<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:arial,helvetica,sans-serif;font-size:10pt"><div>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.<br><br>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! <br><br>73<br>Bob Groh, WA2CKY<br>Heathkit Engineer from 1978 to 1982.<br></div><div style="font-family: arial,helvetica,sans-serif; font-size: 10pt;"><br><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;"><font face="Tahoma" size="2"><hr size="1"><b><span style="font-weight: bold;">From:</span></b> "ChrisIwata@aol.com" <ChrisIwata@aol.com><br><b><span
style="font-weight: bold;">To:</span></b> gggdds@js-net.com<br><b><span style="font-weight: bold;">Cc:</span></b> heath@puck.nether.net<br><b><span style="font-weight: bold;">Sent:</span></b> Wed, February 2, 2011 1:00:51 PM<br><b><span style="font-weight: bold;">Subject:</span></b> Re: [Heath] IC availability<br></font><br>
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<div>In a message dated 2/2/2011 8:32:57 A.M. Pacific Standard Time,
gggdds@js-net.com writes:</div>
<blockquote style="border-left: 2px solid blue; padding-left: 5px; margin-left: 5px;"><font style="background-color: transparent;" color="#000000" face="Arial" size="2">Chris,<br>Do you have any ideas on a <i>design</i> for the SCR
protection you described below, for those of us who can't design
circuits...<br>Guy<br></font></blockquote>
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<div>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!</div>
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<div>The circuit I used for the HP1144 PS is very similar to that in the
following link:</div>
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<div><span><a target="_blank" href="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</a></span></div>
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<div>This link is from the old Radio-Electronics magazine, and the article shows
a schematic and has some tutorial info.</div>
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<div>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.</div>
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<div>For the HP1144 the components and values I used were:</div>
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<div>12V 1/2W zener in series with 2 1N4007 diodes</div>
<div>1K ohm 1/2W resistor</div>
<div>2N1842 SCR</div>
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<div>With these components the Crowbar trips at 16V. </div>
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<div>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.</div>
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<div>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. </div>
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<div>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. </div>
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<div>Chris, KL7DM </div>
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