The Fordson Tractor Pages Forum

Brian: I need help on my Battery charger? Rectifier burnt up.



While this a sears unit it was made by Century I found it both on the sears and Century site. But this rectifier plate is no longer available from either. Century shows a couple of replacements with diodes in them that may work. One has 8 diodes the other has 12 diodes in 2 bridges. Either would cost about $65 by the time I got them. The diodes look very small to me.

So Iam thinking of getting the diodes and building up the units my self. This is where I need help. I need help in choosing the size (power rating) of the diodes.

How is it that Century can use such small in size diodes. They dont give any detail as to the actual power rating of the diodes they use. But it always seems that the higher power rating the larger the diode.

To have an actual output of 250A/intermittent wouldnt I need that much in ratings of the diodes? Secondly how do ratings of amps total in a bridge? i.e. wouldnt if you used 4 10A diodes in a single bridge you would have a 20A bridge? Being 2 would pass neg and 2 would pass positive. Or would you have a 40A bridge with a total of all diodes? Or something else like only the 10A of the rating of a single diode.

I need this charger and use it a lot on the farm here. If Iam going to build it I need it more rugged than original. Tempture and dust is the big problem here.
I believe it is a combo of both and lots of use that caused this rectifier to fail.

I was wondering if I could use these to form the bridge?

http://cgi.ebay.com/2PC-EUPEC-DD82S08K- ... 2a0c8ea468

Terry, my electronics training was more years ago than I want to admit, but from what I remember:
(anyone else that actually uses this stuff every day - feel free to correct me. I will not take offense)

In a bridge diode, at any given moment two diodes are passing current - one in a positive direction, one negative.
When the a/c reverses, the other pair of diodes is active. I think this means that each diode has to be rated for the max current in that part of the circuit...they are not additive.

It is (was) common in power supplies to use multiple windings in the step down transformer - in effect multiple tranformers - ,each with it's own diode bridge, wired in parallel to divide the current load. This allows smaller components to be used.
IE: two 30a supplies in parallel can provide 60a total. This probably explains why you are seeing multiples of 4 (8 or 12 diodes) in the replacement bridges.

So part of what you need to determine is how many sets of windings are in your transformer. Keep in mind that each set probably has a center tap (for 6v nominal) output.

Sorry Terry, my expertise does not cover chargers in depth. However, Jack has covered it as well as I ever could. If you could post the numbers from the diodes ( could be something like 1N4001) I can tell you what rating you have and suggest higher rated ones.

Terry, I'll correct myself a little bit here.
I found my copy of 'The Radio Amateur's Handbook' . From the section on bridge rectifiers....."Each rectifier in a bridge circuit
should have a minimum load-current rating of one-half the total load current to be drawn from the supply"

The plate rectifiers are not widely used anymore. I remember seeing a few of these (selenium type) 30 or so years ago.
I once owned a Norton motorcycle that used one to supply d/c to the battery - along with a zener diode as the voltage regulator.

It's snowing here today...I've got too much time on my hands.

Merry Christmas

Hi Terry,

Without some sort of idea of current rating or at least the physical size of the transformer in your charger it is difficult to advise on diodes. However given the description you give of the diode that has failed it sounds like it is a big charger with a selenium rectifier in it. If I am right then we may be talking 100 amps or more.

Selenium rectifiers are still being made in small numbers (probably means you will have to sell you fist born into slavery to pay for it) and battery chargers is one of their main applications. http://www.cougarelectronics.com/ do them.

If you are replacing a selenium rectifier with a modern silicon replacement, don't let the size fool you. The plates on the selenium diode were a built in heat sink and the silicon diode will need to be attached to a heat sink with approximately the same cooling capacity so the size of the bridge assembly with heat sink may even end up bigger than the rectifier you are replacing.

Beware of bridges with more than 4 diodes if you only have one pair of wires coming out of your transformer. You can not simply hook silicon diodes in parallel to increase the current rating. You will end up with a thing called thermal runaway happening. The diode that is carrying the most current will get hotter than the others. It's forward voltage drop will get lower as it gets hotter than the others from carrying more current causing it to carry an even larger current and so on until the electronic smoke gets let out.

The way modern chargers get away with running more diodes is by running more separate windings, the resistance of the windings evens out the current and the overload doesn't happen. The other side effect is that the wire for each winding can be smaller which makes the transformer easier to make, even though you end up with the same amount of copper in total.

The other effect that you will get is that the output voltage of the charger will be slightly higher with silicon diodes. This is generally not a huge problem for quick charges, but it may be a problem if you are leaving a battery hooked up for a longer time as the higher output voltage will cause the battery to run dry if left on too long.

Post some more details such as current output or transformer size and I will look up some suitable diode bridges and advise on heat sink size.

Regards, Frank.

Hi Tom,
Glad to see that you're still posting on here. We haven't heard from you in quite a while.
I don't know a darned thing about diode bridges or any other kind of rectifier, so I can't help you there.
How's the weather in Washington?