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Transient power rating of a small (24VA) 50Hz pcb mounted potted transformer?
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schmitt trigger:
So that is what "impedance protected" means.

Thanks for clarifying the term.  :-+ :-+
I had seen it, mostly as applied to small electrical motors, but wasn't sure of its meaning.
T3sl4co1l:
Indeed, the same applies to motors, which are merely rotating transformers. :)  They tend to have poorer coupling though (e.g., shaded pole type, where the core wraps around the rotor -- a leakage path), so the impedance is mostly inductive in that case.  Same mechanism applies though!

I think for transformers, it's mostly the huge primary resistance (100s to kohms?), so they still get hot under fault conditions.  You've probably seen a somewhat melty wall wart before.  Well, UL doesn't much care if it melts, or too hot to touch, just that it doesn't burst into flames...  Well, they do care if it's too hot to touch, but that's another matter I suppose...

Tim
max_torque:
Ok interesting thanks all.  I looks like the best option is to test a few transformers and see what the output looks like vs load, and to perhaps make a stab at  measuring there thermal stability (self heating) under various loads!  I won't be able to recreate the actual installed environment, but i can at least look ratiometrically at the thermal performance etc
soldar:

--- Quote from: T3sl4co1l on January 15, 2019, 09:00:43 pm --- I would be more concerned that you won't be able to draw nearly as much power as you want!  These transformers usually have quite high impedances, sometimes being short circuit tolerant as suggested above ("impedance protected").
--- End quote ---
I agree. I would put a zener diode just after the rectifier bridge and the transformer resistance was high enough that a half watt diode could handle the peaks on its own. Then someone tried to improve my design by putting a much bigger transformer and it was blowing zeners like crazy.

I have quite a few of these transformers in my junk box ready to be used in future projects. Unfortunately they flow into the junk much faster than I get to use them. It seems traditional PSU's with transformer, bridge and filter cap are going the way of the dodo bird. It is easier to provide a wart SMPS.

Another thing about these small transformers is that people really do not understand what X amps means. It is not X amps DC after rectified and filtered. It is X amps AC and if you have a rectifier and cap with very poor power factor that's your problem.


--- Quote from: T3sl4co1l on January 15, 2019, 10:30:50 pm ---Indeed, the same applies to motors, which are merely rotating transformers. :)   
--- End quote ---
Indeed, they transform electrical energy into mechanical energy. :)
soldar:
I just got to use one of my tiny blue transformers. I wanted to monitor a 230 Vac doorbell and close a contact when it was activated. I could have used a direct 230 Vac relay but I just prefer an extra degree of separation between 230 V mains and delicate electronics. On the pins of a relay they are just too close for comfort. So I put the transformer in parallel with the doorbell and the secondary is rectified and activates a 12 Vdc relay.

I guess I could substitute a 12V SMPS wart if I had to start from zero but I already have the tiny transformer, diodes, etc.

The ones I have are generally good up to about 100 mA and are good for simple battery chargers and other simple things like that.
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