Voltage regulator circuit - How to test the transformer

Thread Starter


Joined Mar 23, 2021
I got my first voltage regulator (0 - 30V) circuit, it comes with a transformer (see the picture I attached). I'm trying to test the transformer with a multimeter, it should convert the 220V AC from the wall outlet into 30V AC but the multimeter isnt showing anything? The lady at the shop says it wont show anything until I connect it to the rectifier and voltage regulator. Is this true? I dont understand why I cant test the AC output of the transformer with my multimeter



Joined Jun 4, 2014
With the primary connected to 220VAC you should be able to measure the secondary voltage with only the meter connected to it and the maeter on the AC volts range (it will read zero if you measure on a DC voltage range.


Joined Sep 24, 2015
The lady at the shop says it wont show anything until I connect it to the rectifier and voltage regulator. Is this true?
No! As AlbertHall said, if you set your meter to AC you can check the output of the transformer. ALWAYS! AC in - AC out.

As for the regulator - you must first rectify the AC to DC. Otherwise you'll blow the rectifier REGULATOR [edited] out. Then you must take into account converting AC to DC. You use a factor of 1.414 to ascertain the final filtered DC voltage. Filtered by the first capacitor. 30V x 1.414 = 42.42VDC. Then you subtract the common forward voltage drop through two of the diodes since current is always flowing through two of the four rectifier diodes. 42.42 - (0.6Vf x 2) = 41.22VDC filtered and rectified. If your regulator can't tolerate that high a voltage then it will blow out. Here's a basic diagram. It includes filter capacitors to smooth out the DC ripple. The exact value of capacitors you use may vary.
As you can see, the 220VAC input is transformed into 30VAC. That's still AC voltage. Once it goes through the rectifiers it is changed (rectified) into pulsating DC voltage. The 10µF cap filters that DC ripple (pulses) into a closer to pure DC. The rectifier then sets the output to your final regulated voltage. A 7805 will regulate the output to 5V. Because the DC input to the regulator is still a little bumpy the 100µF cap will filter it further and provide a strong backup for any changes in the load. Being so large it doesn't react very fast to changes, so the 0.1µF does the same thing as the 100µF, but faster, thus making a nice smooth DC output to your load.

Again, remember, rectifying AC into DC and filtering it results in a much higher voltage. If you don't filter it the rectifier will suffer and fail in short order. Unfiltered pulsating DC voltage goes from zero volts to 30 volts RMS. It won't like that at all. Filtering is the proper way to do it. AND using a transformer that is not going to put out so high a voltage that when rectified and filtered won't exceed the input rating of the regulator.
Last edited:


Joined Sep 17, 2013
I'm trying to test the transformer with a multimeter
Make sure the meter is set to measure AC voltage, NOT current.
Don't be tempted to connect your meter, set to measure current, directly to the transformer secondary 'to test how much current it can provide'!


Joined Oct 2, 2009
voltage regulator circuit.jpg
Some comments on the above circuit are warranted.

I would replace the 100μF with 10μF. Let the voltage regulator do what it is designed to do.
I would put 100μF or even 1000μF after the rectifiers. Leave the 10μF where it is.

Standard 78xx three-terminal voltage regulators require headroom >2V.
Give it 4-8V but not more than that. Remember that the headroom x load current = wasted wattage that the regulator will have to dissipate as heat.

41V is way too high for a 7805 voltage regulator.


Joined Oct 2, 2009
On testing the transformer, check that your meter can measure resistance and continuity properly.
With the transformer disconnected from any power source and load, measure the resistance across the primary windings.
Repeat across the secondary windings.

You should read about 100-500Ω across the primaries and very low resistance across the secondaries.
Also check that there is infinite resistance between primary and secondary windings, as well as no conductivity to the transformer metal frame.


Joined Mar 30, 2015
The lady at the shop says it wont show anything until I connect it to the rectifier and voltage regulator. Is this true?
It depends. But she probably doesn't know what she's talking about.

You can measure the secondary output with no load and your meter on AC. The voltage will likely read 10% higher as transformers secondaries are usually measured at full load.


Joined Sep 24, 2015
Some comments on the above circuit are warranted.
Those caps were just thrown in there without regard for how they're working. It's merely an example.

I agree, wasted voltage is wasted heat. I again used 7805 as an example, though I probably shouldn't have. The TS didn't even state what was the final voltage, so we're left to assume. And you know what happens when we make assumptions - - - .