Car battery charger - faulty

[tmcw]

Had a bit of a mishap with a car battery charger today, some magic smoke when I plugged it in.

So I opened it up, and it's a fairly straightforward unit, a transformer, 2 switches (12v/6v, Hi/Lo charge), a simple LED indicator for battery health and charging, a bridge rectifier and some type of device on the negative lead that goes to the battery being charged.

The smoke was coming from the bridge rectifier, the casing is now a bit cracked. So i thought maybe a nice easy fix. But when I tested the switches (with BR disconnected), I found that there was continuity between all the switches, no matter what position each switch was in

So I'm thinking there is a problem with the transformer. The outlet side of the transformer has 5 tabs, one connected to the negative of the BR, the other 4 wired to the switches, so I'm presuming these 4 are supplying the 12v/Hi, 12V/Lo, 6V/Hi, 6V/Lo. So with all wires disconnected from the transformer, I'm getting continuity between all 5 tabs.

I presume this is not the way it should be? (I'm a bit clueless with transformers, dangerous yokes!)

So, I suppose I'm wondering how this happened? I'm guessing the lamination on the windings has gone bad, but would that happen because the BR went bad?

I did test the BR, and there is full continuity between 3 of the pins. I didn't really take notes because of the smoke and visual damage to it, but I know that continuity like that (out of circuit) isn't right. On the pin that didn't have continuity with the others, I think it was open on it one way, and showing 0.7 on a diode test, so one of the diodes inside may still be ok, but at least 2 others probably not.

Or maybe the transformer went bad first, sending (12V x2) + (6V x2) at 10+ amps, and that's what cooked the BR? What would cause the transformer to go bad? Could a 3 foot drop onto tarmac cause it to go bad? Or if the positive and negative battery clamps were connected for a short time, could that cause the windings to bake off the lamination?

Very grateful for any insight. I need to understand more about transformers.

[tw2005]

Re: Car battery charger - faulty

It will be a multi tapped transformer, and I would read up a bit just to improve your knowledge but essientially a winding which has pick off points and I would expect the result you had.

Start with some good photos of the design. The cracked/smoking rectifier is probably a good place to start for replacement.

[tmcw]

Re: Car battery charger - faulty

Thanks for the link. It looks like I should expect continuity between the pick-off points after all. I do need to read up more about them. I was thinking that there were discrete windings for each of the voltage outs.

I'll check the resistance between the points tomorrow. I took some photos, but also drew the circuit, I found that helped me understand more about the circuit.

I'll see about getting a replacement BR82, I doubt I have any in the stash. The charger is probably 15+ years old, would be nice if that was all that was wrong with it.

Is it safe to connect the transformer to the AC, and test the voltage on the outlet (secondary?) side between the lead going to the BR negative, and each of the other 4 tabs?

Should the multimeter be set to VAC when doing this test?

[budm]

Re: Car battery charger - faulty

The DC resistance of the transformer windings will be very low, you have to use Ringer tester to find out if it has shorted turn or not, using Ohm meter to test transformer will only show if it has open winding, shorts between windings, but not for shorted turns.
Show us the pictures of the unit.
"Or maybe the transformer went bad first, sending (12V x2) + (6V x2) at 10+ amps" transformer does not actually sending the 10A current, the load will determine how much current it wants to draw from the transformer and if the transformer and the circuit will be able to supply the current demand or not.
To understand it better is to draw the path of the current flow from the source to the load and return back to the source.

[eccerr0r]

Re: Car battery charger - faulty

For power transformers, if you disconnect the secondary and only power the primary, if the transformer gets warm/hot when it didn't when it was new, it has shorts winding in it...

But just like any other high voltage circuit testing... it's dangerous! One hand rule/isolation transformers/etc. all apply.

However I kind of doubt winding failure happened. Make sure that primary and secondary are still isolated - if this is ok, then most likely I don't think the transformer is faulty enough to damage the rectifier. Quite possibly the output was shorted and someone kept it on for a period of time, cooking the bridge...

Or perhaps the switch broke and shorted everything together?

[tmcw]

Re: Car battery charger - faulty

The switches test ok after I disconnect them from the transformer.

There is the possibility the battery leads were touching when power was applied, not today when I first noticed the smoke, but when it was elsewhere. My rule is generally connect the battery leads, make sure the switches are in the correct positions, plug into mains, switch mains on.

There is something in-line on one of the battery leads, not sure what it is, maybe a thermal overload, or one-way current device? I'll have another look later, I don't have it here right now.

The photos I took were with a camera phone, just for quick noting of where the wires are going. I'll take and post some decent photos later tomorrow.

Thanks for the replies.

