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Crowbar SCR Usage in Astron Linear Regulated Power Supplies By Robert W. Meister WA1MIK

I looked at all the Astron Linear Power Supply schematics on repeater-builder and recorded the crowbar SCR part number. The result is summarized below and is sorted by the supply's current rating, with the 28V "LS" supplies listed at the top. I made no attempt to merge the data by current rating, so you'll find entries for RM35 as well as RS35, even though the supplies are identical except for rack mounting.

At the top are the actual SCR ratings and whether the mounting tab (for the TO-220 style) is isolated or not. The stud-mounted SCR must be insulated from the chassis, as the stud itself is the anode that connects to the positive output line in the supply. Stud-mounted SCRs are very expensive these days. You could use either an isolated or non-isolated tab-mount SCR as long as whatever it's mounted to (a small heat sink) is isolated from ground, as the tab is connected to the anode, just like the stud on the 2N681. The isolated package can have a metal tab or be coated entirely in plastic.

You can use an SCR with a higher voltage or current rating than what Astron used as long as it fits into the available space. If you have to replace the stud-mounted SCR, it might be easier to purchase one of the TO-220 styles and an insulated mounting kit and drill a new hole in the chassis, or see if holes are present to mount the SCR on the regulator board. Also purchase a small finned heat sink for the SCR, similar to what is already on the TIP29 driver transistor. If the heat sink is floating and not mounted to the board, you don't need an insulated mounting kit.

Model	Amps	2N6505	2N681	MCR225	SO535W	SO565J
Volts	---	100	25	50	50	50
Amps	---	25	25	25	35	65
Isolated	---	NO	STUD	YES	NO	YES
VLS10	10				X
LS25	25					X
LS35	35				X
VLS35	35					X
RS7	07	X
RS10	10	X	X
SL11	11	X
RS12	12	X	X
SL15	15	X
RM20	20					X
RS20	20	X	X	X	X
VS20	20	X	X	X
RM35	35	X				X
RS35	35		X	X	X
VS35	35			X
RM50	50					X
RS50	50					X
VS50	50		X			X
RM60	60					X
RS70	70				X	X
VS70	70					X
# of uses	---	8	6	4	5	10

Possible available substitutes:

25V, 25A, Stud: switch to an isolated SCR.
50V, 25A, Isolated: TXN625RG (600V, 25A)
50V, 65A, Isolated: S4065JTP or S4065KTP (400V, 65A)

At the time this article was written, all of these are available from Digikey and Mouser.

The SO565J SCR can also be bought direct from Astron for about $12 including shipping. Astron has parts for all their power supplies.

Contact Information:

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.

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This page originally composed on 11-Feb-2017

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This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.