Simple power supply overheating components, shorting zener

[Paul E. Schoen]

I have a simple power supply in a special voltage sensing relay that
detects two valid voltage ranges, 90-130 VAC and 200-270 VAC. I have tested
it at up to 280 VAC, 60 Hz, and everything is fine. It is in a well vented
plastic octal relay enclosure. We have had two of these fail in the field
(possibly in the same unit), where the 15 ohm 2W limiting resistor (R1) has
gotten very hot, and the zener (Z1) has shorted. The schematic is about as
follows:

R2
+-------------/\/\/---------------------> To A/D circuit
| 100K 1W
| C1 C2 D2
ACHI-+-)|---)|-----+--->|---+----+---78L05---> 5 VDC to PIC
2uF 2uF | 1N4004 | | |
Film | | | |
D1--- Z1/-/ -C1 |
1N4004 ^ 12V ^ -100uF|
| | | |
R1 | | | |
ACLO---/\/\/-------+--------+----+-----+----- GND
15R 2W

The circuit draws about 110 mA at 250 VAC 60 Hz, for about 200 mW on R1.
The zener diode (1N4744) should see only about half that, for about 600 mW.
This device is in a circuit breaker test set that may be powered from
nominal 120 VAC or 208/240 VAC supplies. It is unlikely that it would be
powered from anything other than normal AC sine wave power, although it
could possibly be from a generator. The only way I can see high current
flowing through R1 would be from high frequency components on the AC line.
I don't want to use resistors in place of C1 and C2 because they would drop
25W and get quite hot in the small package, and there is no room for a
transformer. The test set would not work properly on a modified sinewave
inverter or PWM supply, and I doubt that the customer would be using
anything like that.

There are only about a dozen of these test sets in the field. A previous
unit had an 18.2 ohm 1/4 W resistor for R1, which was marginal, so it was
replaced with the 15 ohm 2 W. Any ideas about how this circuit might fail
as described?

Thanks,

Paul
www.pstech-inc.com

 

[Homer J Simpson]

There are only about a dozen of these test sets in the field. A previous
unit had an 18.2 ohm 1/4 W resistor for R1, which was marginal, so it was
replaced with the 15 ohm 2 W. Any ideas about how this circuit might fail
as described?

What are the temperature limits on the various components and what
temperature are they experiencing?

 

[Paul E. Schoen]

What are the temperature limits on the various components and what
temperature are they experiencing?

The ceramic power resistor got hot enough to discolor, and it melted a
plastic IC socket next to it. I ran the circuit for about an hour, and the
resistor was just a bit warm. I did not notice any other components with
elevated temperatures. There is only an 8 pin PIC, and two dual solid state
MOSFET relays in DIP packages. Ambient conditions are normal
industrial/commercial temperature ranges, about 0 to 40 C. The inside of
the test set might get hotter than that, but it is forced air cooled. I
plan to run the test set at maximum output and try to see if the area near
this component gets unusually hot.

Thanks for the quick reply.

Paul

 

[Daniel A. Thomas]

| I have a simple power supply in a special voltage sensing relay that
| detects two valid voltage ranges, 90-130 VAC and 200-270 VAC. I have tested
| it at up to 280 VAC, 60 Hz, and everything is fine. It is in a well vented
| plastic octal relay enclosure. We have had two of these fail in the field
| (possibly in the same unit), where the 15 ohm 2W limiting resistor (R1) has
| gotten very hot, and the zener (Z1) has shorted. The schematic is about as
| follows:
|
| R2
| +-------------/\/\/---------------------> To A/D circuit
| | 100K 1W
| | C1 C2 D2
| ACHI-+-)|---)|-----+--->|---+----+---78L05---> 5 VDC to PIC
| 2uF 2uF | 1N4004 | | |
| Film | | | |
| D1--- Z1/-/ -C1 |
| 1N4004 ^ 12V ^ -100uF|
| | | | |
| R1 | | | |
| ACLO---/\/\/-------+--------+----+-----+----- GND
| 15R 2W
|
| The circuit draws about 110 mA at 250 VAC 60 Hz, for about 200 mW on R1.
| The zener diode (1N4744) should see only about half that, for about 600 mW.
| This device is in a circuit breaker test set that may be powered from
| nominal 120 VAC or 208/240 VAC supplies. It is unlikely that it would be
| powered from anything other than normal AC sine wave power, although it
| could possibly be from a generator. The only way I can see high current
| flowing through R1 would be from high frequency components on the AC line.
| I don't want to use resistors in place of C1 and C2 because they would drop
| 25W and get quite hot in the small package, and there is no room for a
| transformer. The test set would not work properly on a modified sinewave
| inverter or PWM supply, and I doubt that the customer would be using
| anything like that.
|
| There are only about a dozen of these test sets in the field. A previous
| unit had an 18.2 ohm 1/4 W resistor for R1, which was marginal, so it was
| replaced with the 15 ohm 2 W. Any ideas about how this circuit might fail
| as described?
|
| Thanks,
|
| Paul
| www.pstech-inc.com
|
|
Paul,
How do you control the inrush when power is first applied?
As drawn, if 120 V ac is applied at the +ve peak the inrush
current will be in the order of 10 Amps.

