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Transformer in reverse current doubt


2008-07-04 10:35 am
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?
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.
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.


2008-07-04 10:35 am
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.


2008-07-04 10:35 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.


2008-07-04 10:35 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.