Diode identification?

But one that is used by manufacturers world wide for exactly that purpose ...

Arfa

so? just because it gets used doesnt mean its suited for the task. think bell curve....

BTW in that position its probably a 47V zener, clamping the peak drain voltage.

Cheers Terry

Terry Given sez:

I'd been turning over in my mind that this is indeed a zener, not simply a "plain" rectifier. It is indeed a 47 volt zener.

Why was this diode chosen in the design? I'm familiar with the standard diode being used to short-circuit the back-EMF from the solenoid, but I can't figure out the purpose of a zener used in this location.

Vdd /\ | | SS SS Solenoid SS | +-----+ | | | | BUZ72 | /---/ ZY47 FET |--+ /\ Diode

-------| | |--+ | | | \ | 0.27R / | \ | | | | | /// ///

I think that should show proper in Courier or Monaco... or Paris (c:

I must add that Vdd is *reported* to be 42vdc. I was handed this board with scribbled specs. May be higher or lower or in a parallel universe.

Thanks,

ZY47 diode, from the data sheet: Vz(min) = 44 Vz(max) = 50 I test = 10A Dynamic R @1khz = 24 (typ) Vrev = 24

If Vdd was 42V, then a 47V zener sticks 5V reverse voltage across the coil, so the current will decay 5/42 times faster than it built up.

Whereas if you just use a conventional freewheeling diode, Anode to Drain, Cathode to Vdd, there is 0.7V(ish) reverse voltage across the coil when the FET turns off, so the coil current decays 5/0.7V times slower than the 47V zener.

Of course the actual zener voltage wont be 47V, it'll be higher, depending on the actual current.

One can achieve the same objective at lower loss with a 4.3V zener in zeries with a freewheeling diode, but thats 2 parts.

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.

Or perhaps the designer was a bit stupid, used no freewheeling diode, then discovered the FET broke, so added the zener. You might be surprised how many shit designs make it to market.

Cheers Terry

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.

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

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,

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.

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.

Terry Given sez:

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?

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?

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:

What makes you say that ?

Is it against your religion to substitute ?

Graham

Consider how it works !

Graham

The usual trick is to stand them 'on end' with their common connection 'up in the air'.

Graham

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 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.

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