# How to estimate the maximum current provided by a transformer?

I have a transformer that outputs 14.3V (AC of course) and the current in short-circuit is about 13A (made a beautiful spark when I measured it :) ). Can I guesstimate from here what is the 'normal' current that I can get from this transformer without overheating it? I need 1.3A.

Other data:

Input voltage: 230V
Output 1: 14.3V
Output 2: 18V
Transformer's iron core: 4.5 x 5.5 x 3 cm.
It is written on the transformer that it has an internal thermal fuse.
Transformer name: transformer SF-57E/11B/18A
From an music player (tape, cd, pickup, radio) with speakers of about (2x) 4-8W.

• Guesstimates are always possible. – Andy aka Jul 26 '16 at 9:54
• It's directly related to your wire thickness – Mohammad Farahi Jul 26 '16 at 9:58
• @MohammadFarahi-I know but I can open it. But I think a trafo with very thin wire won't be able to output 13A in shortcircuit. – Ultralisk Jul 26 '16 at 10:01
• go look at similar sized 50 Hz trnsformers, the VA rating will be the same. – Jasen Jul 26 '16 at 10:16
• How did you measure the output voltage? With a DVM or an O-scope? Is it RMS, 0-Peak or Peak-Peak? – FiddyOhm Jul 26 '16 at 10:31

You need 14.5 V X 1.3 A = 18.9 VA. Look for 20 VA transformers on seller sites. Here is one that I found:

The one that you have is about that size, so there is a good chance that is will be ok, but you don't have much safety factor in the estimate.

• the dimensions you give are milimeters? – Ultralisk Jul 26 '16 at 11:26
• Yes. The dimensions are millimeters. – Charles Cowie Jul 26 '16 at 11:30
• Them my transformer is pretty close to yours :) Accepted. Thanks. – Ultralisk Jul 26 '16 at 11:32
• Note that Charles assumed your AC output current is to be 1.3A RMS- if you need 1.3A of DC you'll need substantially more VA from the transformer (~60%+ more for a full wave bridge+capacitor). – Spehro Pefhany Jul 26 '16 at 12:45
• I missed the edit where you revealed that there are two secondary windings. The approximately 20 VA total capacity will be divided between the two windings. It is possible that one winding could handle 20 VA if the other is unused, but I don't think that is very likely. – Charles Cowie Jul 26 '16 at 12:46

Short-circuit current is not a useful measure.

Instead, pick a tolerance — say, 10% — and gradually increase the load current until the output voltage drops by that amount (e.g., from 14.3 V to 12.9 V). This gives you one estimate of the transformer's capacity.

Pay attention to how the voltage drops as you increase the current. If there's an abrupt "knee", then you want to stop there and back off a bit. This indicates that the transformer core has started to saturate, and you don't want to operate in that region because losses go up dramatically.

Let the transformer run at that load for a while and see how warm it gets — and whether the thermal fuse trips (hopefully, it's self-resetting). If it gets too hot to touch, you need to back off on the load.

• Saturation occurs as you describe when the voltage in increased above the rated voltage. When the load current increases, the voltage drop is much more linear. The core does not saturate. – Charles Cowie Jul 26 '16 at 11:37