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[AMPS] Zener screen supplies

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Subject: [AMPS] Zener screen supplies
From: (Tom Rauch)
Date: Fri, 13 Apr 2001 21:46:59 -0400
> Fortunately that's not true, because VDRs act on a timescale of
> nanoseconds. Their problem is - as identified by Steve - that they
> don't clamp hard at the limiting voltage. That simply means the supply
> circuit has to be designed to survive that situation (which Steve's
> inherited supply evidently couldn't).

That's right, they don't clamp hard.

The initial arc is also discharging capacitance beyond the current 
limiting resistor.

VDR's are poor clamps, plus the initial transient can be very large 
compared to the time it takes to wipe a zener out.
> Gas-arc surge protectors are slower to strike, but after the arc
> strikes it clamps at a lower voltage than the striking voltage. It's
> very difficult to tell which is better for screen protection, VDRs or
> gas tubes. I tend to use multiple VDRs - at least one large one
> connected right at each screen terminal - but wouldn't argue strongly
> against gas tubes. Either can work well.

You need enough capacitance near the screen to absorb the initial 
spike. Neither a MOV or gas tube will work well by itself. Even a 
modest overvoltage is too much for regulators. Slow the transient 

Actually, the MOV is a last resort just to protect the socket would be a gas tube. Neither will do much for 
protecting a screen supply...since most series regulators won't like 
voltage applied to the output and shunt regulators will hard clamp 
and protect the MOV and gas tube.
> Returning to strings of zeners, one useful feature is a large
> electrolytic capacitor (say 50uF) in parallel with the zeners. This
> will help to absorb the rising edge of any voltage surge. In the last
> amp I had that used zeners, I had several arcs from B+ but never lost
> any more zeners after fitting the capacitor.

That's a good idea with almost any regulator. I can't imagine NOT 
using one!
73, Tom W8JI 

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