Dan Thomas

 

[Paul E. Schoen]

| I have a simple power supply in a special voltage sensing relay that
| detects two valid voltage ranges, 90-130 VAC and 200-270 VAC. I have
tested
| it at up to 280 VAC, 60 Hz, and everything is fine. It is in a well
vented
| plastic octal relay enclosure. We have had two of these fail in the
field
| (possibly in the same unit), where the 15 ohm 2W limiting resistor (R1)
has
| gotten very hot, and the zener (Z1) has shorted. The schematic is about
as
| follows:
|
| R2
| +-------------/\/\/---------------------> To A/D circuit
| | 100K 1W
| | C1 C2 D2
| ACHI-+-)|---)|-----+--->|---+----+---78L05---> 5 VDC to PIC
| 2uF 2uF | 1N4004 | | |
| Film | | | |
| D1--- Z1/-/ -C1 |
| 1N4004 ^ 12V ^ -100uF|
| | | | |
| R1 | | | |
| ACLO---/\/\/-------+--------+----+-----+----- GND
| 15R 2W
|
| The circuit draws about 110 mA at 250 VAC 60 Hz, for about 200 mW on
R1.
| The zener diode (1N4744) should see only about half that, for about 600
mW.
| This device is in a circuit breaker test set that may be powered from
| nominal 120 VAC or 208/240 VAC supplies. It is unlikely that it would
be
| powered from anything other than normal AC sine wave power, although it
| could possibly be from a generator. The only way I can see high current
| flowing through R1 would be from high frequency components on the AC
line.
| I don't want to use resistors in place of C1 and C2 because they would
drop
| 25W and get quite hot in the small package, and there is no room for a
| transformer. The test set would not work properly on a modified
sinewave
| inverter or PWM supply, and I doubt that the customer would be using
| anything like that.
|
| There are only about a dozen of these test sets in the field. A
previous
| unit had an 18.2 ohm 1/4 W resistor for R1, which was marginal, so it
was
| replaced with the 15 ohm 2 W. Any ideas about how this circuit might
fail
| as described?
|
| Thanks,
|
| Paul
| www.pstech-inc.com
|
|
Paul,
How do you control the inrush when power is first applied?
As drawn, if 120 V ac is applied at the +ve peak the inrush
current will be in the order of 10 Amps.

Dan Thomas

That is a good point. It might be as high as 25 amps at 280 VAC input. The
time constant should be only about 15 uSec, however, so I don't think that
would cause the resistor to overheat. It might explain the shorted zener,
however, although the electrolytic capacitor across it should absorb the
spike. Maybe the customer has a very noisy connector or switch that causes
the voltage to turn on and off rapidly.

Paul

 

[ksoner]

I would doubt that if 7805 turns on and off for some line conditions
and draw more current and power when it restarts its operation than in
steady state. Did you try to increase the value of C1 and use 105C
capacitor. You may also decrease the value to see if the problem occurs

 

[colin]

I have a simple power supply in a special voltage sensing relay that
detects two valid voltage ranges, 90-130 VAC and 200-270 VAC. I have tested
it at up to 280 VAC, 60 Hz, and everything is fine. It is in a well vented
plastic octal relay enclosure. We have had two of these fail in the field
(possibly in the same unit), where the 15 ohm 2W limiting resistor (R1) has
gotten very hot, and the zener (Z1) has shorted. The schematic is about as
follows:

