Diode identification?
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From: John E. on 4 Mar 2007 12:37 Palindrome sez: > I'd go for the continuous - every time where component and circuit board > burning were evident. The reason this machine drew attention in the first place was because the solenoid valve was gummed up and sticking. I wouldn't think that this would cause problems with the drive circuit. Au contraire, it would result in no back-emf. -- John English
From: John E. on 4 Mar 2007 12:37 Palindrome sez: > As I and others have written - the diode didn't burn up because of > transient energy. There is a supply problem, somewhere. The pcb had failed 'lytic caps (ends pushed out), so that could have added to the problem. Or, being beyond a certain age, the 'lytic problem may lie in the PS as well. I'll see about 'scoping the PS voltages in the machine. -- John English
From: John E. on 4 Mar 2007 12:37 Palindrome sez: > Hence why I wrote "series switching element" rather than FET. If the > designer was brought up designing using pnp/npn transistors, he may have > always protected them this way. Interesting you mention this... the silkscreen for the FET says "b-e-c". Seems that the original design was for BJT, but component specs were changed to include FET sometime in production with little regard for the confusion it would cause service personnel who saw these markings... Does this shed any different light on the choice of zener for this purpose? And the possibility for a different replacement part? The 47v, 2W part is looking like unobtanium... Thanks, -- John English
From: John E. on 4 Mar 2007 12:38 Terry Given sez: > So it is possible that the zener was used to get a suitable rate of > decay (although ramp down is more accurate) of coil current. I'm told that the solenoid this circuit operates is for a vacuum valve that must operate quickly and repeatedly. It was thought by the person who handed me this pcb that the solenoid was operated with 2 voltage rails, switching between opposite opening voltage and closing voltage. But according to measurements by him (and the fact that there's only 1 FET), the purpose of the zener here seems to make sense. But how can a 4.7v zener and one diode drop serve similar purpose as a 47v part? -- John English
From: John Popelish on 4 Mar 2007 13:09
John E. wrote: > Palindrome sez: > >> I'd go for the continuous - every time where component and circuit board >> burning were evident. > > The reason this machine drew attention in the first place was because the > solenoid valve was gummed up and sticking. I wouldn't think that this would > cause problems with the drive circuit. Au contraire, it would result in no > back-emf. I would think that a partially shorted zener would keep the solenoid energized, giving a "gummed up" symptom. If the board allows space for the modification, I would replace the 47 volt zener with a series combination of a 4.7 or 5.1 volt zener in series with a 1N400X or similar small rectifier diode, connected directly across the coil, instead of across the fet. Such a low voltage zener will be a lot more rugged (dissipating only a small fraction of the power dumped into the 47 volt zener, since it discharges only the solenoid energy, rather than that energy plus lots more from the supply). The energy dump per solenoid discharge is so much lower you may get by with a .75 or .5 watt zener and a 1N4148 diode, if the solenoid current is less than about .1 amp. The rectifier cathode connects toward the positive supply end of the solenoid, but the zener cathode points toward the fet drain. Can you find a place to put those two components? |