AC to DC power converter issue

[Flaviking]

To power my Amp meter, I am using this converter:

http://www.amazon.com/dp/B00BXAM694/?tag=skimlinks_replacement-20

I've got a doorbell transformer, bringing down the AC voltage from 120 to about 19v, based on my multi meter readings, However, the converter is reading ~21 VDC on the out side... I've turned the screw about 30-50 times counter clockwise and it continues to read ~21V VDC out..

Is this thing broken? or am I not using my multi meter correctly? Is it even possible to have a higher DC voltage then the AC voltage coming in?

Already burned up 1 AMP meter, trying to get the out put reading in the right spot, before I hook the new one up.

Thank you in advance for your help.

 

[RufusBrewer]

Hard to say what is going on. The description is an LM317 power supply. An LM317 is a DC In/DC Out variable voltage regulator. If they advertize it as AC and DC inputs, I am guessing the black component on the left side of the board, behind the green terminal block is a diode bridge rectifier. (right behind the IN silkscreen) A diode bridge rectifier turns AC into nominal DC. If it is a diode bridge, typically it have terminals marked "+" and "-" and two terminals with the squigly sine wave symbols. If you see that, then this is a AC or DC input device.

To answer one question: Yes you can put an AC voltage in and have a higher DC voltage out.

I would expect that the screw can turn a maximum 10 turns. A 1 turn pot would be typical and all you need. If you are able to turn it 30-50 times, something is wrong.

To use your meter correctly, make sure you have it on the right setting, AC on the input, DC on the output. And if AC and DC has range setting. make sure you are set to a range above what you are measuring. On the output, you can also use the teh AC setting to see if there is any AC component. A signal can be "both" AC and DC,

Lastly, a meter alone may not tell you the whole story. To meaure an output properly you want to measure the output under load conditions. A 150 ohm 5 watt resistor should work. Depends on what you can get your hands on. Put the resistor across the output and then measure the output.

 

[coyotebrew]

Bad from china?

 

[BigFloppy]

A true RMS voltmeter readout will give you higher DC than AC

 

[Wynne-R]

I suppose your ammeter is looking for 12 volts to power it. I don’t think 21 VDC would smoke it, at least not immediately.

Check your wiring. Unplug that bad boy and look for a short. Use your ohmmeter.

As for the voltage, a DC power supply reads a higher voltage than the applied AC because of the way AC voltage is measured. AC mains voltage is 120V RMS nominal. The actual voltage varies from 0 to ±170 volts peak. Most meters actually measure the peak voltage and are calibrated to read RMS voltage based on the properties of a sine wave. As long as the AC is a sine wave it’s a good approximation.

So 19v RMS is 27v peak, which is then regulated down to your set voltage. The LM 317 is pretty easy to blow with a momentary short.