R2
+-------------/\/\/---------------------> To A/D circuit
| 100K 1W
| C1 C2 D2
ACHI-+-)|---)|-----+--->|---+----+---78L05---> 5 VDC to PIC
2uF 2uF | 1N4004 | | |
Film | | | |
D1--- Z1/-/ -C1 |
1N4004 ^ 12V ^ -100uF|
| | | |
R1 | | | |
ACLO---/\/\/-------+--------+----+-----+----- GND
15R 2W

The circuit draws about 110 mA at 250 VAC 60 Hz, for about 200 mW on R1.
The zener diode (1N4744) should see only about half that, for about 600 mW.
This device is in a circuit breaker test set that may be powered from
nominal 120 VAC or 208/240 VAC supplies. It is unlikely that it would be
powered from anything other than normal AC sine wave power, although it
could possibly be from a generator. The only way I can see high current
flowing through R1 would be from high frequency components on the AC line.
I don't want to use resistors in place of C1 and C2 because they would drop
25W and get quite hot in the small package, and there is no room for a
transformer. The test set would not work properly on a modified sinewave
inverter or PWM supply, and I doubt that the customer would be using
anything like that.

There are only about a dozen of these test sets in the field. A previous
unit had an 18.2 ohm 1/4 W resistor for R1, which was marginal, so it was
replaced with the 15 ohm 2 W. Any ideas about how this circuit might fail
as described?

Spikes on the AC line, or ACLO having a substantial voltage with repsect to
gnd ...
if this is connected ot the mains it can see harsh voltage spikes from other
eqpmnt turning on/off etc..
I would suspect a basic generator is just as bad if not worse unless there
is some sort of filter/protection.

Any spikes would cuase large currents to flow through the capacitors etc,
maybe it might be worth using a capcitor from the end of c2 down to aclo to
form a capacitor divider.

Colin =^.^=

 

[[email protected]]

I have a simple power supply in a special voltage sensing relay that
detects two valid voltage ranges, 90-130 VAC and 200-270 VAC. I have tested
it at up to 280 VAC, 60 Hz, and everything is fine. It is in a well vented
plastic octal relay enclosure. We have had two of these fail in the field
(possibly in the same unit), where the 15 ohm 2W limiting resistor (R1) has
gotten very hot, and the zener (Z1) has shorted. The schematic is about as
follows:

R2
+-------------/\/\/---------------------> To A/D circuit
| 100K 1W
| C1 C2 D2
ACHI-+-)|---)|-----+--->|---+----+---78L05---> 5 VDC to PIC
2uF 2uF | 1N4004 | | |
Film | | | |
D1--- Z1/-/ -C1 |
1N4004 ^ 12V ^ -100uF|
| | | |
R1 | | | |
ACLO---/\/\/-------+--------+----+-----+----- GND
15R 2W

The circuit draws about 110 mA at 250 VAC 60 Hz, for about 200 mW on R1.
The zener diode (1N4744) should see only about half that, for about 600 mW.
This device is in a circuit breaker test set that may be powered from
nominal 120 VAC or 208/240 VAC supplies. It is unlikely that it would be
powered from anything other than normal AC sine wave power, although it
could possibly be from a generator. The only way I can see high current
flowing through R1 would be from high frequency components on the AC line.
I don't want to use resistors in place of C1 and C2 because they would drop
25W and get quite hot in the small package, and there is no room for a
transformer. The test set would not work properly on a modified sinewave
inverter or PWM supply, and I doubt that the customer would be using
anything like that.

There are only about a dozen of these test sets in the field. A previous
unit had an 18.2 ohm 1/4 W resistor for R1, which was marginal, so it was
replaced with the 15 ohm 2 W. Any ideas about how this circuit might fail
as described?

Thanks,

Paul
www.pstech-inc.com

I cant read the circuit too well but it looks like C!/2 are failing.
Why do you have 2 caps in series? Are they rated for direct mains
connection (X2) ?

