It's shorted, burned on the side against the board, the side of the diode
that has part of the p/n printed (of course)...

Best I can make out is (reading around the diode:
ITT
4?
47

Physically it resembles a typical 1A black epoxy rectifier.

Would this be 1n4147? The "47" is clearly visible, and I think I can make out
a "4" in the first part of the poorly-legible digits. No telling how many
digits between the two "4"s.

Any possibilities other than 4147?

Thanks,
--
John English

John E. wrote:
It's shorted, burned on the side against the board, the side of the diode
that has part of the p/n printed (of course)...

Best I can make out is (reading around the diode:
ITT
4?
47

Physically it resembles a typical 1A black epoxy rectifier.

Would this be 1n4147? The "47" is clearly visible, and I think I can make out
a "4" in the first part of the poorly-legible digits. No telling how many
digits between the two "4"s.

Any possibilities other than 4147?

Not wishing to trach granny, but this would be my approach:

Reverse engineer parts of the associated circuitry until I am reasonably
confident of what sort of application it is being used for, eg lf
rectifier, hs switch, flywheel, etc. Or more importantly, if it is a
zener.. It is normally not to difficult to work out what the diode is
doing and what sort of currents, voltages and frequencies are happening
to it.

At that point, wire in an external diode with a much, much higher spec
than the original - and measure the actual running parameters. I keep a
few huge and very expensive semiconductors just for this.

Then match a diode to that requirement, by measuring what is actually
happening - with any luck the spec will match to something with a lot of
4s and the odd 7 in its product name.

Assumptions about what things may be tend to bite..

--
Sue

"John E." wrote in message
obal.net...
It's shorted, burned on the side against the board, the side of the diode
that has part of the p/n printed (of course)...

Best I can make out is (reading around the diode:
ITT
4?
47

Physically it resembles a typical 1A black epoxy rectifier.

Would this be 1n4147? The "47" is clearly visible, and I think I can make
out
a "4" in the first part of the poorly-legible digits. No telling how many
digits between the two "4"s.

Any possibilities other than 4147?

Thanks,
--
John English


Unfortunately a 1N4147 is a small signal glass diode.......

I would take a guess that what you have got is a zener diode.

Try googling for a 1N4747.

If it's ****ed then something else might have ****ed it so check the rest of
the circuit. However it sounds like it might have been slowly ****ed so you
might want to re-design the circuit.

DNA

Ah, another ITT diode on the board starts with "ZY" (where "Z" could be
mistaken for a "4" on the charred carcass). So I looked up ZY47 and get a
4.7v power zener diode, 2W.

http://www.allcomponents.ru/diotec/zy47.htm

Also, according to a National data sheet I found, a 1N4147 (a.k.a. 1N914A) is
glass, and way too lightweight physically compared to my charred sample.
Wrong turn, I think.

So it looks like ZY47?
--
John English

"John E." wrote in message
obal.net...
Ah, another ITT diode on the board starts with "ZY" (where "Z" could be
mistaken for a "4" on the charred carcass). So I looked up ZY47 and get a
4.7v power zener diode, 2W.

http://www.allcomponents.ru/diotec/zy47.htm

Also, according to a National data sheet I found, a 1N4147 (a.k.a. 1N914A)
is
glass, and way too lightweight physically compared to my charred sample.
Wrong turn, I think.

So it looks like ZY47?
--
John English

John
You can't just guess at this. As sue suggested, you need to look at the
circuitry and figure out what that diode function is. What is the circuit
for? What components are near the diode? Can you trace the diode connections
to the next devices? What are they? Once that is determined, there are
likely substitute devices that will work just fine, even if you can't
identify the original. I do this all the time as I restore a lot of old test
equipment. It can be a challenge when you don't have the schematic, but
generally you can get there with enough patience and logical thinking.

Ben Miller

--
Benjamin D. Miller, PE
B. MILLER ENGINEERING
www.bmillerengineering.com

Ben Miller sez:

You can't just guess at this. As sue suggested, you need to look at the
circuitry and figure out what that diode function is. ...

OK, here goes:
German-made paper-handling machine, c. 1989. Circuit activates solenoid,
taking several inputs from other sensors, signals, etc.

Anode of unknown diode is to ground, cathode to drain of BUZ72 MOSFET. 1uF
cap also from drain to ground. Drain connects to solenoid. Drive voltage is
42vdc.

Thanks,
--
John English

"John E." wrote in message
obal.net...
Ben Miller sez:

You can't just guess at this. As sue suggested, you need to look at the
circuitry and figure out what that diode function is. ...

