Transformer power?

[nenad88]

So I went to local flea market and bought this for $4, ripped it apart and found nice speaker, heatsink and transformer. Transformer was main reason for buying this type of audio systems, name of system was Durabrand 5.1 sp-690w. Heatsink is 9x14cm speaker says 50w 8ohm and transformer is 8x4.5x7cm how can I know how much power can this transformer handle. I will use it only for headphone amps so I know i wont stress him at all but Im curious how to calculate. In this system transformer was powering 7 lm1875t's. Transformer fuse is 250V/1.5A.

 

[Mark Tillotson]

If you measure all the winding resistances you can roughly calculate the dissipation at any given load. You'd want the dissipation to be no more than other transformers with the same physical size, preferrably derated somewhat for uncertainties.

 

[Galu]

A 1.5A/250V primary fuse may suggest a maximum primary current (at 230V) of 0.65A, and a transformer rating of 150VA.

But, don't take my word for it - study the information contained in this source: https://m.littelfuse.com/~/media/el...elfuse-transformer-protection-white-paper.pdf

 

[nenad88]

Thanks!

 

[FauxFrench]

Really good bargain. And, 7 "genuine" LM1875.
I cannot find any good photo on the Internet of this system. Normally on the rear of the cabinet, near the power input, you have a label stating the maximum power consumption. Do you have such a label?

 

[DF96]

Weigh the transformer. Compare with catalogue figures.

 

[nenad88]

Really good bargain. And, 7 "genuine" LM1875.
I cannot find any good photo on the Internet of this system. Normally on the rear of the cabinet, near the power input, you have a label stating the maximum power consumption. Do you have such a label?

I throw everything out except radiator speaker and transformer. I didnt want to bother to desolder lm1875t's, speaker was working before ripping apart.

Weight is 1.8kg

 

[FauxFrench]

With the information you give 150VA, as suggested by Galu, seems a good guess. The power capacity of a transformer is not an exact value but depends on winding resistance and the cooling ability.
Good luck with the potential projects.

 

[nenad88]

Couldnt find any better picture so far

I dont see any rating on backplate.

 

[FauxFrench]

I see "AC power" and then I cannot read the rest.

 

[nenad88]

It says 230V 50hz

 

[FauxFrench]

Many thanks, that won't give us a hint.
If the maximum input power is stated and no important consumption outside of the transformer is known, I assume the transformer power to be 80%-90% of the maximum input power.

 

[nenad88]

yea probably

 

[Elvee]

Search this forum and the WWW: there are many hints, based on weight, dimensions, resistances.
A recent example found here:
Age-old question: reverse, perverse engineering of surplus power xfmrs.

 

[LateraLiz]

Weigh the transformer. Compare with catalogue figures.

Is this ballpark 'accurate', have you used this method to gauge the power rating of a mystery xmfr? Seems almost too easy to be true, ha!

 

[PRR]

Power transformers are historically 5VA/pound.

The properties of iron have changed little (in ways that matter for our types of iron) and copper has changed not at all. Insulation is better and commercial pressure is much higher, so ratings/pound have gone up from 5VA/lb. But comparable types will all fall in a narrow band. E-I versus torroid makes some difference. Cheap-junk versus lasts-forever can be a big difference. Industrial transformers often fall under industry guidelines and may seem under-spec for the weight (but will stand hours of overload).

 

[nenad88]

Transformer with no load gives me 14.5-0-14.5 and after rectifier filter board -20-0-20 or 40v. I assume this is 12-0-12 transformer because voltage would drop to 12-0-12 unrectified under load. Am I correct?

 

[FauxFrench]

Transformer with no load gives me 14.5-0-14.5 and after rectifier filter board -20-0-20 or 40v. I assume this is 12-0-12 transformer because voltage would drop to 12-0-12 unrectified under load. Am I correct?

Sounds right. Nominally some 13Vac. The 20Vdc rectified is unusually high for nominally 12Vac but you may have peak rectification of some spikes as well. With a slight loading of the 20Vdc it should drop to some 18Vdc.

 

[PRR]

> 20Vdc rectified is unusually high for nominally 12Vac

He reports 14.5VAC no-load. Times root-2 makes 20.5V peak. Allow a half-volt for a no-load Silicon rectifier, the math works out clean.

I would expect it to droop to 15VDC-17VDC depending on load and size of iron.

 

[FauxFrench]

I evidently agree with your math. But, the question was if it is a (nominally) 12-0-12 transformer. My impression is that " 20Vdc rectified is unusually high for nominally 12Vac". The nominal value is rather 13Vac-14Vac.