 

[Spehro Pefhany]

I have a simple power supply in a special voltage sensing relay that
detects two valid voltage ranges, 90-130 VAC and 200-270 VAC. I have tested
it at up to 280 VAC, 60 Hz, and everything is fine. It is in a well vented
plastic octal relay enclosure. We have had two of these fail in the field
(possibly in the same unit), where the 15 ohm 2W limiting resistor (R1) has
gotten very hot, and the zener (Z1) has shorted. The schematic is about as
follows:

R2
+-------------/\/\/---------------------> To A/D circuit
| 100K 1W
| C1 C2 D2
ACHI-+-)|---)|-----+--->|---+----+---78L05---> 5 VDC to PIC
2uF 2uF | 1N4004 | | |
Film | | | |
D1--- Z1/-/ -C1 |
1N4004 ^ 12V ^ -100uF|
| | | |
R1 | | | |
ACLO---/\/\/-------+--------+----+-----+----- GND
15R 2W

The circuit draws about 110 mA at 250 VAC 60 Hz, for about 200 mW on R1.
The zener diode (1N4744) should see only about half that, for about 600 mW.
This device is in a circuit breaker test set that may be powered from
nominal 120 VAC or 208/240 VAC supplies. It is unlikely that it would be
powered from anything other than normal AC sine wave power, although it
could possibly be from a generator. The only way I can see high current
flowing through R1 would be from high frequency components on the AC line.
I don't want to use resistors in place of C1 and C2 because they would drop
25W and get quite hot in the small package, and there is no room for a
transformer. The test set would not work properly on a modified sinewave
inverter or PWM supply, and I doubt that the customer would be using
anything like that.

There are only about a dozen of these test sets in the field. A previous
unit had an 18.2 ohm 1/4 W resistor for R1, which was marginal, so it was
replaced with the 15 ohm 2 W. Any ideas about how this circuit might fail
as described?

There might be something else on the circuit that is injecting nasty
high-frequency crap into the line, and series line inductance is
allowing a lot of energy into your unit. For example, a noisy AC-motor
inverter drive or something like that. Short of a proper
transformer-isolated design, for a fix, my suggestion is an external
power resistor of at least 100-200R (maybe a 5W-10W ceramic type).
Check your zener surge rating and substitute a TVS if necessary to
stay within the maximum ratings under all conditions (including
transient). For future designs, I suggest you use a bigger octal case
and put a suitable transformer in there if this is an industrial
application. That kind of capacitive dropper 'stuff' is really only
(barely) suitable for low-end consumer applications.


Best regards,
Spehro Pefhany

 

[Ancient_Hacker]

I have a simple power supply in a special voltage sensing relay that
detects two valid voltage ranges, 90-130 VAC and 200-270 VAC. I have tested
it at up to 280 VAC, 60 Hz, and everything is fine.

Several potential problems:

(1) What is the voltage rating of those 2uF capacitors? It better be
at least 600V each. Power lines have spikes on them! As a real-world
example, when I open up old tube radios, about 20% of them have
exploded capacitors across the 120V power line. Those caps are
sometimes 200VDC rated paper capacitors, sometimes even 600VDC.

(2) Did you know that many types of metallized film capacitors are
unabashedly designed to short out several times a second? The metal
film is very thin, so the short clears itself as the film vaporizes,
but you do get a heck of a current pulse.

(3) Imagine if the guy turns off the power switch just as the line
voltage is at its negative peak. At 280 VAC that's 396 volts. Then he
waits a second plus 1/120th of a second and flips the switch back on
again. Now you have 392 volts positive from the line going into the
lead of a capacitor that is charges to minus 392. That feeds your
diode, the zener, and the 15 ohm resistor with SEVEN HUNDRED NINETY
TWO volts. Current could be as high as SEVENTY EIGHT amps. Waay too
much current for all the components involved.

(4) Even if the components can take the current, imagine what happens
to that poor resistor. It has 700+ volts across its terminals. If
it's one of those spiral-cut resistors, it could arc over from one turn
to the next, likely leading to the same thing happening to the next
turn, and the next, and the next.... leading to really high currents
there for a few milliseconds. You need a resistor there with a
continuous voltage distribuution insiude, like a carbon comp type.
Even then I'd worry about it blowing apart.

Methinks you're in a no-win situation. How's about you buy a tiny
wall-wart transformer, or such? See www.digi-key.com. They're not
expensive and a whole lot cheaper than pissing off your customers.

 

[Ancient_Hacker]

oops, a few afterthoughts:

(5) I hope you're not making a 1uF non-polar capacitor out of two 2uF
polarized reverse connected electrolytics! Many electrolytics are
designed for smoothing applications where the ripple voltage is a small
fraction of the applied voltage. In the two-capacitor case, that's not
true. Plus since electrolytics have an up to 200% tolerance, the
smaller one may end up with up to 50% reverse voltage applied across
it. That's not a good thing.

