Electronics help

Update: So fuse arrived today, soldered in to the charger, switched on and POP! That will be for the bin then as now beyond my understanding of electronics. At least it never exploded this time.

Ok now waiting on replacement fuse & holder for the other charger, fingers crossed.

Yes. There are usually two failure modes. In one, the wire vapourises, coating the inside of the glass jet black or, in more violent cases, the entire guts of the fuse disappear, leaving only the two end caps. In such cases, I've never found any trace of the glass or wire anywhere!

I've never seen one where half the glass and wire remain but that glass does look unusually thick

If it makes it easier for you.......

When you say the lead acid battery battery limits the current "much of the time", just how much of the time?

At what point does the lead acid battery stop limiting the current and why doesn't your Voltage regulation just reduce the Voltage to the point where you are not reliant on the environmental variables of the surroundings to influence current consumption?

I trust this helps?

AB

I'm glad you said enthusiast!

In my dim and distant, working as a TV Engineer, another fuse went in first.

Some of those spring type antisurges had an impressive delay time, far longer than the regulator, feed rectifiers and printed circuit track.

:-)

AB

As some predicted it might. ;-)

Before you do ... do you or a friend have a Digital Multimeter (or if you have anything with a battery in it, get one yourself anyway). Put it on Diode test, or the lowest value of ohms and test both ways across all four of the diodes (with it disconnected from the mains of course). If all is well with the diodes you should see a reading in one direction and different in the other (equally on all 4 diodes), but not a short circuit across any. Any showing as a short circuit are probably faulty and can be replaced easily and cheaply. (You might also just desolder one end of each diode that shows as a short and test it like that in case the short is coming from elsewhere).

It's not difficult to do and might only cost pence if it turns out to be a diode (or two).

;-)

So time to test the diodes in the other charger. ;-)

Cheers, T i m

You crafty little devil you!!

And here was me thinking that the light bulb was to help you see the fuse blow.

If you wish to share your wisdom with a few hundred thousand manufacturers that use them for mains fusing, I'm sure they will be thankful.

AB

Thanks Tim, yes I have a multi meter but basically use it for continuity and mains V checking, I`ll try what you suggest, its about time I got some better use out of it.

While prodding around under the board, stick the meter across the legs of the device attached to the heatsink.

There should be a high reading from possibly the center pin to the other two. If Q1 is a transistor that is!

The bit that has the Volts on will go to the ferrite transformer.

All three legs reading low Ohms to each other is a fault and may be more likely than a blown rectifier.

Having said that, if you have a dodgy plug connection or are using one of those adapters using copper foil to make contact, it's yer diodes.

If the transistor is S/C, it may just be that but its worth checking the low Ohms resistors in the emitter. These often fail when the full whack of current goes through Q1.

Finally if the "transistor" is S/C, it is also worth checking the shaping network on the collector.

On this diagram the items are R12 and C9.

The resistors here frequently went o/c and this led to a very short lifespan for Q3 in the diagrams case, or Q1 for your board.

On a final note, it would appear that Q1 is a bog standard transistor, but all sorts of things get included in epoxy packages these days.

On the plus side, it may have a part number stamped on :-)

The last SMPS I bought from Ebay was less than £5-00.

When I repaired them for a local industrial electronics company, the repair would cost the client forty time that sum.

Happy days :-)

AB

====snip====

Just as I expected and tried to advise you of this most likely outcome. Once that safety fuse has blown, you can guarantee that the cause was an HT switching transistor[1] going short circuit for one reason or another which may well involve a fault in any of the components in the driving and control circuitry. Just blindly replacing a blown HT switching transistor is far from guaranteeing a successful repair.

And mine, too (and I'm far more knowledgeable of SMPSU basics than you [2]).

Probably just a matter of "dumb luck". :-)

From what I can recall, that looks a much better bet.

worth replacing the 4 mains diodes if you know how, those are the most likely cause.

NT

I'm sure they know what they're doing just fine. You plainly don't.

NT

The same when I was in the trade but remember, we weren't powering up sets that hadn't seen the light of day, let alone power, for 20, 30 years or more!

Usually a Variac is used as well as the main smoothing, at least, usually needs reforming so needs to be powered up gradually.

I don't dabble myself but do follow a couple of forums where most members do. It is surprising how often the smoothing reforms OK - a bonus as you can't whistle up a replacement from RS Components these days!

In fact, before even switching on, there is a visual inspection and the suppressor capacitor across the incoming mains supply snipped out!

Readings from diodes, hope it makes sense:

Readings from diodes, hope it makes sense:

Oops! Forgot to add the [notes]

Something wrong with number one. They should be similar. Check for any derbies from the explosion on the tracks/components that may be shorting it,

Then consider unsoldering/cutting it out and testing as a bare device to make sure its the faulty device before trying a new one.

Well, sort of. Normally on the diode range the meter reports the Vf of the diode. However the bottom right one is clearly very different to the others, so it looks like they're suspect. One shorted diode could be enough to take out a fuse.

NT

OK. On diodes 1 & 2 showing the readings 157.7 & 1.4 on the underside the solder is connecting the 2 diodes, is this correct? Image below.

Roger Hayter brought next idea :

I agree entirely, but my experience with a permanently on trickle charge, will dry a battery out eventually.