Fast zener diode substitute

I decided to built this:

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It keeps the battery a little bit active, preventing sulphation by loading the battery with a hefty current (40 A) for a short interval (50 ?s) approximately every two minutes.

But they used a fast zener diode (BZT03 27V) across the FET for protection, as rather large voltage spikes can occur when the FET switches off.

I can not find this diode where I am, so can you suggest a substitute? I guess I can not just put regular 27V zener diode here?

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Reply to
Yvan
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A zener is pretty much a zener. I've never heard of a FAST zener. Use any similarly-rated (voltage, power) zener.

I wonder if the circuit actually works, chemistry-wise.

John

Reply to
John Larkin

look for TVS diodes as your replacement.. They can handle some junk. Look at that one. the 12V version is most likely what you want. it has a clamp of 22 amps at 27V and only a 5ma of leakage at around

14v or so.

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Mouser has them. Most like you can get them in Axle leads, these are smt.

Reply to
Jamie

is the zener really needed?, if that little zener can handle the spikes, surely a big avalance rated fet can too

-Lasse

Reply to
langwadt

"Yvan" schrieb im Newsbeitrag news:hkkol8$fo4$ snipped-for-privacy@news.eternal-september.org...

Yes put a 3-5W 27V zener. If diodes use the zener effect, they are always very fast, because no charge needs to be removed. This is valid for z-diodes above 8V rating. The circuit discharges the battery, in one year with more than 30Ah, together with the self discharge rate there will be 0 capacity left after a year for a normal 45Ah, this is certainly not a good idea and won't prolong battery life at all. Better to get a trickle charger and keep the battery fully charged during the storage time. The article goes back to the seventies BTW and was extensivly discussed and the circuit was considered pretty much useless.

ciao Ban

Reply to
Ban

The BZT03 is a big junction in a small package - it can dissipate 10W continuously, or 600W for a 0.1msec.

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I don't recall that it was ever advertised as particularly fast, though Vishay now claims that it can turn on quickly, which may be true, once the first charge-carriers have appeared in the junction. "Zener" diodes with breakdown voltages above about 8V are in fact avalanche diodes, and the avalanche starts with a single charge- carrier pair, which may come from thermal noise or radioactive decay.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

A 2K2 gate resistor pretty much prevents fast enough switching to cause spikes. The fet swots it out, zener or no.

RL

Reply to
legg

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