Fusing toroids - primary or secondary?

If you have a low-wattage (rather high resistance) winding on the secondary, a short on this may not draw enough current to blow the fuse, but will still destroy the transformer by overheating. E.g. 48V- and HT-windings. So it can be a good idea to fuse these individually..

Jakob E.

Fuse goes at the power entrance, to protect against the many-many different ways things go wrong: broken insulation on wires, inside transformer, even inside the switch.

The ultimate is the UK custom of putting fuses in the wall-plug.

usually I'll put a fuse on the AC inlet. on the circuitboard i'll use a PTC(resetable thermal circuit breaker).

Hi friends!

Hopefully i found this thread!

I'm building a summing mixer with these elements:

1. Toroidal trafo 30 watts 0.83 amps 18-0-18 v

2. ssl9k psu with 15 volts output

3. the summing amplifier around two op275 wich after the specs draw 5ma each.

So, which fuse value do i need on the mains inlet? (220volts ac)

¿how is the fuse calculated? ¿which circuit elements determine the value?

Thank you for your help!

JAY X

Toroidal transformers are inductive, so you will probably need to use an anti-surge fuse - they can demand a surge on start-up that's many times the current they draw on average.

Calculating the optimum fuse value is non-trivial, as you have to factor in power supply efficiency. Probably the easiest thing to do is put an ammeter in series with the live leg on the primary and measure current draw. Choose fuse value nearest to draw.


Justin

Fuses are there to protect against catastrophic failure, usually shorts or near-shorts. You will also find that most toroids will have a thermal fuselink in the primary winding to protect against overheating. Once this goes you have to replace the toroid. It also means there's something serious wrong with either the toroid or the circuit attached to it.

To select an appropriate fuse for a small toroid, calculate the power required on the secondary side (you can measure or estimate this), add about 20% to account for transformer efficiency and divide this by your RMS mains voltage. Then select the next highest fuse value (0.5A, 1A, 2A etc). For a toroid use a slow-blow fuse. If you have large caps attached on the secondary after the rectifier you may need to increase the size of the fuse another step to account for large startup currents.

Thank you Friends!!

I didn't figure that this would be so complicated. Just for a fuse!!  ???

Thank you for your advise.  I have 1 and 2 fast fuses, but as this is a critical step in my DIY, i will let this stage to a qualified tech to check it.

JAY X