OK, here goes:
German-made paper-handling machine, c. 1989. Circuit activates solenoid,
taking several inputs from other sensors, signals, etc.

Anode of unknown diode is to ground, cathode to drain of BUZ72 MOSFET. 1uF
cap also from drain to ground. Drain connects to solenoid. Drive voltage
is
42vdc.

Thanks,
--
John English


Not then, I would suggest, a 4.7v zener ... Sounds like it's just a
flywheeling diode and a 1N4007 would do the job just fine. Does the FET
source go to ground ? Many power MOSFETS have a diode internally in that
orientation across from the source to the drain. It gets there as an
integral side effect of the manufacturing process.

Arfa

flywheeling diode and a 1N4007 would do the job just fine. Does the FET
source go to ground ? Many power MOSFETS have a diode internally in that
orientation across from the source to the drain. It gets there as an
integral side effect of the manufacturing process.

Newer designs seem to use a fast-recovery diode (e.g. FRED or HEXFRED
or HiperFRED) for MOSFET flyback protection. 1N4007s aren't
particularly fast, and reverse-conduction losses can be significant if
the switching frequency is high (in e.g. an SMPS). That's probably not
all that much of an issue in a solenoid driver, though.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Palindrome wrote in
:

Not wishing to trach granny, but this would be my approach:


I hope it doesn't come to that, if she has any breathing difficulties at
all, I imaging it's just due to the rarified air of this place.

Anode of unknown diode is to ground, cathode to drain of BUZ72 MOSFET. 1uF
cap also from drain to ground. Drain connects to solenoid. Drive voltage
is
42vdc.

The diode is a snubber to bypass the inductive kick from the solenoid when
it deenergizes. A 1N4007 will work fine.

Ben Miller

--
Benjamin D. Miller, PE
B. MILLER ENGINEERING
www.bmillerengineering.com
"John E." wrote in message
obal.net...

"John E." wrote:

It's shorted, burned on the side against the board, the side of the diode
that has part of the p/n printed (of course)...

Best I can make out is (reading around the diode:
ITT
4?
47

Physically it resembles a typical 1A black epoxy rectifier.

That's not what either a 1N4147 or 1N4447 looks like.

Graham

"John E." wrote:

Ah, another ITT diode on the board starts with "ZY" (where "Z" could be
mistaken for a "4" on the charred carcass). So I looked up ZY47 and get a
4.7v power zener diode, 2W.

http://www.allcomponents.ru/diotec/zy47.htm

Also, according to a National data sheet I found, a 1N4147 (a.k.a. 1N914A) is
glass, and way too lightweight physically compared to my charred sample.
Wrong turn, I think.

So it looks like ZY47?

BZY47-C47?

2 watt zener 47V ? The number after the C is the voltage. 4.7 volts would be
BZY47-C4V7 btw.

Graham

On Fri, 02 Mar 2007 16:49:16 GMT, John E. put
finger to keyboard and composed:

It's shorted, burned on the side against the board, the side of the diode
that has part of the p/n printed (of course)...

Best I can make out is (reading around the diode:
ITT
4?
47

Physically it resembles a typical 1A black epoxy rectifier.

Would this be 1n4147? The "47" is clearly visible, and I think I can make out
a "4" in the first part of the poorly-legible digits. No telling how many
digits between the two "4"s.

Any possibilities other than 4147?

Thanks,

Go here ...

http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf/$$Search?OpenForm

.... and type the following into the search box:

1n4*47

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

"Dave Platt" wrote in message
...
flywheeling diode and a 1N4007 would do the job just fine. Does the FET
source go to ground ? Many power MOSFETS have a diode internally in that
orientation across from the source to the drain. It gets there as an
integral side effect of the manufacturing process.

Newer designs seem to use a fast-recovery diode (e.g. FRED or HEXFRED
or HiperFRED) for MOSFET flyback protection. 1N4007s aren't
particularly fast, and reverse-conduction losses can be significant if
the switching frequency is high (in e.g. an SMPS). That's probably not
all that much of an issue in a solenoid driver, though.



That was my thought, and the reason that I suggested a KISS approach with a
1N4007

Arfa

Arfa Daily sez:

Not then, I would suggest, a 4.7v zener ... Sounds like it's just a
flywheeling diode and a 1N4007 would do the job just fine. Does the FET
source go to ground ?

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.

Many power MOSFETS have a diode internally in that
orientation across from the source to the drain. It gets there as an
integral side effect of the manufacturing process.