[Agent24]

Re: Car battery charger - faulty

Bridge rectifiers sometimes just short and blow up for fun, too.

If you used diode\continuity test on the transformer you will get false readings, as already mentioned.

[tmcw]

Re: Car battery charger - faulty

I did the AC test on the 4 outlet taps, and I'm getting 5.90v-6.50v on two, and about 11.80v-12.50v on the other 2, so it looks like the transformer is ok.

I was reading a discussion elsewhere, and they were saying that that sort of variance for a car charger was fine, the battery can handle the fluctuation, and hence why they don't use smoothing caps in these designs.

I'll post a few photos later, for anyone who's interested.

The BR is a BR86. Might be a few weeks before I can get a replacement, but I'll update when/if I get it replaced and repaired.

[tmcw]

Re: Car battery charger - faulty

I was having a quick look for a replacement, and it seems the BR86 might be a less used number now.

Would the 806 I see mentioned more commonly, be an equivalent part?

http://www.ebay.co.uk/sch/i.html?_od...0v+8a&_sacat=0

http://ie.rs-online.com/web/p/bridge...fiers/8172617/

http://ie.rs-online.com/web/p/bridge...fiers/2278586/

The specs seem to be similar, though I can't compare them all.

[joshnz]

Re: Car battery charger - faulty

I was having a quick look for a replacement, and it seems the BR86 might be a less used number now.

Would the 806 I see mentioned more commonly, be an equivalent part?

http://www.ebay.co.uk/sch/i.html?_od...0v+8a&_sacat=0

http://ie.rs-online.com/web/p/bridge...fiers/8172617/

http://ie.rs-online.com/web/p/bridge...fiers/2278586/

The specs seem to be similar, though I can't compare them all.

BU1508-E3/45
any with a current of 8A or greater and voltage 20volts or greater will work

[budm]

Re: Car battery charger - faulty

"I was reading a discussion elsewhere, and they were saying that that sort of variance for a car charger was fine, the battery can handle the fluctuation, and hence why they don't use smoothing caps in these designs" It is positive pulse charging (no filter cap), if it is full wave rectifier then the pulse will be 120 Hz (100 Hz for 50 Hz power), half wave will be 60 Hz (50 Hz).

[tmcw]

Re: Car battery charger - faulty

I found a S10VB60 in an exotic power supply.

It's rated at 600V and 10A (original was 600V and 8A). As far as I can tell, it's rated the same or slightly better for most of the parameters that I can compare from the datasheets I've seen for both. Same package type as well.

So until I get a direct replacement, I might give this one a shot.

Edit: I've attached a poor photo of the unit. The bridge rectifier is to the bottom left side (brown wire attached, so directly to the left of the circuit board). The heatsink the bridge rectifier was attached to it at the top-left, a metal plate that runs about three quarters the length of the case. The Hi/Lo and 12V/6V switches are at the bottom (just above the label)

So a fairly simple unit: Mains inlet to top of transformer, secondary outlets to 12V/6V switch, then to Hi/Lo switch (with some loops to the circuit board for battery condition and charging status), next through the bridge rectifier, and from there to the leads that go onto the battery. I didn't figure out what the yoke on the negative battery lead is, it can be seen to the left of the transformer, it's insulated inside a thick plastic cylinder. One-way device, or thermal protection?

[tmcw]

Re: Car battery charger - faulty

Doing a bit of additional searching, I found this page:

http://www.vetco.net/catalog/product...oducts_id=4408

It's a page for a bridge rectifier, but it lists equivalent replacements, of which my original BR86 (CP806) and the alternative S10VB60 I found a few hours ago are listed as replacements.

It got me wondering if there is a site where comparisons/equivalents for components (new and historical) can be easily determined. I know I can input specifications on some supply sites and get alternatives, but they might not list a component I might happen to have in my stash or on a board I need to check. Searching for and downloading a datasheet isn't such a big deal, but if there was a useful page like the one linked above, where I could make a quick determination whether a component was equivalent or not, it would greatly help finding replacement parts.

[stj]

Re: Car battery charger - faulty

most of us would just go to farnell or whatever supplier we use and put in the minimum voltage & current and see what they have.

[Agent24]

Re: Car battery charger - faulty

I can't be sure from the photo, but that thing on the negative lead looks like it might just be a wire joiner of some kind.

Don't worry about the bridge rectifier too much, as long as it can hande the voltage and current, that's about all you need to care about with it, especially at low frequencies like the mains.

[keeney123]

Re: Car battery charger - faulty

I would think about putting a inline fuses in the output circuit so if you touch the two leads together again you will only have to replace the fuse. Perhaps a fuse like one in a car that you just pull . They have holders that can be solder right in line with the output leads.