# Voltage across a resistor

#### L33Lee

Joined Dec 6, 2019
3
Hello,
I have an AC power supply and I am wanting to measure the volt drop across a resistor that is in the circuit. Maybe a rather obvious and silly question but would i expect to see AC or DC volts across the resistor?
Many thanks
Lee

#### ericgibbs

Joined Jan 29, 2010
10,200
hi Lee,
Welcome to AAC.
Do you have diagram showing the position of the resistor.?
E

#### L33Lee

Joined Dec 6, 2019
3
On sheet 3 we have AC across wire number 43 and 29 and then we are measuring across the voltage shunt wires, btm, right of the diagram.
Many thanks,
Lee

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#### ericgibbs

Joined Jan 29, 2010
10,200
hi,
Wire 29 is the 0v reference, looking the switching options it is hard to work out
What is the circuit design to do.?
E

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#### DickCappels

Joined Aug 21, 2008
6,459
You might find a little of each. If you have access to the resistor you can measure the DC voltage across it then you can measure the AC component of the voltage across it too, though you might need to use a DC blocking capacitor depending upon whether your meter can read DC on the AC scale or not.

#### L33Lee

Joined Dec 6, 2019
3
Hello E, putting aside our circuit design, i have found a v.simple example of a cct on the web. basically on the lower circuit shown, attached, would we see DC volts across that resistor?

Hello Dick, thank you for your reply. at the risk of sounding dumb ( i am just the software guy on this project looking to help the electrical team) a little of each, i don't understand. what we are hoping to do is to take the voltage read across the resistor into a proprietary little din rail mounted amplifier which accepts a 0 -10 VDC signal. But because the value we see on the amplifiers display is 'jumping around' it lead to ask well are we getting AC or DC across the resistor? Hopefully i am making sense

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#### DickCappels

Joined Aug 21, 2008
6,459
Maybe Eric has a better handle on this than I, but I am now not sure what you see and what you want to determine, Can you explain a little more about what you are trying to understand, or even better, what problem you want to solve?

#### MisterBill2

Joined Jan 23, 2018
6,330
When you say it is an AC power supply does that mean that the output is AC, or does it mean that the supply is powered by the AC mains? That matters. Without that all of us are just guessing.
Is the meter that will be reading the voltage across the resistor a digital multimeter, or an analog multimeter? OR, is this a meter that iis installed across an ammeter shunt resistor?

Use the AC volts setting first, and note the reading, then set the meter to rad DC volts and take a reading.