[Heath] IC availability

[Mike Morris]

At 11:00 AM 02/02/11, you wrote:
>
>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

There is some crowbar info and general linear power supply info
on the Astron introductory info page at <www.repeater-builoder.com>
There is also a library of Astron power supply circuits, from 7 to 75
amps, and almost every one has a crowbar.

Read the Introductory article, then go back to the section titled
"Linear Power Supply Design and Theory", then read the linked
article titled ""Power Supply Analysis".

One interesting comment from the article...
Another fault in the Astron is that they under-engineered the crowbar 
circuit - they used a design appropriate for a 7 amp or 12 amp supply 
in every size supply up to and including 50 amps !!!   The active 
part is a 2N681 that is rated at 25 amps peak.   They later went to 
an SO565J device, but that is rated at only 50 amps or 70 amps (it 
depends on which manufacturers data sheet you read)... and on a 
crowbar you ALWAYS go for at least two times the maximum current 
expected, if not four or even ten.   After all, the crowbar triggers 
because the regulator failed (or somehow exceeded the crowbar trip 
voltage), and the purpose of any crowbar circuit is to blow the fuse 
and shut down the supply.   To do that the SCR has to simultaneously 
short the full current of the supply AND dump all of the stored 
energy contained in the fully charged caps, and do it without 
comitting suicide by overcurrent !   Using an undersized SCR or small 
diameter wiring (i.e. current limiting) is defeating your own 
purpose. Personally I'd use a heftier crowbar device and bigger 
internal wire if I was repairing or rebuilding a high-current Astron.

Mike WA6ILQ
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