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

Black ? What size ? I guess 1n4007

1KV , ?1amp , 40 amp surge

____________________________________






On Mar 2, 9:49 am, John E. wrote:
It's shorted, burned on the side against the board, the side of the diode
that has part of the p/n printed (of course)...

Best I can make out is (reading around the diode:
ITT
4?
47

Physically it resembles a typical 1A black epoxy rectifier.

Would this be 1n4147? The "47" is clearly visible, and I think I can make out
a "4" in the first part of the poorly-legible digits. No telling how many
digits between the two "4"s.

Any possibilities other than 4147?

Thanks,
--
John English

"John E." wrote:

Arfa Daily sez:

Not then, I would suggest, a 4.7v zener ... Sounds like it's just a
flywheeling diode and a 1N4007 would do the job just fine. Does the FET
source go to ground ?

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.

Many power MOSFETS have a diode internally in that
orientation across from the source to the drain. It gets there as an
integral side effect of the manufacturing process.

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?

It seems my post aboutt his didn't reach the group.

It's most likely a BZY47-C47 2 watt zener 47V

Graham

Werty wrote:

Black ? What size ? I guess 1n4007

You would be wrong.

Late at night, by candle light, Eeyore
penned this immortal opus:



"John E." wrote:

Ah, another ITT diode on the board starts with "ZY" (where "Z" could be
mistaken for a "4" on the charred carcass). So I looked up ZY47 and get a
4.7v power zener diode, 2W.

http://www.allcomponents.ru/diotec/zy47.htm

Also, according to a National data sheet I found, a 1N4147 (a.k.a. 1N914A) is
glass, and way too lightweight physically compared to my charred sample.
Wrong turn, I think.

So it looks like ZY47?

BZY47-C47?

2 watt zener 47V ? The number after the C is the voltage. 4.7 volts would be
BZY47-C4V7 btw.

Graham

Spot-on I think. It'll normally have 42 V across it, not enough to
conduct. When de-energizing there'll be a spike wich it clamps at 47
V. Dunno why the 1 uF cap.

- YD.

--
Remove HAT if replying by mail.

YD wrote:

Dunno why the 1 uF cap.

German overkill engineering !

Graham

YD wrote:
Late at night, by candle light, Eeyore
penned this immortal opus:



"John E." wrote:


Ah, another ITT diode on the board starts with "ZY" (where "Z" could be
mistaken for a "4" on the charred carcass). So I looked up ZY47 and get a
4.7v power zener diode, 2W.

http://www.allcomponents.ru/diotec/zy47.htm

Also, according to a National data sheet I found, a 1N4147 (a.k.a. 1N914A) is
glass, and way too lightweight physically compared to my charred sample.
Wrong turn, I think.

So it looks like ZY47?

BZY47-C47?

2 watt zener 47V ? The number after the C is the voltage. 4.7 volts would be
BZY47-C4V7 btw.

Graham


Spot-on I think. It'll normally have 42 V across it, not enough to
conduct. When de-energizing there'll be a spike wich it clamps at 47
V. Dunno why the 1 uF cap.

- YD.

So, if all the conduction it should get is from the odd spike, how come
it got the San Quentin hairdo? It takes rather more than static to fry
chips.

My thoughts would be that the supply rails are screwed. If the unit has
a positive and negative rail, it may be that the ground return to the
psu has problems and thus more than the 42v is appearing across the
zener, as the ground point has shifted towards the negative rail.


--
Sue

Dave Platt wrote:
flywheeling diode and a 1N4007 would do the job just fine. Does the FET
source go to ground ? Many power MOSFETS have a diode internally in that
orientation across from the source to the drain. It gets there as an
integral side effect of the manufacturing process.

Newer designs seem to use a fast-recovery diode (e.g. FRED or HEXFRED
or HiperFRED) for MOSFET flyback protection. 1N4007s aren't
particularly fast, and reverse-conduction losses can be significant if
the switching frequency is high (in e.g. an SMPS). That's probably not
all that much of an issue in a solenoid driver, though.


1N4007s can take a long time to turn _on_ as well as off. You can get
several volts of overshoot in some applications. Definitely not your
ideal catch diode.

Cheers,

Phil Hobbs

"Phil Hobbs" wrote in message
...
Dave Platt wrote:
flywheeling diode and a 1N4007 would do the job just fine. Does the FET
source go to ground ? Many power MOSFETS have a diode internally in that
orientation across from the source to the drain. It gets there as an
integral side effect of the manufacturing process.

