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Brit01 6th October 2010 11:45 AM

Transformer in reverse current doubt
If I have a small 220/6-0-6 v tranny (300mA) in reverse connected to a 12.3v (1.5 amps) heater supply what current can the small tranny deliver?

I'm using this for a 10mA FET and grid bias but it's pulled the voltage down to a tiny 20 volts from about 130 volts without load!!

Is the current reduced significantly when a tranny is used in reverse?

darryl_h 6th October 2010 12:15 PM

If the small transformer is designed to deliver 12 volts at 300mA, then it is rated at approximately 4VA. The current in the primary at maximum load will be 4/220, or about 18mA. If the transformer is reverse-connected and its output is full wave rectified and filtered, the available DC current is only 0.7 times the AC current, 18 x 0.7 = 12.6mA.

Small power transformers generally have poor regulation, and reverse connecting them is very inefficient, but the voltage drop you are experiencing seems excessive. Perhaps a circuit diagram would help us troubleshoot.

Ian444 6th October 2010 12:21 PM

300mA from 6-0-6 (lets call it 12V) is 12 x 0.3 = 3.6VA (volt-amps, or watts). When you feed 12 or so volts back into the 6-0-6 (12V) winding, you can only get 3.6VA from the primary, less actually, due to transformer losses.
Watts = VI, 3.6 = 220 x I, I = 3.6/220 = 16mA. So 10mA would be realistic I would think.

I don't understand how you only get 130V without load from a 220V transformer, sounds more like a 110/6-0-6 transformer? The rectified AC voltage from the tranny should give a DC voltage around 1.28 times the AC voltage, with a load, or 1.4 times without load.
Are you sure your multimeter is reading correctly?

If you can check the AC voltages of each transformer winding you are using, that will give you a good idea of which part isn't doing what you are expecting.

Edit - sorry Daryl, missed your post.

darryl_h 6th October 2010 12:51 PM


Originally Posted by Ian444 (
...sorry Daryl, missed your post.

Fortunately we're both singing from the same hymn book. :D

Brit01 6th October 2010 01:33 PM

Thxs guys.

Conclusion the current available from a tranny in reverse is significantly less.

Not enough for my application. Juts realised I'm running 2 FETs drawing 10mA each and also the bias.


Have to think of another way of creating the 130 volts. New transformer or a form from my 275 tranny.

Brit01 7th October 2010 10:13 AM

Reconfigured things.

I'm now drawing the grid bias from a tap off the 275 B+. Single diode and cap in reverse, 1 fixed resistor of 1K to ground and a trimmer. Works perfectly to get -40 V.

Also rewired the small tranny in reverse, just postive, with smaller caps.
around 360 volts!
Then added a bipolar configuration to tap a negative voltage.

I get +145 volts on the positive side of the FET(drain) and only 15 volts on the neg(source) through a 10K resistor.
I will readjust the resistor to a smaller value as the current was only 1mA flowing through it.

Or could I use a CCS?

I have some small DN2540's.

Can I use a DN2540 on the Drain of the fet and no resistor on the source, just direct to the negative voltage?
Not sure how to configure a transistor CCS on a FET.

Brit01 7th October 2010 10:47 AM

I think it will be better to tap -100+ volts of the B+ tranny, with the same principle, a diode or 2 in reverse, cap in reverse, and appropriate resistor to create the negative voltage.
Tubelabs powergrid has the same example to get -150 volts.
Worked for the bias perfectly.

And use the small tranny in reverse for the positive supply on the drain.

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