A simple, Poor Man's Variac???

[t1d]

This is a long thought to make a short point...

Variacs are AC transformers that can vary their AC output. They are another significant monetary investment for an electronics lab. So, a work-around may be needed, at times, as I do not have one.

I am working on an AC converter for a friend's camper. It keeps the batteries charged up. It is an all-in-one device, having an on-board AC transformer on the same board as the regulator section.

The converter is blowing fuses, so I can not have the unit on long enough to find the short. This would be the perfect time to employ a variac... To use it to inject a low value of AC and look for a component that is heating up. But, as said, I do not have one.

So, what is the work-around. Well, I need a low value of AC voltage at low amps. Having this definition in mind, it occurred to me that this device is just a common transformer. Duh...

If I had a variac, I would be setting it at about 5 volts and maybe 1 amp. 5 volts should be enough to drive the transistors of various types. 1 amp x 5 volts = 5 watts; that's enough to create discernible warmth, without melting things.

So, the short point... A simple, low value transformer may be a perfectly acceptable substitute for a variac, correct? As I have tons of them, the price is certainly right...

Cautions...
- Great care is needed, if you intend to use an oscilloscope to look at the primary side of the circuit. See Dave's video on blowing up your scope and google isolation transformers.
- Remember to use a resistor in series to absolutely limit the amount of current that is allowed to enter the circuit.

Thoughts, or suggestions?

[t1d]

It does occur to me that the 5V/1A supply might do for checking the primary side, before the DUT transformer. If the fault is not found on that side, the substitute transformer's rating will need to consider the DUT's transformer's reduction.

For example, the battery monitor in question is likely to have a secondary-side output of ~14.4v. 120v/14.4v = 12 times reduction. So, if I want 5v on the secondary, then I need 5 x 12 = 60V. But, still with the 1 amp limit. So, testing may require two, single value transformers... Correct?

[Stray Electron]

   I guessed you missed the point that a Variac is nothing but a variable transformer. You can replace it with a fixed, non-variable, transformer for a use like this.  An old 6 volt filament transformer would be fine for a use like this. In fact a real transformer will be better than a Variac since a Variac is an auto-transformer and only uses a single winding for both the primary and secondary so the AC line is still connected and it is possible to get shocked if you touch the wrong part or if there's an electrical short. Since a filament transformer is a true transformer with a separate primary and secondary windings, it is safer to use.

  If you can't find a filament transformer, there are plenty of cheap AC output wall warts available.  You should open them up and look or measure between their inputs and their outputs be be sure that they do use a transformer and not just some form of resistor or capacitive AC voltage dropper.

  And you should use some form or current limiting, such as a light bulb or a large resister, between your power source and your DUT regardless of the current rating of the transformer.

[drussell]

So, the short point... A simple, low value transformer may be a perfectly acceptable substitute for a variac, correct? As I have tons of them, the price is certainly right...

Sure, a fixed-voltage transformer can be used in place of a variac, you just can't vary it without swapping transformers or taps, but that probably isn't much of an issue for a repair project like this.

What you probably really need, though, in this case though is something that limits your input current and also gives you a visual indication of the current draw.  Have you considered constructing a dim-bulb tester?  That is the perfect apparatus for this kind of sleuthing and the current limit can be changed by switching light bulbs.

(Note that you obviously need to us real tungsten-filament incandescent light bulbs, the halogen kind are generally readily available if you don't have a stash of standard bulbs.)

[brabus]

Take a powerful audio amplifier, drive it with a 50Hz sinus and put a 24V 50Hz transformer backwards on the output (aka driven on the 24V side). You can set the amplitude with the volume knob.

Luxury mod: change the log pot with a linear one.

[t1d]

   I guessed you missed the point that a Variac is nothing but a variable transformer.

Yes, that is the light that came on... Pun intended.

Since a filament transformer is a true transformer with a separate primary and secondary windings, it is safer to use.

Yes, I have plenty of wound transformers and wall warts.

If you can't find a filament transformer, there are plenty of cheap AC output wall warts available.  You should open them up and look or measure between their inputs and their outputs be be sure that they do use a transformer and not just some form of resistor or capacitive AC voltage dropper.

Yes, I do verify that my warts are wound transformers.

And you should use some form or current limiting, such as a light bulb or a large resister, between your power source and your DUT regardless of the current rating of the transformer.

Yes, I had already considered that caution = Good safety point.

I have a DIY Isolation Transformer and I will be using it for this project. Even though I have the IT, I am just beginning to work with the AC primary side of circuits and I am using much caution and asking lots of questions.

[t1d]

What you probably really need, though, in this case though is something that limits your input current and also gives you a visual indication of the current draw.  Have you considered constructing a dim-bulb tester?  That is the perfect apparatus for this kind of sleuthing and the current limit can be changed by switching light bulbs. (Note that you obviously need to us real tungsten-filament incandescent light bulbs, the halogen kind are generally readily available if you don't have a stash of standard bulbs.)

Yes, I am considering a DBT. I have researched them a good bit. The light changing with the amperage draw seems to be a great benefit. I am just considering the smaller rated transformer as an alternative to a DBT. Maybe I need both...

[t1d]

Take a powerful audio amplifier, drive it with a 50Hz sinus and put a 24V 50Hz transformer backwards on the output (aka driven on the 24V side). You can set the amplitude with the volume knob. Luxury mod: change the log pot with a linear one.

This is a very interesting approach, brabus... I am having to think through its operations... The amplifier is the source. The sine wave (60Hz, for the USA) creates the needed AC variation. The amplifier circuit allows for the variation of the amplitude with the volume knob. Do I have that correctly? Is there a name for this device (other than "audio amplifier") that I could research?

[drussell]

Take a powerful audio amplifier, drive it with a 50Hz sinus and put a 24V 50Hz transformer backwards on the output (aka driven on the 24V side). You can set the amplitude with the volume knob. Luxury mod: change the log pot with a linear one.

This is a very interesting approach, brabus... I am having to think through its operations... The amplifier is the source. The sine wave (60Hz, for the USA) creates the needed AC variation. The amplifier circuit allows for the variation of the amplitude with the volume knob. Do I have that correctly? Is there a name for this device (other than "audio amplifier") that I could research?

Not really, you just use a standard audio amplifier.  If you happen to have a PA amplifier that will do 1800W or more into an 8 ohms load (which is 15A at 120V, the same a standard household circuit) or higher when bridged, you can just run things directly off the speaker outputs.  Otherwise you would need to step up the voltage with an appropriate transformer.  A QSC RMX-2450 works great for this kind of stunt, as do many larger Crown or pretty much any other decent quality amp. 

When you stack three of them together, you can do things like this arbitrary-frequency 3-phase AC converter/re-generator: