Re: 12V Charger
 

Try and pull out the pin in the middle, a pointy thing sometimes gets them out. Otherwise just punch the pin right through. The back of the fastener is spread by the pin.

Re: 12V Charger
 

Pins removed with a small drill and reusable but centres missing. Mains TRX only gives 13V AC when isolated and rectifier gives various different resistances depending on which way probes connected on all 4 pins, disconnected .
I now suspect TRX has a short as I reckon about 15v should be minimum. what does anyone else say? TRX has 80R on primary and about 0.8R on secondary, making it about 10:1 .This should produce 24V ???

Re: 12V Charger
 

I have used and fixed many "Old" chargers. If the meter gives a flick, as mentioned in the first post, it generally means an O/C. Typical is fuse/ fuse holder/ Low voltage (6/12) selector or why?
A modern one will aim for 14 volt output, which means about 10v secondary (10 X sq rt 2 = 14.1v, minus 2 X 0.6v drop (bridge rec) = 12.9v. In other words, about 10-11v on secondary is just about correct.
Now with the MUCH older chargers, with selenium rectifiers, the secondary voltage would be higher, not sur what, but possibly 14v. As suggested earlier, this allowed a much higher voltage at low current, usefull with a dead flat battery.
I have fixed up a few of these, but I ALWAYS add a few resistors, allowing the possibility of various charge rates.
Back in 1960, I had a surprise when I went to charge my M/bike battery at work. We had a very old charger which was used to charge an identical battery for a kiln safety alarm. After the big surprise right up my arm, I discovered it was simply a half wave rectifier direct off the mains. I think it was a copper oxide recifier. Whatever, it gave about half an amp into just about any string of batteries you cared to connect to it. It was just wiser to switch it off before connection.
Les.

Re: 12V Charger
 

10:1 resistance does not mean 10:1 voltage, however you can find the ratio either by energising it as you have done or measuring the inductance.

A shorted turn will dissipate a lot of energy and the transformer will get hot even with no load. I think that the rectifier is the culprit. If you have the courage, you could measure the voltage before and after the rectifier. The voltage measurement will be low unless a battery or capacitor is put across the output to select the peak value.

As a quick check you could have measured the primary resistance and the rectifier volt drop (two diodes) without any dismantling. What type of rectifier do you have?

Re: 12V Charger
 

Not sure what type of rectifier I have but here is a photo..

Re: 12V Charger
 

Selenium or copper oxide, about 1 Amp?

Re: 12V Charger
 

Ammeter reads to 4 Amps.

Re: 12V Charger
 

Hi Pete, as a temporary trial use 4, 1N400 series diodes in a bridge and feeding about 100uF of capacitor. With luck you will now have about 14 or 15 volts DC.
If that is so it is worth investing in a 4A silicon bridge, or use 4, 4A rated diodes. In this application you do not need to add additional resistance in series with the lower loss Si rectifiers.

Ed

Re: 12V Charger
 

If the transformer is not overheating when disconnected, it's probably OK whatever it measures. It the charger doesn't work the rectifier is the most likely culprit (there isn't much else in a simple charger) I can't find anything about it, but you know it's rated at 3A, and it is almost certainly Selenium.
Any bridge rated at over 3A should be OK, if you want to pursue this repair. You may struggle to get a Selenium replacement. Silicon may give different charge rates?

Re: 12V Charger
 

In my(limited) experience of converting these simple battery chargers from selenium to silicon rectifiers the rise in charge current is significant. Adding series resistance will reduce the charge current but will not stop the battery overcharging if left connected for long periods. A possible solution would be to build a bridge rectifier with multiple diodes in each leg so that the forward voltage drop matches that of the original.

Al

Re: 12V Charger
 

Or just a string of diodes in series with the output?

You need to end up with the battery sitting at about 2.4V per cell (14.4V for 12V battery) for a single cycle recharge or for long term trickle at 2.3V per cell (13.8V).

Re: 12V Charger
 

I have now been persuaded that the TRX is OK and rectifier is suspect as it gives almost similar resistance readings in either direction eg 18K then 12k or 6K and 3K, on all combinations of 4 pins.

Re: 12V Charger
 

If you do switch to using modern silicon diodes or a 'bridge' to replace the old selenium/copper-oxide thing you need to introduce some extra series-resistance to limit the current, in place of the high forward-resistance of oldtimer rectifier-stacks.

I'd suggest a nice big metalclad resistor of 1.5 Ohm and at least 5 Watts would be a good start. You also need to use rectifiers/a bridge with decent current capability: it's fortunate that such beasties [typically 25A, 400V] are available in nice encapsulated blocks at pocket-money prices.

http://www.maplin.co.uk/p/kbpc2504-2...ectifier-ar85g

Re: 12V Charger
 

The original rectifiers look something like these (in vintage chargers). The one in the first pictures has a bi-metal 'cutout' attached to its heatsink. The second is a classic Selenium type both of these are workers and rated at around 4 amps.

Re: 12V Charger
 

The suggested remedy is to replace the original rectifier with a more modern Silicon type with the addition of a wirewound resistor to limit the current and 'mimic' the higher forward resistance of Selenium types. The devices shown are a 20Amp bridge rectifier and 0.47 and 1 Ohm wirewounds.

If you want to try either of these options I'll send you either the red selenium or the modern stuff for £4 inc P&P. P.M. me if you're interested,

Nick

Re: 12V Charger
 

To answer your question. Extra diodes in series with the output of the bridge would also work.

Al

Re: 12V Charger
 

That is quite true up to a point; depending on the transformers voltage output and the output impedance, the steep curve of the silicon diode's forward conduction can quite noticeably distort the sine wave out of the transformer, can cause core saturation and a quite hot transformer. Personally I've always found the low value resistor a 'kinder' and more predictable method + it helps limit destructive currents should there be a short.

You takes your choice but my home one has wirewounds in it.

Re: 12V Charger
 

Belt and braces then. Extra diodes after the bridge to limit peak volts to around 14.4 and some series resistance to limit short circuit current.

Or go the whole hog and fit a full house regulator based on something like a UC3906 plus a large capacitor to bring the dc up to peak volts.

Re: 12V Charger
 

I have bought a KBPC604 Si single phase bridge . Is this a suitable part ? I can return it, if I wish, as it is local shop. The data sheet says 6A max and 35 V max input.

Re: 12V Charger
 

I read the data sheets differently. Voltage will be adequate.

Maximum current is 4.6A when feeding a capacitor or battery with temperature of case under 100degC. Bolting the device to a heat sink would be necessary.

It is a bit lightweight but should do.