Diode identification?
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From: John E. on 4 Mar 2007 14:42 John Popelish sez: > 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. What I know of the design goal of this circuit is that it must activate the solenoid quickly from off to on and quickly from on to off with as little "ramping" as possible. With the given circuit, what does this knowledge say about the selection of possible replacement component(s)? > The rectifier cathode connects toward the positive supply > end of the solenoid, but the zener cathode points toward the > fet drain. Anode-to-anode, with the rectifier "on top", the pair being connected across the solenoid? > Can you find a place to put those two components? Yes, pretty easily. It's not too heavily populated. Lots of "vertical implementation" possible (c: Thanks for your suggestions, John. -- John English
From: Terry Given on 4 Mar 2007 14:54 John E. wrote: > John Popelish sez: > > >>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. > > > What I know of the design goal of this circuit is that it must activate the > solenoid quickly from off to on and quickly from on to off with as little > "ramping" as possible. With the given circuit, what does this knowledge say > about the selection of possible replacement component(s)? > V = L*dI/dt, so dt = L*dI/V L & dI are constant, you are increasing V to get a nice low dt. the BUZ72 is a 100V part, so you have PLENTY of headroom there. the existing circuit turns the solenoid off about 8x slower than it turns it on. > >>The rectifier cathode connects toward the positive supply >>end of the solenoid, but the zener cathode points toward the >>fet drain. > > > Anode-to-anode, with the rectifier "on top", the pair being connected across > the solenoid? it doesnt matter if the rectifier is on the "top" or "bottom", only that its cathode faces towards the supply, so it prevents the zener from working when the FET is on, and allows the zener to work when the FET drain voltage rises above the supply. so a K-K connection with the recitifer at the bottom and the zener at the top, or A-A with the zener at the bottom and the rectifier at the top. > > >>Can you find a place to put those two components? > > > Yes, pretty easily. It's not too heavily populated. Lots of "vertical > implementation" possible (c: > > Thanks for your suggestions, John. Cheers Terry
From: Eeyore on 4 Mar 2007 15:11 "John E." wrote: > 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. You mean that in the entire USA there is no such thing as 47V 2-3W zener diode ? Graham
From: John Popelish on 4 Mar 2007 15:35 John E. wrote: > What I know of the design goal of this circuit is that it must activate the > solenoid quickly from off to on and quickly from on to off with as little > "ramping" as possible. With the given circuit, what does this knowledge say > about the selection of possible replacement component(s)? Well, there is nothing these diodes can do about the turn on time. That is a function of the supply voltage and the coil inductance. You would have to raise the supply voltage and add enough series resistance to limit the steady state current to a safe value to speed up turn on. >> The rectifier cathode connects toward the positive supply >> end of the solenoid, but the zener cathode points toward the >> fet drain. > > Anode-to-anode, with the rectifier "on top", the pair being connected across > the solenoid? Order doesn't matter, only orientation. Higher zener voltage means faster current ramp down. But you will probably have to go quite a bit higher to see much difference. The resistive drop of the coil is already starting the ramp down with a 42 volt reverse voltage. But that drop falls as the current falls, so the zener is really there to speed the tail of the process, unless its initial voltage is on the order of the supply voltage. So you might consider one as high as 22 to 39 volts. But then I would look for a 1 watt unit, to handle the power pulse that will end up more there than in the coil resistance. But you should definitely see some decrease in the power down time, to about 37% if what you will get from a 4.7 volt zener if you switch to a 33 volt one. So you can see that the turn off time is not dominated by the zener till its voltage gets near the supply voltage. But increasing the zener voltage drop helps.
From: John E. on 4 Mar 2007 16:14
>> Not at all. Here in USA I checked my 3 regular sources: Mouser, DigiKey, and >> Jameco with nil results, subs or not. Keyword here is "regular". > You mean that in the entire USA there is no such thing as 47V 2-3W zener > diode ? > > Graham Nyet. But point is moot, it seems. See recent posts to thread re. design change. -- John English |