Idea for Cheap Variac

[Nicwix]

Variac are highly recommended here for testing amps, for good reason - beats applying your 1kVA PSU to an untested board

Can anyone see a problem with using a standard domestic light dimmer before the primary of your transformer. I have used dimmers with LV lighting transformers for years without problems.

Obviously, there would be higher than normal RF noise on the output, but that's a good way to test your filtering

You would of course need to check the rating of the dimmer - most here in Aus are rated at 300 to 500VA. That should be enough for one channel of most amps - especially since it is the total power, not amperage rating that counts (unlike a normal variac). A fuse would be good too

Thoughts ???

Nic

 

[pinkmouse]

Most domestic dimmers cannot handle inductive loads like a transformer, you would need to get one that is rated to drive low voltage lighting. Triac dimmers also put a huge amount of noise on the rails, much more than an ordinary amp type psu can handle.

 

[paulb]

A lamp dimmer does not reduce the line voltage. It delays the turn-on each phase in the AC line cycle. What ends up at the lamp is not anything like a sine wave. Applying that to a transformer would have odd results. It's not the same as a variac, which actually reduces the voltage.

 

[Nicwix]

Most domestic dimmers cannot handle inductive loads like a transformer, you would need to get one that is rated to drive low voltage lighting.

I have been running a standard domestic dimmer with 6 x 12V 50W halogen light transformers every night for eight years - should last out amp testing ...

Triac dimmers also put a huge amount of noise on the rails, much more than an ordinary amp type psu can handle.

As per my post, a stress test of the amp ...

A lamp dimmer does not reduce the line voltage. It delays the turn-on each phase in the AC line cycle. What ends up at the lamp is not anything like a sine wave. Applying that to a transformer would have odd results. It's not the same as a variac, which actually reduces the voltage.

Yes obviously, but at the end you will get reduced DC voltage on your rails ...

I am going try anyway and report results

 

[woody]

Heck! if you look on ebay you can buy a vairic cheap say $15 + shipping. I am in favor of being frugal but variac's can be had cheap. I have seen a few 2kw rated ones at ham fest for as low
as $10. If you live near zipcode 30290 I have a 500 watter you can have.

 

[Nicwix]

Bob

Thank you for your kind offer. Unfortunately, the distance between 30290 (Tyrone) and 2086 (Frenchs Forest, Sydney, Australia) is 9,366 miles (15,072 km) as the crow flies.

The few variacs on ebay in Australia typically sell for US$80 to $100, plus shipping.

Regards
Nic

 

[Duck-Twacy]

If I may abuse this topic to ask a rather noob question.
If you have a Variac where can you best apply it??

- Connect it to the amps toroid AC input side and sweep the AC from 0 - 230V (here in Europe)
- Or connect it to the bridge rectifiers en sweep it from 0 - 30 V or something.

Does this matter??

 

[skaara]

You connect output of variac to primary of your transformer.

 

[Circlotron]

If you phase control the AC input to a transformer e.g with a light dimmer it can work, but..... with an el cheapo light dimmer the chopped positive and negative half cycles coming out of the dimmer are unlikely to be absolutely equal. That means you will get some percentage of dc current flowing in your load. For a light globe - big deal, but for a transformer this is very naughty. You can burn out the transformer without even having any load applied if too much dc flows causing the tranny to saturate.

 

[Bernhard]

For amp-testing you can connect a 100W light bulb in series with the primary of the transformer.

that will protect...

I have two 110V transformers in series, one secondary is unloaded.

does the same thing.

 

[djk]

"If you phase control the AC input to a transformer e.g with a light dimmer it can work, but..... with an el cheapo light dimmer the chopped positive and negative half cycles coming out of the dimmer are unlikely to be absolutely equal. That means you will get some percentage of dc current flowing in your load. For a light globe - big deal, but for a transformer this is very naughty. You can burn out the transformer without even having any load applied if too much dc flows causing the tranny to saturate."

Absolutely correct!

Bob Carver uses triac control in his 'Magnetic Field' amplifiers. The triacs are obtained on a selection basis to insure 4th quadrant triggering, plus, the SBS used as a trigger is built from discrete parts. Off-the-shelf parts equaled disaster on earlier models.

 

[gammelmalle]

I've seen triac in MIG-weldingmachines (?)(sorry bout my "english") It sat on the input of the transformer.
BUT!!!!! I don't think (remember) that it controlled from zero to full power. just a part of it. And whwn it comes to welding I dont think any spikes or other mess on the sekundary will matter. But in a PSU !!!! ???? The idea is good but I think you will end up with problems.
A variac is expensive like gold over here. And I dont even recommend it to a PSU. Especially not on the front end. The transformer needs the current in the primary to have a magnetic flow. If you lower it too much the transformer will not work properly. (Of course you can vary the primary so some extence maybe down to 50%... but I dont KNOW)
But what/where is the problem of controlling the low voltage?

gammelnalle
ps
forgive me for my poor english