Diode identification?
in [Repair]
|
Prev: RCA Flyback for sale. 257805 1585161a $20.00 + shipping
Next: Phillips 27" TV - Closed Captions are always on
From: John Popelish on 4 Mar 2007 13:21 John E. wrote: > 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? In the present circuit, when the fet turns off, the coil generates a voltage in the direction that tries to keep the current going. That means that the end that had been pulled negative to ground suddenly goes more positive than the 42 volt rail. At 47 volts the zener comes on, and provides a path for the decaying coil current. So, during that energy dump process, there is about 47-42=5 volts reverse voltage across the coil, driving the current toward zero. But the energy in the zener is being fed from both the coil (the 5 volt part of the 470 and by the supply the 42 volt part of the 47), since the coil current is also passing through the supply. The only advantage I can see to this wasteful and stressful (to the zener) method of driving the coil current to zero, is that the supply current ramps down to zero, smoothly, rather than switching off as the fet does. But I doubt that is a consideration in this circuit. If you put a rectifier and zener directly across the coil, the rectifier keeps the zener out of the circuit when the fet is on, but connects it as a voltage clamp when the fet switches off. Now, the only energy going into the zener is that being dumped out of the solenoid, as its current ramps down to zero. The supply stops contributing the moment the fet switches off. You can adjust the ramp down time by swapping zeners with different break down voltages. But I would start with a 4.7 or 5.1 volt unit to get things back about the way they were to start. But a 6.8 or 7.5 volt unit may make the solenoid work better with an insignificant additional voltage stress for the fet. The supply should also have some bypass capacitance connected very close to the fet source and the positive supply connection of the solenoid, to make sure the fast interruption of the current (that didn't happen with the old zener) doesn't bounce the supply rails around enough to unset either the fet gate drive or some other load connect to the 42 volt or ground rails. A microfarad or 10 would do it. I 1 microfarad, 50 or 63 volt stacked film type would do it well. see: http://www.panasonic.com/industrial/components/pdf/abd0000ce8.pdf
From: Eeyore on 4 Mar 2007 13:53 "John E." wrote: > The 47v, 2W part is looking like unobtanium... What makes you say that ? Is it against your religion to substitute ? http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=9398430 Graham
From: Eeyore on 4 Mar 2007 13:54 "John E." wrote: > 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? Consider how it works ! Graham
From: Eeyore on 4 Mar 2007 13:59 John Popelish wrote: > 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? The usual trick is to stand them 'on end' with their common connection 'up in the air'. Graham
From: John E. on 4 Mar 2007 14:03
Eeyore sez: > Is it against your religion to substitute ? Not at all. Here in USA I checked my 3 regular sources: Mouser, DigiKey, and Jameco with nil results, subs or not. But it looks like design changes are afoot (see other recent posts in this thread). Thanks, -- John English |