Re: Simple power supply overheating components, shorting zener

Paul E. Schoen wrote:
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?
--
ciao Ban
Apricale, Italy


.

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