Bridge rectifier connected to variac

[Frankentronics]

Greetings,

I am currently equipping my workshop to start experimenting with audio amp circuits and power supplies for amps.

I already built my own Variac and have tested a few transformers. I was going to start testing some bridge rectifier circuits and stopped before going any further.

Before I go further I would like to know if there are any reasons why I should not connect a bridge rectifier directly to the Variac, other than me accidentally turning the knob too high. The reason why I would want to do this in the first place is to simplify my experiments and also because I don't have isolation type transformers for all voltages.

I really couldn't find the answer by Googling so I hope to get the answer here.

Thank you...

[Gyro]

Yes, a very simple reason. Connecting the bridge directly to the variac output would not give you any safety isolation from the mains supply. Don't Go There! 

[Damianos]

Instead of searching for an answer, take a piece of paper and make the schematic of the circuit. Now start studying what will happen when each type of load is connected to the output.
This way you will understand what you can do and what not.
For any question, here we are!

[Cliff Matthews]

For that application, I have a suggestion. The variac can power a pair of cheap as chips 16 to 24v bell transformers from most hardware stores.

Parallel the primaries and series connect the secondaries to form a cheap classical center-tapped supply with twice the VA rating of each bell transformer. With the typical 24v 50VA bell transformers and the variac set to max (say 135vac) you should get close to +- 40V rectified.

[danadak]

Google "diy isolation transformer microwave oven", a number of hits
on how to build a cheap isolation transformer.

When done be sure to megger isolation has been met, no nicked or
grounded turns....google "megger test" for reference material.


Regards, Dana.

[james_s]

I've attached a bridge to a variac lots of times, when I wanted a high-ish variable DC bus voltage. It works fine, but be aware of the limitations and safety risks.

[Frankentronics]

First of all, thank you all.

After reading your replies I did some Googling and found something interesting.



http://www.furmanpower.com/furman-technologies/discrete-symmetrically-balanced-power

It seems like this would be a good solution for anyone planning to experiment with audio and audio power supplies.

Some more Googling revealed the bad news: the price.

Now I wonder how feasible it would be to wind my own 1:1 toroidal isolation transformer. I guess some more Googling and YouTubing will provide the answer, but I wonder what you guys think?

And if that's not an option then I guess I can do that MOT hack.

Thanks...

[Damianos]

To use a variac in electronics lab it is necessary to use one or more transformers...
- An 1:1 mains transformer, at the input...
- One or more transformers at the output of the variac, depending of the application(s)...
- Both the above, to be possible to have simultaneously an isolated mains and adjustable output(s)...
To decide which is the best solution depends on what the needs are...
Put down what you have, what you want to do and start studying/designing the solution. By searching here and there, vaguely without understanding, the result will not be the best...

[Cliff Matthews]

In the US, $18 will get a pair of 24v transformers at any Home Depot. That's 80va of safe, split-bobbin, class-2 isolation safety (up to 80vdc or +- 40v rectified when Variac drives them at 135v).  For 160va, get 2 more and series or parallel each pair for either more voltage or current..  And with 120-208-240vac flexible inputs, you get the idea.. One could dream-up a whole lot more to do with these...

[David Hess]

There are two reasons not to use the output of a variac directly:

1. A variac does not provide isolation from the power line.
2. A variac's wiper has a maximum current limit so the rated power of a variac is proportional to output voltage.  If you have a variac set to low output voltage and short the output, excessive current will destroy it instead of blowing the input fuse or circuit breaker.

So either use a step down transformer on the output of the variac to solve both problems or use an isolation transformer on the input and place either a fuse or current limiting in series with the output wiper.

[Zero999]

In addition to the lack of isolation, on a variac, it's also possible for it to fail dangerously. If there's a break in the winding, below the wiper's set point, it will output the full mains voltage.

[IanMacdonald]

For audio amp experiments or repair you really need a dual polarity, current limited supply. Otherwise a simple mistake like a transistor the wrong way round can destroy the whole output stage. The inexpensive option is a 'dim bulb tester' and dual output transformer/rectifier - the more professional one is a current limited dual supply.

[Frankentronics]

Thank you all for your valuable inputs.

I did fuse my variac, using a 1A fuse, even if the variac coil is rated at 2.5A. I was thinking, precisely, about protecting the wiper.

...An 1:1 mains transformer, at the input...

I'm glad you pointed this out because I thought the 1:1 should go after the variac.

I will probably have to put all this in a new enclosure because I'm not sure I can fit it all in. That said, what should be the minimum distance between transformers so the magnetic fields don't interfere with each other? The distances I am finding through Google are not applicable to this project. And I'm assuming it would be best to use shielded transformers connected to earth ground, correct?

Thanks...

[Zero999]

Thank you all for your valuable inputs.

I did fuse my variac, using a 1A fuse, even if the variac coil is rated at 2.5A. I was thinking, precisely, about protecting the wiper.

...An 1:1 mains transformer, at the input...

I'm glad you pointed this out because I thought the 1:1 should go after the variac.

I will probably have to put all this in a new enclosure because I'm not sure I can fit it all in. That said, what should be the minimum distance between transformers so the magnetic fields don't interfere with each other? The distances I am finding through Google are not applicable to this project. And I'm assuming it would be best to use shielded transformers connected to earth ground, correct?

Thanks...

Transformers are normally well-screened enough for that to be a non-issue. A shielded mains transformer has an earthed screen between the primary and secondary windings, so stop capacitively coupled leakage from the mains to the secondary.