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zener diode resistors remarkably hot

J

James Evans

Bedroom Bodger
Just noticed tonight that the zener resistors (680R, 3W, welwyn W21) in my claymore are pumping out vast amounts of heat. The voltage drop is 25V. Not sure if this heat is correct or not - not noticed it before. Any ideas? The voltage output should also be +-16V and I'm only getting +-14.5, which doesn't bode well I assume for my TPR regs outputting +-10V (lt1086 and lt337).

Based on calculations, IR is 35mA. I'm supplying 4 lots of TPR regs (2 lots of +-) with voltage refs, into 2 AD8620 opamps. Now if the current draw of all that is more than 35mA what would happen, would this cause the excessive heat?

I have 2 sets of zeners (identical) running from the same +-41V lines - the 2nd set are outputting +-16V no problem, although the resistors here are also v hot - they are attached to the other resistors at the 41v end though, so I guess it may just be transferal of heat....

One thought - I could always dump the zeners altogether and input the 41v supply to the tprs, but they'll be dropping 31v, so I'm assuming I'd need to invest in some serious heatsinking?

ta

James
 
James,

With R=680R and Vdrop=25V, then the power disipation by the resistor is ~1W.
So....yes, they will get hot.

Jim.
 
Thanks Jim

I still wonder about the current supply from the resistors though - they're feeding 8 regs, with 4 voltage refs and 2 opamps - is 35mA going to be enough? or should I stick a lower R resistor in there? The diodes current rating is 75mA so I guess I could half the R value and not be in any danger?

ta

James
 
Just remember that this would double the disipation in the resistors - ie they will get a lot hotter than they do now.....

Best thing to do would be to keep an eye on the voltage out from your zener circuit, if it drops below the zener voltage then you have stolen all of it's current and it is no longer 'zenering'.


I think it would be a good idea to select a higher zener voltage, say 25V.
This would help you reduce the disipation in your zener resistors, and also give your TPRs a bit more headroom to work in.
 
Jiiim said:
I think it would be a good idea to select a higher zener voltage, say 25V.
This would help you reduce the disipation in your zener resistors, and also give your TPRs a bit more headroom to work in.
Click to expand...

Exactly what I was thinking....anyone got the spec sheet for the zener's available from maplins? dont seem to have any on site, and I can't be arsed spending £20 at farnell just to get some zeners & resistors!
 
Dunno, but they're 5W @ 24v so, using my rudimentary electronics knowledge, that would mean:

Amps = Watts / Volts
=5/24
=200mA approx

But then, I'm shit at that sort of thing and could be totally wrong ;)
 
Looks right to me. Good idea to use over-rated parts, since under-running things is a good idea to improve reliability and tolerances. And as you've found out James, resistors can be at 175-200deg C at rated power, you want to allow plenty of space to avoid scorching things :eek:
 
cheers fellas. What is it with p&p eh - rapid want £4 on a 90p order... might as well go and spend £20 at farnell and get something else to tinker with....
 
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