Battery charger electronics help please

[Steve Maskery]

I found my dad's old battery charger last week. It hasn't got a date on it but dad's been dead 18 years so it is at least 20 years old and quite possibly 40. ANyway, it's a basic thing and so I've been able to do some derusting with it. Very pleased with the results, until it stopped working.

There isn't much inside, and I've established that there is continuity on both sides of the transformer, so the coils are OK, I think. But there is nothing across the croc clip terminals. The meter doesn't work, so I can't rely on that to tell me if current is flowing. But all the bubbling has stopped and my AVO meter is not reading anything across them.

The black thing on the right is a 6V/12V selector, so the only thing that is left is the little black square thingy in the middle:

It has 4 wires connected to the far side, one black and three red.

Can anyone tell me what it is, what it does and have I knackered it. I've been careful not to short out the terminals deliberately, but that doesn't mean that it is impossible that it has happened.
Any help gratefully received.

Edit - I forgot to mention that is is mounted to the case on a piece of aluminium, which I assume is acting as a heatsink.

 

[Racers]

Hi Steve

Its a Bridge rectifier for converting the AC to DC, its a series of 4 diodes, you can test it with a meter you should get some resistance on way between each pair of terminals, anything else (short or open circuit) means its faulty.

I have replacements if its duff, if you are stuck I can test it for you.

Pete

 

[Racers]

Just thought, it could be the meter as its in series with the output try linking it out.

Pete

 

[Eric The Viking]

Wot Pete M. said.

Bridge rectifiers are usually pretty robust - I doubt it's that. The hole in the middle though is for mounting it on a heatsink. If it's a metal case, that usually serves. If plastic there ought to be a chunky metal bracket or a proper heatsink it was fixed to (side with writing to heatsink, bolt through hole). If you look at the writing there's a "+" and "-" these are the rectified outputs. The "~" (squiggle) ones are the AC side, from the transformer. There may also be a "charge rate" switch - either between two secondary tappings on the TX or via a high-current resistor (for slow).

They sometimes also have a low voltage cutout to stop short-circuit damage (or blowing-up a flat battery). Mine is a push-in reset on the front.

There might also be a thermal cutout on the transformer (the blue bit the spade terminals are on). I can't tell from the picture. Sometimes these are single-use, sometimes they reset after it cools (my office shredder is the latter type).

Finally, I've had the leads themselves fail: corrosion where the cable crimps to the croc clip inside the plastic sleeve.
Last time this happened it got me good and proper - I stripped the whole bloomin thing and rebuilt it before realising what the problem was.

I suggest two things:

Draw out the circuit carefully.

Systematically test everything, including the fuse in the mains plug and the mains cable (powered off!). If the mains cable is OK. check the secondary with mains on the primary. You should see somewhere around 15V AC (a DC meter will show nothing). It should be about the same out of the bridge rectifier (try an AC range for this too).

Finally, the meters fitted are usually really cheap, moving-iron ones. They're unlikely to go open-cirtcuit, but they do stick. Is it plausible it's working but the meter is just stuck?

E.

PS: just looked at the rectifier picture properly: the + and - indicated corners are the rectified output. The other two have 'virtual' squiggles!

 

[Steve Maskery]

Ah, that makes sense. No readings across any pair of pins, so I guess that's the problem. Thanks for the offer, I'll take you up on it. No rush.
Why would it suddenly stop working though? I've done a couple of those chisels and a bolster chisel and am very pleased with the results. Stupidly I didn't take a Before shot, but this is what they look like now.

Eric - all good ideas. The heatsink is just a piece of aluminium plate, you can just see it at the bottom of the picture. The rectifier was not screwed very tightly to it, rather loose in fact, so I don't suppose it was doing its job very well.
I can't see any reset button anywhere.

 

[Racers]

Sounds like it over heated and went open circuit.

I have a chunky 25 amp one that should sort it out.

