Power supply: Fusing to protect circuit

[Johnson777717]

Heya folks. I need some advice on protecting my power supply from accidental shorting.

Here's the situation:
I built a multi-output power supply, which is basically 3 different power supplys which run independantly, stuffed into a single enclosure. The outputs are:
1. Regulated 5VDC positive and negative
2. Regulated variable output (both positive and negative)
3. Regulated 12VDC

The supplies will only be capable of about 1.5 amps for each output.

The voltage output is fed to a 7 segment display which is driven by a ICL7107 in order to display the output voltage and amperage.

Currently, there are 2A fuses in between the mains and the transformers. (3 fuses, 3 transformers)

Now, the power supply is a prototype of a PS that will be used by students. With that said, there may be accidental shorting of the output leads from the power supply.
Are the mains fuses capable of protecting the entire circuit from these probable shorts? If not, what is the best way to protect the power supply from accidental output lead shorting?

I appreciate your time, and any assistance that you may provide.

 

[Sebi]

The fuse cannot protect the secondary circuits (only the transformer and - maybe - the bridge rectifier). Post Your circuits for considerations. The safe way of protecting the supply and circuits on outputs possible only with variable current limiter. If the IC-s have big enough heatsink(s), the momentary shortcut not damage it.

 

[andrew2022]

how bout a low voltage, 1.5A (or 2A) fuse on each output?

 

[stevez]

Are all of the paths for current appropriately sized? Wires inside the supply, on the secondary side, can be too thin, foil traces on a board can be insufficient. If all of that is in good order then it would seem that you have some choices - some of them already mentioned.

1. Current limiting.
2. Fuse
3. Circuit breaker

Note that current limiting can happen pretty quickly. Fuses and circuit breakers can take time (very short but maybe significant) to operate though many packaged regulators have internal protection that make them tolerant of this. Current limiting can foul you up if you aren't paying attention - as current is limited the voltage drops and student may not see the drop. A fuse or breaker opens and there is nothing. With fuses you need to have replacements on hand however the inconvenience may be useful in promoting some discipline in the lab.

Back to current limiting - if you've used something like an LM723 it already has current limiting capability that you could exploit with a few additional components.

 

[Johnson777717]

Thanks for the replies folks!

I feel that the internal wire guage and foil traces are sufficient for the voltage and current they will supply.

I like the fuse Idea, because the fuse holders can be mounted on my panels without taking up too much room. Also, as stated by Stevez, blowing fuses may encourage better lab discipline. The current limiting may add too much "complexity" to the supply, leaving the students spending time watching the voltage rather than concentrating on the task at hand.

I think I'll use fuses on the outputs and see where that takes us, although a circuit breaker would be less expensive in the long run. I suppose I'll do some research on breakers to see what I can use.

Thanks to all for your input! Have a great weekend