I Need a Plug-In Ammeter

[Ricardus]

I need an ammeter that I can plug into my variac to test current draw at low voltages for bringing up vintage gear the first time.

I have found things that you can plug into the wall, but I think those might be expecting 120 volts at the outlet to even work properly. I need a self powered ammeter that can tell me how much current a device is drawing when even when I bring it up slowly on the variac at lower voltages. Yes I could use my DMM but I'd rather have a solution with standard male and female plugs on it.

I haven't found much.

 

[MagnetoSound]

AC ammeters are not trivial. Probably easier to just put a low value resistor (1-ohm 50w?) in series with the line conductor and measure the voltage drop across that. Calculate the current using Ohm’s Law.

 

[Ricardus]

Well I have seen variacs with ammeters built in (I think heathkit used to have one) so it can't be too hard. Surely there must be something out there.

 

[pucho812]

I built a variac with volt and amp meter built in. They are handy. Basically I can measure the voltage at the variac output and current draw. This can tell me a lot about equipment if I am powering it for the first time. Really helps point out psu issues.

You get some inexpensive digital meters that do both voltage and amperage ac that track fairly well and quickly.

 

[radardoug]

heres an Ebay search for ac ammeter.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=ac+ammeter&_sacat=0

 

[Philbo King]

A sort of hack:
I used to use a 60W light bulb in series with the AC main when testing stuff. If it lights bright, there is a short. If not, things are probably good.

 

[shabtek]

put an appropriate old analog panel mount ammeter in a small enclosure with a socket to plug in, maybe add a switch with different shunt resistors to give you milliamp or amp range

 

[Ricardus]

A sort of hack:
I used to use a 60W light bulb in series with the AC main when testing stuff. If it lights bright, there is a short. If not, things are probably good.

Oh yeah, I know about the dim bulb testers. Those are good too.

 

[Ricardus]

heres an Ebay search for ac ammeter.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=ac+ammeter&_sacat=0

I've searched there and you'd think what I'm looking for would be something lots of guys like us would want, but there's nothing there like it.

 

[pucho812]

You want this…

https://www.ebay.com/itm/194042812013?hash=item2d2dda566d:g:kogAAOSw1JVZ69SC
Can build that.

 

[Audio1Man]

Look @ Ebay, "ampmeter" modules and display for less than $20.00
Read all details, if you only want "AMPS" and not VOLTS and or POWER most are good.
For VOLTS & POWER where low or 0 volts you will have to modify the display for external display power.

 

[Ivan K.]

A couple of photos show my "current" solution and a older, backup. The Voltech PM1000 is old, but a reliable AC analyzer, often found on eBay for parts, or cheap-not hard to repair. A free PDF Manual is still available from them. As Audio1Man mentions, most eBay voltmeters won't read correctly until they get to 40-60VAC In the US, Variacs are available with dual built-in digital voltmeters from Circuit Specialists - Electronics Supply Store. $80 for a 5A Variac with dual volts/amps. Also, these eBay meters only show average power, NOT TRMS!

Treat yourself to a true isolation transformer ahead of the Variac. A Variac is an Autoformer, NOT isolated. Make sure everything is well grounded, including your wall outlet. The dim bulb approach is well worth having too. There are many how-to articles. I recall a recent one in "Nuts and Volts" magazine.

On the Voltech photo, the top right dual meters monitor incoming line voltage & current. the Variac shows analog voltage, and the Voltech, the variac output voltage, current power, VA harmonics, etc

 

[MagnetoSound]

As I stated earlier, you can build an AC ammeter with just a 1-ohm power resistor in series and an AC voltmeter strapped across it. 1 volt on the scale per ampere of current drawn. By all means put a plug and socket on it. Does it need to be more complicated than that? This is GDIY after all, isn’t it?

 

[Audio1Man]

Look on Ebay for "Valhalla Scientific 2000 Autoranging Digital Watt-Ammerter" $60
several other models 21xx types.

If you buy 2000 you need to add a new AC Line cord to power the instrument as the orignal has a single 15A cord that powers instrument & load.

Duke

 

[Ricardus]

Look on Ebay for "Valhalla Scientific 2000 Autoranging Digital Watt-Ammerter" $60
several other models 21xx types.

If you buy 2000 you need to add a new AC Line cord to power the instrument as the orignal has a single 15A cord that powers instrument & load.

Duke

Oooh. YES. Good call!

 

[Ivan K.]

I'd stay away from any of the really cheap ones, especially the square and round ones. I bought some for a project a couple of years ago and they were not at all accurate.

Look up "AC Power Analyzers" on eBay. It is worth paying more to have an accurate reading!
eBay item number:115441330322
eBay item number:154060034043 (240V)
eBay item number:134698818686

 

[moamps]

As Audio1Man already mentioned, the digital current meters mentioned here will not work for voltages less than approx. 50V unless their power supply is modified. This procedure may not be feasible on the proposed current meters. In addition, there is a question of accuracy because the attached current transformers are probably of low precision, especially for low currents. That's why I would suggest using an analog AC meter that directly measures current ("moving iron" design or "moving coil" design with built-in rectifier) with the addition of one or two shunt resistors (a piece of wire) for multiple measurement ranges. You will recognize such meters by the symbol below the scale next to the number that shows the accuracy of the instrument, and by the scale itself, which is not linear but denser on the left side.
search for the phrase "analog ac meter 100ma"
ebay: 230918972240, 275941383763

 

[Khron]

As Audio1Man already mentioned, the digital current meters mentioned here will not work for voltages less than approx. 50V unless their power supply is modified.

... Or you could power them from the input, while measuring the output..?

In the case of a variac, that's perfectly ok, since one end is common between the in & out anyway. Just stick the isolation transformer upstream of the variac, call it a day.

 

[abbey road d enfer]

... Or you could power them from the input, while measuring the output..?

In the case of a variac, that's perfectly ok, since one end is common between the in & out anyway. Just stick the isolation transformer upstream of the variac, call it a day.

Indeed, most of these digital panel meters are available as externally powered with about 8-12Vdc or mains.
Actually, many models include an isolation xfmr (current loop).
https://www.ebay.com/itm/2030727935...rand=Unbranded&_trksid=p3458402.c101506.m1851

 

[JohnRoberts]

I have a small cheap ($15 chinese) power meter monitoring the power draw from my crawl space sump pump system.

Over the years I have had more than a little drama from an aftermarket sump pump float controller with a bad predilection to fail with the pump turned on (thats how pumps burn out). i resolved that failure mode by replacing one transistor inside the float controller with a higher voltage device I happened to have sitting around.

With the power meter I can easily see the difference between pulling water and pulling air. Knock on wood it has been working flawlessly for years. I just checked and it indicates 1.5W which is the idle draw from the float controller. A couple years ago I added a low temperature cut off so the pump wouldn't try to work in freezing weather (no bueno to pump water into a frozen discharge hose).

With the power meter I can even see the low temperature cut off in action (draw is less than 0.5W during freezing weather).

Of course my power meter with only a several hundred watt range is likely too small for your needs, but there are myriad choices. We are lucky living in todays world. Just about anything we can think of has already been made by somebody.

My variac, still sitting under my computer desk has terminal posts on the top that I can meter with a VOM to see output voltage. I still used it to get the correct voltage for programming embedded controllers but I haven't used it for years since I shut down my drum tuner company.

JR