Transformer

[ebenai]

I purchased a Hammond transformer rated at 300 VA dual secondaries 25 v. When I measured I only get 12.5 from the secondaries. In looking at the datasheet: you get 25 if you wire the secondaries in paralel. Will I still be able to connect it to the rectifier board? Or should I try to exchange it for another one with 25 V " independent" dual secondaries. I'm a begginer.

[Nuuk]

A diagram showing how you have connected it all up, and where you are taking your measurments from, will (hopefully) help somebody to suggest a solution!

[BWRX]

Dual secondaries 25V usually means the transformer has two 25VAC secondaries. What you didn't mention is the primary configuration of your transformer. Does it have one or two primary windings and what voltages are they?

Since you're measuring half of the rated voltage on the secondary chances are your transformer has either a single 240V primary or two 120V primaries that you've wired in series. If the transformer has a single 240V primary you will need to get another transformer. If it has two 120V primary windings simply wire them in parallel, paying close attention to make sure they are wired in phase. The datasheet should provide a connection diagram.

Finally, be very careful when working around mains voltages. Take your time, triple check your wiring, and ask as many questions as you need to instead of toying around with it.

[BudP]

ebenal,

Perhaps you should go to Duncan amps and down load their free power supply simulation software to help you understand the circuit you are working with. This is a very accurate tool and easy to use.

http://www.duncanamps.com/psud2/index.html

It will eventually allow you to specify transformers in a very precise manner and also allow you to model the ripple characteristics you will get from any particular power circuit design.

Bud

[ebenai]

I have a attached the diagram. With the primary windings connected in series I got 12.5 V from each secondary. With the secondaries in series I get 25 V but then I only have two cables to connect to the rectifier board instead of 4 (2 AC1 and 2 AC2). Maybe I purchased the wrong transformer.

HAMMOND 182T12
Primary 2 x117
50/60 hz
VA 300.

[BWRX]

Hi ebenai.

According to this Hammond page http://www.hammondmfg.com/182.htm the transformer you have has dual 12VAC secondaries.

182T12
300
24V ct @ 12.5A
12V @ 25A
Dual

That means you'll only be able to get about 16VDC rails after rectification. You will need a different transformer if you want one with two 25VAC secondaries for about 34VDC rails after rectification.

[kscharf]

Take a look at this tranny!
http://www.apexjr.com/new.htm (scroll down the page a bit to find it)

It's $45 and 40 POUNDS to ship! over 1000VA!!!!!!

[AndrewT]

Originally posted by ebenai
I have a attached the diagram. With the primary windings connected in series I got 12.5 V from each secondary. With the secondaries in series I get 25 V but then I only have two cables to connect to the rectifier board instead of 4 (2 AC1 and 2 AC2). Maybe I purchased the wrong transformer.

HAMMOND 182T12
Primary 2 x117
50/60 hz
VA 300.

Originally posted by BWRX
Hi ebenai.

According to this Hammond page http://www.hammondmfg.com/182.htm the transformer you have has dual 12VAC secondaries.

182T12
300
24V ct @ 12.5A
12V @ 25A
Dual

That means you'll only be able to get about 16VDC rails after rectification. You will need a different transformer if you want one with two 25VAC secondaries for about 34VDC rails after rectification.

There is something that does not add up between these two posts.

Do you understand the difference between parallel and series?

The hammond literature clearly states that the T12 is a 24V ct transformer.
That means there are ONLY three secondary wires. But their diagram shows dual secondaries.

The voltages measured by ebenai show 12.5Vac per winding.
But this is done with the primaries in series. These primaries add up to 220Vac but what are they fed from? 12.5Vac is suspiciously low for a 12Vac transformer. I would expect 12.7 to 12.8Vac when fed with the correct mains voltage. But, maybe mains voltage was a little low at measurement time.

Ebenai, please clarify what you have and what you are measuring.

[BWRX]

Originally posted by AndrewT
The hammond literature clearly states that the T12 is a 24V ct transformer.

On the Hammond web page it states that the 182T12 is a dual secondary transformer. The "24VCT" in their series rating does cause confusion, but they don't mean that it is a center tapped transformer.

The voltages measured by ebenai show 12.5Vac per winding.
But this is done with the primaries in series. These primaries add up to 220Vac but what are they fed from? 12.5Vac is suspiciously low for a 12Vac transformer. I would expect 12.7 to 12.8Vac when fed with the correct mains voltage. But, maybe mains voltage was a little low at measurement time.

The primaries are 117VAC, so in series that's 234VAC. Feeding 120VAC into the primaries in series would yield about 6.2VAC on each secondary, or around 12.5VAC in series.

I agree that ebenai is confusing series and parallel wiring. Even so, if he has the 182T12 it will not give him the voltages he is looking for.

[phn]

The Hammond trafos have two secondaries. Tie two secondaries together and you have a CT trafo. The tied together leads become ground. There's nothing strange about that.