Re: Simple power supply overheating components, shorting zener
- From: "Paul E. Schoen" <pstech@xxxxxxxxx>
- Date: Wed, 20 Sep 2006 11:48:36 -0400
"colin" <no.spam.for.me@xxxxxxxxxxxx> wrote in message
news:Kj8Qg.24702$0i4.4163@xxxxxxxxxxxxxxxxxxxxxxx
"Paul E. Schoen" <pstech@xxxxxxxxx> wrote in messageThe GND in the schematic is an internal reference ground for the rest of
news:4510d77e$0$19911$ecde5a14@xxxxxxxxxxxxxxxxxxx
I have a simple power supply in a special voltage sensing relay thattested
detects two valid voltage ranges, 90-130 VAC and 200-270 VAC. I have
it at up to 280 VAC, 60 Hz, and everything is fine. It is in a wellhas
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)
gotten very hot, and the zener (Z1) has shorted. The schematic is aboutmW.
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
This device is in a circuit breaker test set that may be powered fromdrop
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
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 =^.^=
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
.
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- From: Paul E. Schoen
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- From: colin
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