Hi,

Transformers do get hot. The difference between unloaded and loaded is also =

less with lower VA. What may seem to be too hot at the touch is not allways=
 too =

hot. You can measure the temperature rise in the windings with the resistan=
ce =

method. Do this with an unloaded and a shorted transformer if it is short =

circuit proof otherwise use the current your limiting device (the fuse or t=
he =

PTC resistor) will allow in contiuous operation.

Trise=3DRhot/Rcold*(234.5+Tambstart) - (234.5+Tambend).

Tambstart is the ambient temperature at the start of the test and Tambend i=
s =

the ambient temperature at the end of the test measured where the generated =

heat in the transformer doesn't affect it. Let the transformer run for at l=
east =

4 hours.

You have to measure the resistance very quickly after it is switched off si=
nce =

the temperature drop can be rather fast the first degrees. For the secondar=
y =

winding you may also have to adjust the resistance with the probe and lead =

resistance.

The temperature rise should be added to your maximum rated ambient temperat=
ure =

and the temperature rise you get inside the enclosure. Then check this =

temperature against the datasheet for the transformer. Check the maximum =

winding temperature, the temperature for the insultaing materials, the pott=
ing =

materials etc.

50Hz will generally produce more heat than 60Hz for mains transformers.

/Ruben

> I have a mains powered PIC based design that is intended for operation in=
 the US
> but I want to test my prototypes here first. Local supply here in New Zea=
land is
> 230V 50Hz.
> =

> I've tried a 230/115V transformer (measured 120V) but my PCB mount transf=
ormer
> on my board gets too hot for my liking (85 deg C, or 185 deg F, no load).=
 FYI it
> is too hot to touch for more than a second or two!
> =

> I was expecting it to be warmer on 50Hz than 60Hz but not this warm. The =

> transformers I'm using are Hammond 183G24, intended for universal operati=
on,
> 10VA, dual primary 115/230V, 50/60Hz, sec 2 x 12V. They have insulation r=
ated to
> 155 deg C, but should I expect it to run this hot? Surely my measured 120=
V is
> not to high? I've tried 5 transformers and they alll run about the same t=
emp.
> =

> http://www.hammondmfg.com/183.htm
> =

> I've checked and double checked that my two primary windings are wired in
> parallel for 115V operation with the correct polarity, the output is also=
 OK and
> my no load test above even had the secondaries completely disconneted. I =
will
> wire one up for 230V operation now and check to see how hot it runs on 50=
Hz.
> =

> Trying to figure out a way of getting a 60Hz supply to test the transform=
er
> before I complain to the transformer manufacturer. I thought about 12V or=
 24V to
> mains inverters, but everything available here seems to be fixed 50Hz. Wo=
uld
> also need to be a sine wave for a proper test, and with 100VA or more I c=
ould
> test my complete product.
> =

> Ideas? Thanks.
> -- =

> Brent Brown, Electronic Design Solutions
> 16 English Street, St Andrews,
> Hamilton 3200, New Zealand
> Ph: +64 7 849 0069
> Fax: +64 7 849 0071
> Cell: 027 433 4069
> eMail:  brent.brown@clear.net.nz
> =

> =

> -- =

> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
> =

> =

> =

> -- =

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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
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