# How can I measure the voltage of 40A transformer?

I need to power 10 meters of 5V LED strip, for which I bought a 40A transformer on AliExpress.

Now, as much as I believe that cheap stuff bought from China can be okay quality, I'd like to make sure I'm sending the right amount of volts in the strip. There is a little screw on the transformer (on the very right) to adjust the voltage but there are no numbers or anything.

My question is, is there a way to measure voltage coming out of the transformer before I plug it in? All multimeters I've seen have maximum limit of 200mA or 10A.

Does this limit apply to measuring voltage as well?

Do you have any other ideas how to make sure I won't burn all my LEDs when I plug them in?

• Connect the black lead to the center terminal and red to the right v, ohms, ma. Put your meter on volts dc 9 o'clock to 11 positions start high and turn down to get better readings, black lead to common and red to v+ they are probably in parallel the 3 sets adjust your voltage then connect the load (lights) now measure the voltage again and if needed increase the voltage. Dec 5, 2017 at 19:57

You're not measuring the amperage, you are measuring the voltage.

Set it to measure DC Volts on the 20 scale, read. The maximum voltage for the meter is printed right on it, 600.

The device you have is not a transformer, it is a DC power supply.

When you connect your lights, measure the voltage again with the lights connected. Don't try to measure the current with this meter.

• Is the distinction that a DC power supply transforms and rectifies? Dec 5, 2017 at 15:25
• Alright, I was just not sure because even though I don't want to measure amperage, I'd plug the jumper wire in the port that says '200mA MAX'. Thanks for confirming this does not matter when measuring voltage. Dec 5, 2017 at 15:33
• @anoniim, some multimeters have 4 ports and two of them are dedicated to amperage. Your lower-level amperage port just happens to be shared with volts and resistance. With a meter like yours, be sure not to twist the dial through the amperage settings while the leads are connected to a voltage source. Dec 5, 2017 at 15:36
• @anoniim Correct, plug the jumper wire in the port that says "200mA MAX" because below that it also says Volts, Ohms, and Milliamps on your meter. The 10 Amps DC port is only for measuring current, so you might never use it depending on what you're doing. Dec 5, 2017 at 15:48
• @isherwood - a 'Transformer' is a relatively simple device which consists of a magnetically permeable core with 2 or more conductor windings on it (the windings may be linked as in an autotransformer) and only really 'works' with AC. That 'Power Supply' is a box full of electronic parts (including 1 or more transformers). Transformers obey the 'transformer law' where input/output voltage/current are 'transformed' according to the ratio of number of turns on the windings... Dec 5, 2017 at 16:02

A is for amps (sometimes shown as I in formulas). They express current, which is exactly what it sounds like: flow. The only way to measure it is by putting a test instrument inline with the flow. It's hard.

V is for volts (sometimes shown as E in formulas) They measure voltage or potential. Volts can be measured easily by touching 2 probes across a measured point.

potential x current = useful power.

Most things are constant-voltage. They self-limit their current drawand you supply them spec voltage. Which is easy to measure. Your LED strips are constant-voltage. (Bare LEDs are not). So as long as you can measure voltage, you are all set.

If you really, really want to measure current, then permanently install as part of the circuit, something called an ammeter shunt. This is a slight constriction in current flow, like a concrete channel in a river, that makes it easy to infer current from other things that are easier to measure, in this case volts. Even your voltmeter has an ammeter shunt, that's what the 10A socket is for.

But I wouldn't ever use it. Despite your faith in cheap Cheese goods, I would not use them for anything related to life safety, and a meter measuring multiple amps, or volts over 40 - definitely counts as life safety. If you are serious about electricity, at some point you should buy your Fluke.