Diode identification?



You blokes have forgotten R and L, and L/R. :-)
I couldn't be bothered to do the sums so just LTspice'd a quick 42V supply, 100mH and 42 ohm coil, switched by a MOSFET and clamped by a Schottky diode to a variable voltage.
The current Risetime at switchon, from 0.1A to 1A was about 5.5mS, as per the L/R exponential sum.
Below is a little table of LTspice current Falltimes.
Vclamp. Falltime (1A to 0.1A).
42 5.3mS <-- nearly equal to the L/R Risetime. 47 3.9 <-- only 1.3x 5.3mS. 57 2.6 84 1.4 <-- Changing from an L/R sum to mainly a V = L.dI/dT sum.
--
Tony Williams.

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Terry Given wrote:

Tony Williams just pointed out my mistake.
I'm so used to dealing with SMPS inductors I forgot we were talking about a solenoid.
In a SMPS inductor (or transformer) some external circuit is used to limit the current - pulse width, peak current control etc, and in order to minimise losses, Rdc is very small. In which case V = LdI/dt is the "right" equation to use (as I*R is very small)
but a solenoid or relay isnt (generally) used that way - Rdc sets the current, and is most assuredly not "very small", and of course I*R = Vcc which is not "very small"
in which case its more about L/R time constants. I = Vcc/R, so when you switch the solenoid off, the voltage across the internal inductance rises to Vcc (because of I*R) + Vclamp.
So the difference between 42+5 and 42+0.7 is bugger all, and the difference in decay time is, likewise, bugger all - well not bugger all, but certainly not 7x.
Oops.
Cheers Terry
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John E. wrote:

The zener does a better, but more expensive, job of protecting the series switching element. It limits both positive and negative transients. A diode across the switched inductor does stop most (but not all) of the switching transient - but doesn't protect the series element from transients on the supply rails, caused by other inductances elsewhere reacting to the sudden change in current. It is usual to combine these sorts of design with reasonably fast (eg tantalum)electrolytics placed locally - to act as energy "tanks" to supply and sink transient power.

As I and others have written - the diode didn't burn up because of transient energy. There is a supply problem, somewhere.
--
Sue


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Palindrome wrote:

its pretty hard finding a FET without a body diode, so negative transients are invariably taken care of regardless of the type of clamp circuit.
A diode across the switched inductor does stop most (but not

by "series element" you must be referring to the FET. Yep, the zener will protect the FET against voltage spikes on the 42V bus. Of course FETs nowadays are rated for avalanche energy.....
caused by other inductances

Que?
It is usual to

seeing as Im being a pedantic sod, I'll point out that tantalums are not electrolytics (and vice versa).
I once had a serious brain fart in this regard, making a small motor controller at Uni. It ran from a 3-phase supply, and seeing as full-wave-rectified 3-phase AC has ~15% ripple, I figured I didnt need a DC bus cap.
Which worked fine, until the first time I turned the H-bridge off with current flowing in the motor :) 30 minutes, 4 FETs and a complete set of gate drive circuits later, I added a large cap. oops.

assuming the thing ever worked properly, which it sounds like it did.
conceivably a shorted solenoid could have stored enough energy to end up snotting the zener, but as you say, a supply overvoltage would definitely kill it. And it doesnt even have to be that much, just continuous.
Cheers Terry
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Palindrome sez:

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


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"John E." wrote:

What makes you say that ?
Is it against your religion to substitute ? http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU 98430
Graham
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Eeyore sez:

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


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"John E." wrote:

You mean that in the entire USA there is no such thing as 47V 2-3W zener diode ?
Graham
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Keyword here is "regular".

Nyet.
But point is moot, it seems. See recent posts to thread re. design change.
--
John English


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Palindrome sez:

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


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John E. wrote:

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?
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John Popelish wrote:

The usual trick is to stand them 'on end' with their common connection 'up in the air'.
Graham
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John Popelish sez:

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)?

Anode-to-anode, with the rectifier "on top", the pair being connected across the solenoid?

Yes, pretty easily. It's not too heavily populated. Lots of "vertical implementation" possible (c:
Thanks for your suggestions, John.
--
John English


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John E. wrote:

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.

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.

Cheers Terry
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John E. wrote:

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.

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.
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John Popelish sez:

Seems we're creeping back up toward the original 47v zener (although it was connected across the FET, not the coil). Any advantage to simply using another 47v part along with the rect. in the configuration you recommend? Is this a case of "bigger (v) is better"?
Thanks again,
--
John English


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John E. wrote:

The advantage in moving the zener is the lower energy absorbed per discharge (for the reason I explained earlier). At 47 inverse volts across the coil, you are getting pretty close to the 100 volt mark, which will stress the fet a bit more. Are you confident in its ability to handle that voltage? And there is a point of diminishing returns. The 37% discharge time I gave above referred to the time for the current to reach zero. But that is not really the time for the magnetic field to reach zero, because the iron parts of the solenoid will circulate eddy currents that support the field for a bit. Then there is the inertial time constant of the mechanism that delay s movement, after the magnetic field stops holding it against the return spring.
If you used a 1000 volt zener, the coil current would hit zero in a really short amount of time, but the valve would close in just about the same time as if you used a 500 volt zener.
My gut feeling is that, unless this solenoid and valve mechanism were designed with fastest possible reaction time in mind, going much above 22 volts on the zener will not pay off in much decreased valve action.
But a handful of 1 watt zeners in the range of 4.7 volts to 47 volts cost only a few bucks, if you want to take the experimental route. Can you rig up some mechanical pickup on the valve, so you can, measure the response time effect of various zeners? That would make it pretty obvious where the diminishing returns come into play.
A better way to speed the release might be to put a parallel resistor and capacitor in series with the coil, so that the coil voltage actually decreases a little after the cap charges to the IR drop of steady state operation. That way, you have the large pick up force to get the valve open, but a reduced holding force to keep it open, so there is less magnetic field to quench when you want it to close. This is called a pick and hold strategy, and there are special driver chips that perform this function with two switches, one on each side of the coil.
At energize, both switches turn on, applying full voltage (often a voltage the coil would not tolerate, continuously) to the coil to ramp the magnetic field up as fast as possible. The current is sensed, and when the required pick current is reached, one of the switches pulse width modulates the current down to the hold value. When turn off time arrives, both switches open, and the coil dumps its energy back into the supply through a diode across each of the switches. So the supply voltage acts like your zener voltage. Very fast and energy efficient (there is minimal heat in the coil, and no intentional power wasted anywhere else in the circuit) but probably not practical as a retrofit in this case. http://www.ortodoxism.ro/datasheets/stmicroelectronics/1331.pdf But something to keep in mind if a board layout comes along.
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Palindrome sez:

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


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Arfa Daily sez:

The source connects to a flame-resistant (blue), less-than 1-ohm, 1/4w(?) resistor (red-violet-gold-gold) that measures about 0.5 ohm. (It should measure 0.27, yes? Maybe candidate for replacement? But maybe it's my Fluke 77's accuracy at that low setting. Resistor doesn't look abused...) The other end of the resistor does connect to ground.

Yes, I've read about the need to short out back-EMF when dealing with relay coils, solenoids, etc.
So, 1N4007 it is.
An after thought... since the diode was cooked (it actually charred the PCB beneath it) but the resistor and the FET are OK, maybe the diode needs to be boosted to a higher A rating? Thoughts?
Thanks to all,
--
John English


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"John E." wrote:

It seems my post aboutt his didn't reach the group.
It's most likely a BZY47-C47 2 watt zener 47V
Graham
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