1. Home
  2. Electronics Repair
  3. Mains fuse for a PSU - rule of thumb?

Mains fuse for a PSU - rule of thumb?

I've just been repairing a linear PSU where the toroidal transformer had failed due to a faulty 0.1µF disc capacitor across one of the rectifier diodes which had gone short circuit. The insulation on one of the low volt outputs from the transformer had melted - so it was obviously drawing excessive current before it failed.

It's a 28 volt regulated 1 amp PS made by a long defunct pro audio company. The regulator is an LM317T, and it has an adequate heatsink and has survived. The transformer a 15 + 15v 30 VA. No LT fuses, only mains (240v, 250mA standard type.) which had blown.

Now toroidal transformers have a high 'startup' current, so I'd guess an anti-surge would be better at perhaps 125 mA?

Is there a 'rule of thumb' for mains fuses for such devices? And this isn't the first disc ceramic I've come across which has failed short circuit. Better type?

--
*Whatever kind of look you were going for, you missed.

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)
Loading thread data ...

Hi,

To fuse a transformer winding I would:

  1. Check manufacturers current rating on the winding.
  2. If unavailable, determine the wire guage and work the amperage
  3. To allow for inrush, I would measure it and pick the right curve on the fuse. If that is impratical, a thermal type resettable protector might be an idea (integrates the current pulse over time).

You may check

formatting link
to see if recommendations for fusing their products are provided. Even though your transformer may be different, the considerations and methodology should be similar.

Regards, Jim

Reply to
james.shedden

Is it built into something or can random external equipment be connected to it? If the latter, an output fuse might be a good idea.

My Tek 465M (and I assume other similar Tek scopes) have an interesting approach. There is a regular mains fuse, then the transformer. On the secondary, there is the winding, bridge rectifier, and main filter cap - then another fuse before the regulator. I think that's kind of an interesting place for a fuse.

AoE 2nd suggests using "a slow-blow fuse with rating 50%-100% greater than worst-case current drain of the instrument." I'm pretty sure that "anti-surge" is rightpondian for "slow-blow". At 30 VA and 240 V, this would suggest a fuse in the range of 188 to 250 mA.

Maybe Mylar or polystyrene? I'm not sure how much better they might be, but those seem to be the other common types in similar capacitance and voltage ranges to small disc ceramics.

Matt Roberds

Reply to
mroberds

It is internal, but looks like they might have used it on other products.

Yup. I'd have thought before the rectifier would make more sense - since as I've found failure there can take out the expensive bit - the transformer. Although it's possible the correct mains fuse would have failed first.

Great. Thanks. Just what I was looking for. Couldn't find this in the couple of text books I looked at.

Yes, IIRC anti-surge and slow-blow are the same. And even time delay?

OK. Strangely, the PCB hole spacing looked more for a Mylar than a disc.

Thanks once more.

--
*Very funny Scotty, now beam down my clothes.

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)

Just to be clear, AoE == "Art of Electronics", 2nd edition.

formatting link
It has the usual textbook stuff but also lots of practical advice like the above. There is rumored to be a

3rd edition in the works; you can harass one of the authors (Mr. Hill) about it on sci.electronics.design .

I'm less sure of "time delay" being the same thing. To me, a "time delay" fuse is a big (>10 A for me, >5 A for you) mains fuse that gets used on things like motors and air conditioners that draw high initial current. It's the same idea as a small "slow-blow" fuse in electronics, but I think the delay time is longer. All fuses have curves that show how long they take to blow at what percent of their ratings; these are usually in the manufacturer's catalogs (notably: Bussman and Littelfuse).

Matt Roberds

Reply to
mroberds

In the UK, time delay is the name given to the sizes we're talking about too.

--
*A hangover is the wrath of grapes.  

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.