How does overload protection in an UPS work during a short-circuit?

[cheater]

Hi djococaud,

Hi !
What about a good old light bulb in series (100W) ?
Lights up if there is a short, but only usable on small loads though...

For one thing, light bulbs introduce a voltage drop during limiting which can destroy the circuit. In specific they will lower the voltage anywhere form 20-50%. If a switching mode power supply is operated with a voltage drop this high, it will try to make up low voltage with higher current, which will drop more voltage because of the light bulb, and eventually it'll self-destruct. To prevent this, you have to make the light bulbs provide much more power margin than is safe for a power supply in fault condition. However, if the light bulb limiter can offer so much current, then the power supply will be able to self-destruct during short circuit. So you're really trading off one self-destruct scenario for another.

A separated power supply with the right fuses will act better in that case. A lot of power supplies are smps nowadays.

On the other hand fuses + bulbs might work as well, except you need a very high margin between the power you want to deliver to your device normally, and the maximum power that your mains can deliver before tripping the fuse. That's an issue here.

[djococaud]

yeah, I agree...It was just an idea...
You should use a small circuit beaker instead... (fuses would be expensive on the long term, and we are always run out of fuses when we need a new one !)

[Strada916]

The light bulb idea is a great one. Used it on many occasion repairing VHS and CRT tv's Granted the load is light.(no pun intended) Nearly all TV I repaired had SMPS, they handled the lower voltage ok. (240mains here is AU)

What's stopping you from having multiple bulbs in parallel? 4 x 250W would be just fine. Now SMPS rated at 1000kW is massive. Sound like you are repairing industrial equipment? Yes I know they create Vd but if the DUT is shorted then you can tell as the lights will be bright. The bulb idea is also ideal as it has very low resistance on start up. once on it rises.

You could put switches in circuit so you can control how many light are in parallel.

Anyway my two cents worth.

[cheater]

Strada, I'm glad you have had success repairing SMPS with this method. This means I'm likely just worrying too much. I'll try it out and tell you where I get. I'll need to come up with a lot of light bulbs! :-)