 

[Spehro Pefhany]

oops, a few afterthoughts:

(5) I hope you're not making a 1uF non-polar capacitor out of two 2uF
polarized reverse connected electrolytics! Many electrolytics are
designed for smoothing applications where the ripple voltage is a small
fraction of the applied voltage. In the two-capacitor case, that's not
true. Plus since electrolytics have an up to 200% tolerance, the
smaller one may end up with up to 50% reverse voltage applied across
it. That's not a good thing.

He says they're film. You can get film caps rated for 250VAC line
voltage, but of course they are larger and more expensive than the
ones that will work for a while then short.


Best regards,
Spehro Pefhany

 

[Fred Bartoli]

Spehro Pefhany a écrit :

He says they're film. You can get film caps rated for 250VAC line
voltage, but of course they are larger and more expensive than the
ones that will work for a while then short.

But these are still not guaranteed to not show momentarily shorts.

 

[Ban]

I have a simple power supply in a special voltage sensing relay that
detects two valid voltage ranges, 90-130 VAC and 200-270 VAC. I have
tested it at up to 280 VAC, 60 Hz, and everything is fine. It is in a
well vented plastic octal relay enclosure. We have had two of these
fail in the field (possibly in the same unit), where the 15 ohm 2W
limiting resistor (R1) has gotten very hot, and the zener (Z1) has
shorted. The schematic is about as follows:

R2
+-------------/\/\/---------------------> To A/D circuit
| 100K 1W
| C1 C2 D2
ACHI-+-)|---)|-----+--->|---+----+---78L05---> 5 VDC to PIC
2uF 2uF | 1N4004 | | |
Film | | | |
D1--- Z1/-/ -C1 |
1N4004 ^ 12V ^ -100uF|
| | | |
R1 | | | |
ACLO---/\/\/-------+--------+----+-----+----- GND
15R 2W

The circuit draws about 110 mA at 250 VAC 60 Hz, for about 200 mW on
R1. The zener diode (1N4744) should see only about half that, for
about 600 mW. This device is in a circuit breaker test set that may
be powered from nominal 120 VAC or 208/240 VAC supplies. It is
unlikely that it would be powered from anything other than normal AC
sine wave power, although it could possibly be from a generator. The
only way I can see high current flowing through R1 would be from high
frequency components on the AC line. I don't want to use resistors in
place of C1 and C2 because they would drop 25W and get quite hot in
the small package, and there is no room for a transformer. The test
set would not work properly on a modified sinewave inverter or PWM
supply, and I doubt that the customer would be using anything like
that.
There are only about a dozen of these test sets in the field. A
previous unit had an 18.2 ohm 1/4 W resistor for R1, which was
marginal, so it was replaced with the 15 ohm 2 W. Any ideas about how
this circuit might fail as described?

Thanks,

Paul
www.pstech-inc.com

This circuit cannot work. It is a charge pump. It will kill the zener with
the inrush current and then will pump up to 600V into the cap(without a
load), yes it is a voltage doubler. It cannot supply more than 30mA(on 230V)
either with a horrible ripple. Or are these 2u caps parallel? Why don't you
use a bridge rectifier instead?

 

[Ancient_Hacker]

oops, right, they're film, the question is what is their voltage
rating, and is it enough to guarantee them not shorting under all
conditions?

Maybe you could fit a transformer in there-- a 1watt one at digi-key is
only 1x1.3x0.8 inches. Downside: $12 !!


Or a complete 5VDC wall-wart from the same place is only $3.95! I'd
seriously consider this unless it is awfully uinconvenient for the user.

 

[Paul E. Schoen]

oops, right, they're film, the question is what is their voltage
rating, and is it enough to guarantee them not shorting under all
conditions?

Maybe you could fit a transformer in there-- a 1watt one at digi-key is
only 1x1.3x0.8 inches. Downside: $12 !!


Or a complete 5VDC wall-wart from the same place is only $3.95! I'd
seriously consider this unless it is awfully uinconvenient for the user.

I believe the caps are 250 VAC polypropylene, although possibly 400 VDC. If
a transformer were used, it would have to be rated at 250 or even 300 VAC
primary, if the secondary is to be accurate enough for measurement (it
cannot saturate). There is limited space in the portable test set. Cost is
not a major factor.

