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Author Topic: ARRL Handbook power supply GOOF  (Read 3664 times)
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W4AMV
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« on: December 28, 2011, 09:31:53 PM »

FYI, and for more than a decade, a popular PS in the ARRL Hdbk, say 2000 edition, a 28 V @ 10 A has a goof. The crowbar ckt as shown does not work. The MC3423P OVP IC is biased from a 12 V Zener  via a 4.7 k series R. Trying to fire an SCR, the Zener falls out and the circuit fails. Change the series R to the Zener to 820 ohm. All is then bliss.  This had me going for awhile in trying to wrap up this supply. Cheesy
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WA1GFZ
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« Reply #1 on: December 28, 2011, 10:02:09 PM »

Wow that is a pretty high value. It would take some real over voltage to latch the SCR.
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Pete, WA2CWA
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« Reply #2 on: December 28, 2011, 10:48:50 PM »

Did you check the Handbook Errata section here: http://www.arrl.org/arrl-handbook-reference
I would also send an e-mail to the Handbook editor with your findings.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
W4AMV
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« Reply #3 on: December 29, 2011, 10:47:49 AM »

Good idea. Nope, so it is not part of the errata list. So the contact for errata items by the way is:sford@arrl.com

Thanks!
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k4kyv
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Don
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« Reply #4 on: December 29, 2011, 06:45:30 PM »

FYI, and for more than a decade, a popular PS in the ARRL Hdbk, say 2000 edition, a 28 V @ 10 A has a goof. The crowbar ckt as shown does not work. The MC3423P OVP IC is biased from a 12 V Zener  via a 4.7 k series R. Trying to fire an SCR, the Zener falls out and the circuit fails. Change the series R to the Zener to 820 ohm. All is then bliss.
Quote
Nope, so it is not part of the errata list. So the contact for errata items by the way is:sford@arrl.com

Goes to show how few hams actually try to build the construction projects described in the Handbook.  After a whole decade, no-one had called attention to the error.

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Don, K4KYV                                       AMI#5
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WQ9E
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« Reply #5 on: December 29, 2011, 07:41:34 PM »

Probably a few built it but hopefully nobody experienced a shorted pass transistor and thus discovered too late that the crowbar didn't work.

I wonder if a drafting error crept in before it made it to print?  One would hope the original designer tested the protection circuit. 
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Rodger WQ9E
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« Reply #6 on: December 29, 2011, 07:52:16 PM »

What does the IC's spec sheet show? Maybe a 470?

Carl
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wb6kwt
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« Reply #7 on: December 30, 2011, 09:23:32 AM »

Wow, thanks for the info, I'm building that power supply now, just need to find a case to put it in.

Bob
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W4AMV
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« Reply #8 on: December 30, 2011, 10:06:56 AM »

Yep! That supply is FB. However, that crowabr if you ever need it, WILL NOT WORK! Sure enough, if you go to the data sheet for the MOTO (ON Semi) and look at the MC3423 data sheet, page 3- Figure 5, see the equation on the right hand side for RS: Rs=(Vs-10)/(25) k OHM. So they actually add about 15 mA overhead to the Zener current for the IC and the gate current for the SCR. So, in the case of the 12 V Zener and operation of the 3423 IC at 12 V, we need (33-12)/(25) or 840 ohms. I used 820 and the supply in my case tops out at 33 V. Your case may differ.

Good luck, 73

Alan  Smiley
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