Astron Linear Power Supply Nuisance Crowbar Tripping

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Back to Home Resolving the Astron Linear Power
Supply Nuisance Crowbar Tripping
By Ray Maynard NØLGR

Background:

Our Astron RM-35A power supply was periodically firing the crowbar SCR and had to be power-cycled to recover. Needless to say, making a trip to the repeater tower site every few weeks was a royal pain in the rump. I saw the reset circuit kludge that Astron makes, but decided to actually try to fix the problem.

Bench Testing:

I swapped the supply and brought it back to the bench for some testing. I found that if I connected a large capacitive load to it, it would instantly crowbar. At first I thought it was the over-voltage protection circuit firing the SCR, but I monitored the SCR gate line with an oscilloscope and saw no turn-on signal. To verify this, I disconnected the gate line and tied it to the SCR's cathode, but it still fired when a load was attached. I used a 6,800uF capacitor to reproduce this, and every time the discharged cap was connected, the supply would shut down.

The Cause:

I concluded that it was a Δv / Δt (dv/dt) firing issue so I made the modifications shown in the schematic below. You can read more about this problem on the web. The false firing is caused by a rapid rise in anode-to-cathode voltage that seems to drag the gate layer along with it, thereby triggering the SCR to turn on.

The Fix:

The parts that address this are the upgraded (higher-voltage) SCR, the R1-C1 snubber, and the L1 choke. It now seems to be bullet-proof. The toroid core for L1 was one I had in my junk box and I don't think it is really critical. You can use any solid insulated or enameled wire for L1. The L2 ferrite bead and 0.1uF capacitor on the gate lead are more for RF immunity. The TVS diode D1 is mounted externally so any catastrophic damage from lightning and power glitches is immediately apparent. It is strange that some Astron supplies have this problem and others don't. We have one that never fails and two that did until they were modified. It could be differences in the SCR. This particular power supply had a Teccor SO565J with the letters B4 by the gate and I don't know what the one that works has, but one day I'll open it up and take a look.

Additional Information:

Some of the modifications are just additional insurance. Since our SCR is remote mounted on the back chassis, the gate lead is long enough to pick up RF, so I added the ferrite bead and the additional gate bypass cap. If the SCR is mounted on the regulator circuit board, these last two parts (L2 and C2) would not be needed. Astron installs the SCR on the regulator board on most current model supplies however the high current models (50 Amp and above) seem to still use a stud-mount SCR remotely wired to the regulator board. The long leads associated with remote wiring may have something to do with this problem.

Contact Information:

The author can be reached at: maps [ at ] century22sales [ dot ] com.

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This page originally composed on 02-Feb-2020.

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.