Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => AmpTools/Tech Tips => Topic started by: terminalgs on March 04, 2014, 02:29:34 pm

Title: lower voltage VAC test box
Post by: terminalgs on March 04, 2014, 02:29:34 pm

About once a month, I end up using my variac to provide 3V to check out a transformer of some type.  While workable, I don't like my method's lack of safety (without going into the details of the rest of the variac test method....).  so I'm thinking of building a little test box like this:

(http://i.imgur.com/dLydAxZ.jpg)

I've got a 16V doorbell transformer that seems like it would work well for this.  I'd use 5 banana plugs,  or a six way switch and 2 plugs.

I think I can build it all with parts on hand.

what should overall resistance of R1+R2+R3+R4+R5 be?  1Kohm? 10Kohm?

any ideas or suggestions? 

I'm not set on voltages, but at least 1 setting for a low voltage of 1-3V is useful for testing transformers, and 6v and 12v seem very useful in general terms.


maybe R5 should be a trim pot,  or a partially a trim pot?

Title: Re: lower voltage VAC test box
Post by: jjasilli on March 04, 2014, 03:00:03 pm

I suggest 10V to test trannies.  It makes the math easier.  1V is small enough to get lost; 100V is way to large.  I just set my variac to put out 10VAC as confirmed by a meter. 

If you also want to use your proposed circuit as a power supply, the voltage divider circuit will tend to load down the device being powered.   Also the 16V tranny will actually put out out 16V only under it's rated load.  Unloaded it may put-out say 20V, which will skew your divider circuit.  I don't see the point of all this.

The beauty of the variac is that you can dial in an exact test voltage regardless of load / no-load, and measure from there.  If you're concerned about isolation use an iso tranny or any suitable 1:1 tranny before the variac.

Title: Re: lower voltage VAC test box
Post by: sluckey on March 04, 2014, 03:08:52 pm

I would suggest just plug your 16V transformer into your variac. Very safe and easy to adjust to a precise low voltage.

Title: Re: lower voltage VAC test box
Post by: terminalgs on March 04, 2014, 03:55:10 pm

sluckey, jjasilli: very good points.  KISS principal needs to be applied, no need to over think it...

Title: Re: lower voltage VAC test box
Post by: HotBluePlates on March 05, 2014, 03:33:21 pm

The problem you would have to think about is how much current could be drawn through those resistors (if you used them). If you use fixed resistors, then the current draw has to be exactly what you expected, or all the test points wind up higher/lower than what you wanted.

So Sluckey's got the right idea...

Title: Re: lower voltage VAC test box
Post by: jjasilli on March 06, 2014, 11:28:41 am

Yes, the "iso" tranny need not be 1:1