Paul

 

[Ancient_Hacker]

Alternatively you can do the whole shebang of voltage detection with a
LP339 quad comparator and a snootful of resistors all which can run
off microamps.

Or a CMOS quad schmitt NAND trigger with even fewer resistors.

 

[John Larkin]

I have a simple power supply in a special voltage sensing relay that
detects two valid voltage ranges, 90-130 VAC and 200-270 VAC. I have tested
it at up to 280 VAC, 60 Hz, and everything is fine. It is in a well vented
plastic octal relay enclosure. We have had two of these fail in the field
(possibly in the same unit), where the 15 ohm 2W limiting resistor (R1) has
gotten very hot, and the zener (Z1) has shorted. The schematic is about as
follows:

R2
+-------------/\/\/---------------------> To A/D circuit
| 100K 1W
| C1 C2 D2
ACHI-+-)|---)|-----+--->|---+----+---78L05---> 5 VDC to PIC
2uF 2uF | 1N4004 | | |
Film | | | |
D1--- Z1/-/ -C1 |
1N4004 ^ 12V ^ -100uF|
| | | |
R1 | | | |
ACLO---/\/\/-------+--------+----+-----+----- GND
15R 2W

The circuit draws about 110 mA at 250 VAC 60 Hz, for about 200 mW on R1.
The zener diode (1N4744) should see only about half that, for about 600 mW.
This device is in a circuit breaker test set that may be powered from
nominal 120 VAC or 208/240 VAC supplies. It is unlikely that it would be
powered from anything other than normal AC sine wave power, although it
could possibly be from a generator. The only way I can see high current
flowing through R1 would be from high frequency components on the AC line.
I don't want to use resistors in place of C1 and C2 because they would drop
25W and get quite hot in the small package, and there is no room for a
transformer. The test set would not work properly on a modified sinewave
inverter or PWM supply, and I doubt that the customer would be using
anything like that.

There are only about a dozen of these test sets in the field. A previous
unit had an 18.2 ohm 1/4 W resistor for R1, which was marginal, so it was
replaced with the 15 ohm 2 W. Any ideas about how this circuit might fail
as described?

Thanks,

Paul
www.pstech-inc.com

Probably huge line transients. There's a low-impedance path through
the input caps and the 15 ohm thing, and it's all silicon after that.

Try replacing the zener with a transzorb, a zener rated for very high
peak power. Better yet, use a small transformer supply or buy a tiny
universal-input switcher.

John

 

[Paul E. Schoen]

This circuit cannot work. It is a charge pump. It will kill the zener
with the inrush current and then will pump up to 600V into the
cap(without a load), yes it is a voltage doubler. It cannot supply more
than 30mA(on 230V) either with a horrible ripple. Or are these 2u caps
parallel? Why don't you use a bridge rectifier instead?

The circuit *does* work. The capacitors (in series) act as a voltage
doubler, but the load of the zener diode and the 100uF filter cap limit the
voltage, and absorb the energy on the forward conduction cycle. A bridge
rectifier cannot be used because the measuring circuit must be referenced
to the ACLO point. The 15 ohm resistor does add some error (about 1.5 VAC)
to the AC voltage reading, however, so it might be better to move the
reference GND to the ACLO line. I am very limited in space, so I cannot add
more (or larger) components, unless I go to a SMT design.

Thanks,

Paul

 

[Paul E. Schoen]

Spikes on the AC line, or ACLO having a substantial voltage with repsect
to
gnd ...
if this is connected ot the mains it can see harsh voltage spikes from
other
eqpmnt turning on/off etc..
I would suspect a basic generator is just as bad if not worse unless
there
is some sort of filter/protection.

Any spikes would cuase large currents to flow through the capacitors etc,
maybe it might be worth using a capcitor from the end of c2 down to aclo
to
form a capacitor divider.

Colin =^.^=

The GND in the schematic is an internal reference ground for the rest of
the circuit. The capacitive divider might be worthwhile to consider, but
I'd rather pump mostr of the energy into the filter capacitor across the
zener. I will look at the waveforms with a scope to see if there is
anything to be concerned about, but this circuit has only failed at the
customer's site. I will see if I can talk to him and find out how he is
powering the unit. I don't think random voltage spikes would cause the
resistor to overheat, but possibly some sort of SCR phase fired regulator
could be a serious problem with a repetitive sharp rise time waveform.

Thanks,

Paul