[Heath] HP 1144 overvoltage crowbar circuit

[Robert Groh]

Regarding the overvoltage question, the HP1144 was updated (to a HP1144A) to 
incorporate a overvoltage crowbar - pretty much implemented exactly as outlined 
below. You can look at the schematic for the HP1144A to see how we did it.

I was, in fact, the design engineer for that little change. The only thing I 
didn't like about it was that it shunted the output with a thud - not subtle at 
all!  


73
Bob Groh, WA2CKY
Heathkit Engineer from 1978 to 1982.




________________________________
From: "ChrisIwata at aol.com" <ChrisIwata at aol.com>
To: gggdds at js-net.com
Cc: heath at puck.nether.net
Sent: Wed, February 2, 2011 1:00:51 PM
Subject: Re: [Heath] IC availability

  
 
In a message dated 2/2/2011 8:32:57 A.M. Pacific Standard Time,  
gggdds at js-net.com writes:
Chris,
>Do you have any ideas on a design for the SCR    protection you described below, 
>for those of us who can't design    circuits...
>Guy
>
 
The SCR over-voltage circuit is generally referred to a Crowbar.   At a preset 
voltage it trips and acts like a short circuit, blowing the fuse  before the 
over-voltage gets to the equipment.  If the PS uses a slo-blo  fuse consider 
replacing it with a regular fuse, else when the Crowbar trips you  might fry 
more regulator components!
 
The circuit I used for the HP1144 PS is very similar to that in the  following 
link:
 
http://www.radio-electronics.com/info/circuits/scr_overvoltage_crowbar/scr_over_voltage_crowbar.php

 
This link is from the old Radio-Electronics magazine, and the article shows  a 
schematic and has some tutorial info.
 
The article is correct, the choice of Zener is somewhat trial-and-error, On  the 
other hand, once you get the right value you're done.
 
For the HP1144 the components and values I used were:
 
12V 1/2W zener in series with 2 1N4007 diodes
1K ohm 1/2W resistor
2N1842 SCR
 
With these components the Crowbar trips at 16V. 
 
Adding the diodes in series with the Zener boosts the zener voltage by  about 
0.7V per diode. This is how you adjust for fudge-factor. You  could use just 
about any silicon diodes, even 1N914s.  But the 1N400X  series is a little more 
beefy current-handling wise.
 
The SCR was surplus, so might be hard to find.  Choose a SCR that has  a current 
rating at least twice the rating of the PS.  The voltage  rating of just about 
any SCR will exceed what you need for a low voltage  PS.  Choose a TO-220 
package or a stud mount, these are the high amp  packages. You don't need a 
heatsink, the time the SCR will be ON is milliseconds  before the fuse blows. 
SCRs are specified for different applications but the  choice for the Crowbar is 
NOT critical. 

 
The capacitor in the schematic keeps the SCR from tripping on a pulse-like  
transient.  But if the PS is regulated, the regulator circuit will never  let 
this happen.  So I omitted the cap. 

 
Chris, KL7DM 
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