[Heath] IC availability

[ChrisIwata at aol.com]

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_ 
(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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