Newer designs seem to use a fast-recovery diode (e.g. FRED or HEXFRED
or HiperFRED) for MOSFET flyback protection. 1N4007s aren't
particularly fast, and reverse-conduction losses can be significant if
the switching frequency is high (in e.g. an SMPS). That's probably not
all that much of an issue in a solenoid driver, though.


1N4007s can take a long time to turn _on_ as well as off. You can get
several volts of overshoot in some applications. Definitely not your
ideal catch diode.

Cheers,

Phil Hobbs

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

Arfa

Arfa Daily wrote:
"Phil Hobbs" wrote in message
...

Dave Platt wrote:

flywheeling diode and a 1N4007 would do the job just fine. Does the FET
source go to ground ? Many power MOSFETS have a diode internally in that
orientation across from the source to the drain. It gets there as an
integral side effect of the manufacturing process.

Newer designs seem to use a fast-recovery diode (e.g. FRED or HEXFRED
or HiperFRED) for MOSFET flyback protection. 1N4007s aren't
particularly fast, and reverse-conduction losses can be significant if
the switching frequency is high (in e.g. an SMPS). That's probably not
all that much of an issue in a solenoid driver, though.


1N4007s can take a long time to turn _on_ as well as off. You can get
several volts of overshoot in some applications. Definitely not your
ideal catch diode.

Cheers,

Phil Hobbs


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:

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

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,
--
John English

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

John E. wrote:
Terry Given sez:


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


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,

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 **** designs make it to market.

Cheers
Terry

John E. wrote:
Terry Given sez:


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


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.

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.


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.

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

--
Sue

Palindrome wrote:
John E. wrote:

Terry Given sez:

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



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.


The zener does a better, but more expensive, job of protecting the
series switching element. It limits both positive and negative
transients.

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
all) of the switching transient - but doesn't protect the series element
from transients on the supply rails,

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
elsewhere reacting to the sudden change in current.

Que?

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.

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.




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.

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


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

Terry Given wrote:
Palindrome wrote:

John E. wrote:

Terry Given sez:

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




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.



The zener does a better, but more expensive, job of protecting the
series switching element. It limits both positive and negative
transients.


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.

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.


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,


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

elsewhere reacting to the sudden change in current.


Que?

The power distribution and supply system connected to the load will,
itself have a transient response (eg have series inductance) and may
easily overshoot following step changes in load.


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.


seeing as Im being a pedantic sod, I'll point out that tantalums are not
electrolytics (and vice versa).
"Tantalums
Tantalum capacitors are also electrolytic, constructed with a very
porous anode made with tantalum powder. This powder is pressed into a
pellet form with a tantalum wire inserted."
http://www.electronicproducts.com/pr...w.may2006.html


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.




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.


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


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.

I'd go for the continuous - every time where component and circuit board
burning were evident.

--
Sue

Palindrome wrote:
Terry Given wrote:

Palindrome wrote:

John E. wrote:

Terry Given sez:

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





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.




The zener does a better, but more expensive, job of protecting the
series switching element. It limits both positive and negative
transients.



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.


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.

its surprising how much stuff ends up being designed that way -
"because", rather than having a good reason.



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,



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

elsewhere reacting to the sudden change in current.



Que?


The power distribution and supply system connected to the load will,
itself have a transient response (eg have series inductance) and may
easily overshoot following step changes in load.


now thats a nicely worded sentence.


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.



seeing as Im being a pedantic sod, I'll point out that tantalums are
not electrolytics (and vice versa).

"Tantalums
Tantalum capacitors are also electrolytic, constructed with a very
porous anode made with tantalum powder. This powder is pressed into a
pellet form with a tantalum wire inserted."
http://www.electronicproducts.com/pr...w.may2006.html


ya got me

common usage of "electrolytic" refers to ye olde carboxylic acid-style
electrolytes, for whatever reason tantalums are always called tantalums.


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.




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.



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


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.


I'd go for the continuous - every time where component and circuit board
burning were evident.

it all depends on how much transient energy there was. when working with
big stuff, components can (and do) disappear completely. If any form of
arc develops as a result of some transient phenomenon, its pretty easy
to burn big holes in things.

and a transient that snots the zener will make it fail short-circuit
(unless it disappears), at which point it will then fry.

however such pontification is essentially meaningless; the supply rail
needs to be checked....

Cheers
Terry

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

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

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

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

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?

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/...abd0000ce8.pdf

"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/endecaSear...sp?SKU=9398430


Graham

"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

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