Pete

 

[Steve Maskery]

Curiouser and curiouser.
I've just reassembled it and it's working again. Just under 11VDC and bubbles. I assume that the reduced voltage is because when it was made we had 240V instead of the current (ha-ha) 220V.

If there is a thermal cutout I have no idea where it is.
The ammeter is definitely NBG though
S

 

[John Brown]

I think you'll find a most places still have 240V. It is within the acceptable range of voltages specified. The lower measured voltage could be because there's no smoothing capacitor - the battery would normally perform that function, sort of...

Here's a quote from Wikipedia regarding the mains voltage:

Voltage levels

Most of Europe, Africa, Asia, Australia, New Zealand and most of South America use a supply that is within 6% of 230 V. In the UK, Australia[1] and New Zealand the nominal supply voltage is 230 -6% +10% to accommodate the fact that most supplies are in fact still 240 V.

 

[Harbo]

Maybe OK for a bit of Electrolysis but I read on the net, that you should not use these old type chargers on modern car systems?

Rod

 

[mind_the_goat]

if you measured noting across the rectifier it could be that the voltage applied by your meter is not sufficient to overcome the forward voltage drop of the diodes, about 0.6 V. Many meters have a setting for testing diodes and transistors, may have a symbol, a black triangle with a bar across one vertex.
You will see a reduced voltage on a flat battery due the internal resistance of the charger, voltage will drop with increased current, as it charges the voltage will rise and the current reduce.
Have never heard anyone suggest that these basic chargers are not suitable for modern Lead Acid batteries, the current supplied is usually very low compared to what the car alternator could deliver, basically just trickle charges the battery, which is why it can take hours to get any significant charge. Don't use it on other types of battery though.

Important, ensure battery is in well ventilated area, undo the caps if you can so gasses can escape quickly, wear safety glasses and check the battery is not getting too hot.
If the fluid levels are a bit low, add distilled water after charging, if they are very low add before charging.

 

[Eric The Viking]

The mains here hasn't changed, but the specification has.

It was an EU thing, theoretically so as to allow manufacturers legally to supply the same items to all of Europe. The 230V is a 'middle-ground' value.

The actual voltage here hasn't (read shouldn't) have changed and isn't supposed to.

 

[John Brown]

"When I was younger I could remember anything, whether it happened, or not."

But not what someone wrote three posts back!

 

[Eric The Viking]

You said:

I think you'll find a most places still have 240V.

Everywhere in the UK has 240V that used to.

What's changed is that we can (must) now buy appliances that were previously deemed unfit for sale here. That's not the point you made.

The lower measured voltage could be because there's no smoothing capacitor - the battery would normally perform that function, sort of...

IIRC, most simple chargers have an open-circuit voltage of around 16V (RMS). It drops on load. Rectified AC is actually a good way to charge most things*. It's a very good way to charge NiCds as they have a propensity to form dendrites when charged with smoothed DC.

The lower measured voltage is more likely to be a high resistance somewhere in the circuit - a faulty lead or switch contact.

E.

*I mean the waveform, not the simple circuit that's used for lead-acid cells, which are pretty tolerant. Good NiCd chargers use asymmetric AC. Later technologies require a high degree of sophistication, as both current and voltage have to be carefully controlled, but I have less experience of them. They don't lend themselves to FWR because of the current control issues.

 

[John Brown]

IIRC, most simple chargers have an open-circuit voltage of around 16V (RMS). It drops on load.

Agreed. But Steve is loading it. It may also depend on how good Steve's meter is at true RMS measurements.

 

[Benchwayze]

Steve.

Is there a 'blade-fuse' in the bottom of the charger.

My charger died on me too. It was so old, I just bought bought a new one, which looked almost the same model. Then I discovered the fuse. I now have two chargers that work ok! :mrgreen:

regards

John

PS Like these,
http://www.ebay.co.uk/itm/60-X-ATC-Car- ... 3a6183d59b

In case you haven't come across them before. I hadn't!!!