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cuibono 4th January 2008 09:30 PM

Mislabeled Signal Transformer?
 
I'm building a run of the mill +/- 18v unregulated power supply. I ordered a 36V transformer with CT from digikey - a Signal Transfomer DST-6-36. Here is the data sheet:

http://www.signaltransformer.com/Dat...ets/1.1-36.pdf

Now, I've got the primaries in parallel, and the secondaries in series, with pins 6 and 7 to ground. The ac off my wall is 125V - the output of the transformer is 48.1V AC, 66.7V DC full wave rectified.

This isn't a 36V CT transformer is it - I think it is the next highest value, a DST-6-48.

Am I getting anything wrong here??

P.Lacombe 4th January 2008 09:53 PM

The transformer is specified 36 V @ 0.55A....

But at very low intensity, voltage rises significantly. It is not unusual in such transformers...

PL.

error401 4th January 2008 09:54 PM

Re: Mislabeled Signal Transformer?
 
Quote:

Originally posted by cuibono
I'm building a run of the mill +/- 18v unregulated power supply. I ordered a 36V transformer with CT from digikey - a Signal Transfomer DST-6-36. Here is the data sheet:

http://www.signaltransformer.com/Dat...ets/1.1-36.pdf

Now, I've got the primaries in parallel, and the secondaries in series, with pins 6 and 7 to ground. The ac off my wall is 125V - the output of the transformer is 48.1V AC, 66.7V DC full wave rectified.

This isn't a 36V CT transformer is it - I think it is the next highest value, a DST-6-48.

Am I getting anything wrong here??

Transformer output voltage is rated under full load. If you're measuring it unloaded, 30% is pretty standard regulation. Under load, the output will drop; at 0.5A you should expect about 36VAC out of it. Unfortunately the regulation is not specified in the datasheet, but I know 30% is a typical value for toroids of this size.

sawreyrw 4th January 2008 09:55 PM

Hopefully, you learn something every day. I didn't know the regulation of these small transformers was so bad.

cuibono 4th January 2008 10:00 PM

Yeah, it could be unloaded voltage rise, but I doubt it could be that big. Especially when it sits right at 48V, which is the next size up transformer in the series. I think its mislabeled.

I'll power it up soon and see if the voltage drops when loaded...

jrockhead 4th January 2008 10:37 PM

Greetings, did you try to load down the outputs yet? Very possible you won't see what your expecting unless almost full load is on the transformer.

On some transformers the output swings wildly from no load to full load. This may be one of them.

Regards, Marcus

AKN 4th January 2008 11:16 PM

Quote:

Originally posted by cuibono
Yeah, it could be unloaded voltage rise, but I doubt it could be that big. Especially when it sits right at 48V, which is the next size up transformer in the series. I think its mislabeled.

I'll power it up soon and see if the voltage drops when loaded...

Hi cuibono,

Hi,

Your unloaded secondary voltage is not impossible at all. I cannot see any regulation specification on the spec sheet you provided so I took numbers from another small 6VA transformer as an example:

Transformer 6VA 2x18V (@170mA), Regulation 25%.

Assuming mains voltage spot on we get maximum unloaded AC:
36*1.25=45V
Add some tolerance and 48V is not far away.

cuibono 4th January 2008 11:56 PM

Yes, you all may be right, but I'm not fully convinced.

With a small load (about 45mA per rail), the AC stays the same at 48V output, and the rectified DC drops to 60V.

Close. I'm willing to concede that the transformer is the correct one. I'm just surprised the voltage rose so much...


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