10:1 setting you can use that and multiply by 10.
No danger with 300VAC , I have never seen s probe that would not withstand that Voltage - at least as long as it take to get the reading.

Kurt

### RE: Measuring Voltage

You can do it!!
by adding a voltage divider of 2 resistors
the resistors value shall be about 1/100 of the input resistance of the meter
that will give you an error about 1%
good luck

### RE: Measuring Voltage

How about a control transformer 600/120 (reading x 5) or something with a center-tapped primary as an auto transformer (reading x 2). Work out the exact ratio on a Voltage that the meter can read directly.

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