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Author Topic: Recommendations for power supply soft-start?  (Read 2954 times)
W7GIF
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Posts: 126




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« on: November 10, 2013, 10:19:14 AM »

Looking for recommendations for soft-start mod for a National NCX-A power supply. The inrush current is such that the spikes on the +280vdc are of a very high level......sufficient to "flash" the OA2, and vaporize 1K bias resistors on the 280v buss in the NCX-5. All power supply components are good (choke, caps, resistors) and the NCX-5 280v buss resistors and bypass caps are good. Bringing the NCX-5/NCX-A up with a variac prevents the inrush spike. Steady-state current draw on the +280v is pretty constant/stable (200ma) for all modes of operation, and the 120vac current draw varies significantly between receive and transmit modes (approx 1A vs. 3A). Any one got experience or recommendations for soft-start mods for this or similar supplies at either the line (primary) input or B+ buss, and/or sources for suitable NTC disc "Globars"?

Does anyone offer surge suppressors with avalanche levels near 300V peak?

Wish I still had some NTC disc "Globars" in my parts trays....like I once used for soft start of 3-500Z filaments!

Thanks in advance......

« Last Edit: November 10, 2013, 10:33:55 AM by W7GIF » Logged
K0BT
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Posts: 196




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« Reply #1 on: November 10, 2013, 10:44:27 AM »

Allen,

The Harbach soft start module for the SB-220 should work.  http://harbachelectronics.com/shop/heathkit-sb220-sb221/ss-221-soft-start-module/

Just curious.  Did you replace the 40uF capacitor at C3?  

Bob
« Last Edit: November 10, 2013, 10:54:27 AM by K0BT » Logged
W7GIF
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Posts: 126




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« Reply #2 on: November 10, 2013, 12:50:11 PM »

Allen,

The Harbach soft start module for the SB-220 should work.  http://harbachelectronics.com/shop/heathkit-sb220-sb221/ss-221-soft-start-module/

Just curious.  Did you replace the 40uF capacitor at C3?  

Bob
Thanks Bob. The Harbach kit sure is a good solution for soft-start, and I sure could have used it when I had an SB-220. WRT C-3, yes, actually I have two NCX-A's and one has a fresh C-3, the other has the original, and both spike the 280v supply although the steady state ripple is very low. I think the spike is due to an initially unloaded buss on the NCX-5 at power-up until the OA2 kicks in with the regulated B+, and the numerous stages using the unregulated +220v come to life.
I found a couple of gas discharge tubes (GDT's) in my parts bin that are rated at 300v breakdown (nominal). I'm going to try one of them, knowing that the GDT's have a wear-out or limited firing life. The "+280v" supply current is limited to under 2 amps, so maybe I can get some reasonable life from the GDT's. The better (longer-life) solution would be the Harbach soft-start kit, but I'll see how the GDT scheme works out. I'm surprised that the OA2 has survived the pounding that it gets at each power-up.

Thanks again, and I'll post results of the GDT scheme for those who may have an interest in solving similar power supply secondary surges.
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