Diodes for large vehicle battery starter/charger

Thread Starter

sambo32

Joined Sep 23, 2011
1
Hi everyone, new here.
The fuses recently went on the battery charger/starter i use. While it was apart decided a general overhaul would be a good idea as its quite old (20+ years). This was when i found out previously some of the diodes had blown and it had been made into a half bridge as opposed to full bridge rectification.
So, I'm trying to find some replacement diodes. IT is a large 12/24 volt capcpale of 80 Amps plus. The current ones are DO-8 package i think. I've never encountered diodes of this size before. There are two sets one for the 12V tap and one for the 24V tap off the transformer.
I've been looking on RS/digikey etc, I can find diodes with a suitable amperage but the rated voltages are all very high and very expensive £40+ each. Am i right in thinking that as they are after the transformer the voltage rating doesnt need to be in the hundred just a safe margin above the peak voltage at 24VAC?
Can anyone help point something suitable out or are the expensive ones i've seen the ones that are required and they are just an expensive part?
Cheers
Sam
 

R!f@@

Joined Apr 2, 2009
9,647
For half wave or full wave the PIV should be rated for the transformer voltage as u mentioned. The current rating needs to be about the same as before.

Typical PIV for that charger is usually 100V
 

Brownout

Joined Jan 10, 2012
2,364
sambo32 said:
Am i right in thinking that as they are after the transformer the voltage rating doesnt need to be in the hundred just a safe margin above the peak voltage at 24VAC?
Not quite. first, the 24VAC will be 24*1.7= 40.8Vpeak. Now, the diodes need to be rated for twice this voltage, remembering they have this voltage on the output side and minus this voltage in the input side during their "off" half-cycle. So, the diodes have to be rated at a minimum of ~82V. Of course, sufficient margin should be added, bringing the total to >100V.
 
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