Bridge Rectifiers

I have a favorite, old battery charger that dates from the 1970s. It switches between 12v/6v, has a hi/lo setting and delivers up to 8A via an inbuilt ammeter.

But the other day it stopped charging. I have some electronics knowledge, so I put a 12v 12w bulb across the output with a DVM in parallel and reasoned that as I seemed to be getting about half the required voltage perhaps the bridge rectifier had failed on one leg.

I found a KBDC 3502 bridge rectifier on the web that was rated to 35A and 200V. That seemed to have the safety margins so I bought and installed it. But the charger is still not right. I seem to be getting the right volts if I put the output onto a charged battery (voltage rise from 12.5v to 13.4v) but it will not deliver much current. If I put the 12v 12w test bulb across the battery and charger the voltage drops to 12v and only half an amp seems to come from the charger.

So have I got the wrong bridge rectifier and is it not switching fully? rather than a 200v maximum should I have got one with a 50v maximum. I don't understand rectifiers.

I cannot see that there can be another fault with the charger. It is a transformer with switches and wires so either there is a circuit or it is blown, and since I can drive 0.5 amps through a bulb it can't be open circuit anywhere.

Edited By norm norton on 16/03/2016 17:57:52

Norm

That style or rectifier has been around for donkeys years and I've used several in the past to fix old style battery chargers with every success. Assuming you haven't been unlucky and been supplied with a faulty unit I have to ask if you are certain its connected the correct way round. Usually one terminal blade is rotated by 90° with respect to the others. That one is the + output and the one diagonallu opposite is - output. AC input is via the tow diagonally opposite terminals that are the same way round.

If your DVM has a diode check function its relatively easy to verify that the innards are OK by disconnecting the rectifier and measuring between between the AC terminals and the DC outputs.

Clive.

Hi Norm

its possible you have a faulty switch or a corroded connection

photos showing how the charger is wired would help

if I remember correctly

decades ago, I looked at a battery charger that had a 6V/12V switch that selected all or part of the secondary winding

a second switch controlled the charge rate by connecting all or part of the bridge rectifier to the ammeter

this gave you the full or halfwave rectified output from the transformer - easy to do with the very old 5 terminal selenium rectifiers

John

Edited By john swift 1 on 16/03/2016 19:17:05

That voltage rating is fine, I often use 1N4007 rectifier diodes which are 1000V and 1A rating for low voltage circuits  but with lower current than yours.

Edited By John Caines on 16/03/2016 19:32:11

Edited By John Caines on 16/03/2016 19:33:27

Edited By John Caines on 16/03/2016 19:34:01

Humm, thanks for those thoughts, and for confirming that a 3502 should work. I'm 99% sure the rectifier is wired in correctly as I checked the same diagram as shown above. If it wasn't I would not be seeing 13v surely?

The Hi/Lo switches tappings to the primary on the transformer, and the 6v/12v switches tappings on the secondary and then feeds the rectifier, so perhaps it does switch half to full wave?

I like the suggestion of a dirty/failing switch. All the wires are soldered on (good old seventies construction) so perhaps I will try a jumper lead on the backs of the switches tomorrow and see if I can get more amps to come out. Just be careful of the switched 250v input.

It has got a reverse connection protection device with a resettable button on the output - I suppose that could be putting some resistance in the line?

I think I will put a 60w bulb across the output and see just what gets hot in the circuit after several minutes, and that will be the failing part.

Battery chargers are deliberately designed to work as you have found. It's usually done in the transformer so that the current drops off as the battery voltage rises.

John

-

Edited By Ajohnw on 16/03/2016 20:07:40

Hi Norm,

Can you compare the old bridge against the new one, with your DVM.

If they are identical, then suspect fault elsewhere.

Regards, Bear..

Hi Norm

two possible charger circuits to go with my previous post

switching between half and full wave rectification saves a terminal on the transformer -

making the bean counters happy if it makes the transformer cheaper

I expect the second diagram is more like your charger

john

Remember the output is 'Full WaveRectification' and may not be smoothed. (If like lower diagram above)

So the output is not a constant 12volts (Its a sine wave but with the negative part flipped to positive - so rises to 12v and falls to 0v 50 times a second if you are in Europe)

So you will not draw 1amp if you put a 12 watt load on it. (Can't remember the equation but I think root 2 comes into it) but my guess is that I would expect to see 0.7 to 0.8 amps.

Does the 6 volt setting work?

Dave

Err, I'd rather hope the output waveform was fundamentally 100Hz rather than 50Hz.

Andrew

Whoops ..... 100 times a second .....its late at night

Dave

Any increase on 100 ?

Do I hear 120 from the one and sixes ??

Going once.

Going twice.

Goin........................

"one and sixes" ??????

Best seats in the picture house at the back 1shilling 6 pence

Ahhh..., but not the dress circle, eh.

Those not fortunate enought to live in these sceptred isles will have 120 times a second, but of course they don't ocupy the best seats in the house!

The reason I understand is that by the time these foreign chaps got AC the manufacurers had improved there kit so it could revolve faster and so get more output for the same money.

One of the big reasons was the improvement of the steels used in transformer laminations (leading on to the silicon steels) which had lower losses, and therefore allowed higher (well, 20% higher, anyway) frequencies. And with higher frequency you can pump more power through a transformer for the same physical size, thereby cutting the distribution costs. This progression has ultimately led to the modern ferrites which allow kilowatts to be processed through tiny transformers using frequencies vastly higher than in the normal utility power distribution network.

To Norm Norton ... as well as possible problems with wiring and/or diode connections there is another problem and that could be the actual transformer itself ... you are the only one who can physically see and touch the charger ... is the unit getting hot ? ... far hotter then it usually does ? ... is there any smoke coming out of anywhere ? ... any smell of burning ? ... if any of the windings have gone short circuit they will mess up the output power drastically ... does the charger get hot even with no load ? ... a dodgy transformer can give some very peculiar symptoms whether it is the input mains side or the output 12 V side ... if the answer is yes to any of these symptoms then it is probably R. I. P.

martin

Using a "12V" transformer to charge a 6V battery by half wave rectifying the output isn't a good idea. The peak open circuit transformer voltage is still the same, so the final float voltage will also end up around the 14V mark.

The other issue is that with half wave rectification, the (DC) charging current flows through the transformer secondary, requiring the transformer to be designed to withstand that flux without saturation. With full wave rectification, there is no DC component, only the AC magnetising current.

The load regulation of these mains transformers is truly appalling, which you discover when you actually design one. Although this means that the current waveform is naturally smoothed and the current naturally tails off to some degree, the open circuit voltage is what sets the float voltage - and damages the battery if left for any extended period, either by conversion of the water to gas (flooded) or generation of heat (for VRLA).

If you are going to generate a 6V output, you really need to use a different tap on the transformer. Either that or remove the charger before the battery starts fizzing....

Dress circle in Ilkeston ???

Are you real man ??