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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: blue 2px solid; PADDING-LEFT: 5px; MARGIN-LEFT: 5px"><FONT
style="BACKGROUND-COLOR: transparent" color=#000000 size=2
face=Arial>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>
<DIV> </DIV>
<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>
<DIV> </DIV>
<DIV>The circuit I used for the HP1144 PS is very similar to that in the
following link:</DIV>
<DIV> </DIV>
<DIV><A
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></DIV>
<DIV> </DIV>
<DIV>This link is from the old Radio-Electronics magazine, and the article shows
a schematic and has some tutorial info.</DIV>
<DIV> </DIV>
<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>
<DIV> </DIV>
<DIV>For the HP1144 the components and values I used were:</DIV>
<DIV> </DIV>
<DIV>12V 1/2W zener in series with 2 1N4007 diodes</DIV>
<DIV>1K ohm 1/2W resistor</DIV>
<DIV>2N1842 SCR</DIV>
<DIV> </DIV>
<DIV>With these components the Crowbar trips at 16V. </DIV>
<DIV> </DIV>
<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>
<DIV> </DIV>
<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>
<DIV> </DIV>
<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>
<DIV> </DIV>
<DIV>Chris, KL7DM </DIV>
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