Jeroen said:
This sort of cicruit is pretty common in cheap low-power
mains-fed gadgets. The way I've seen it is actually like
this:
carbon
C1 220 1W D2
ACHI-----||----/\/\/-------+--->|---+------78L05--->
470n X2 | 1N4004 | |
| | |
Z1--- | |
12V/_\ 100uF=== |
| | |
| | |
ACLO-----------------------+--------+---------+----- GND
The 220 Ohm carbon resistor should hold up pretty well to abuse.
The zener doubles as a rectifier and shunt pre-regulator. If the
load is absent, it gets to dissipate all the power. You can
extend the idea to a full bridge, with two zeners and two ordinary
rectifier diodes. That would halve the ripple, but you'd lose
the ACLO to GND connection.
A fuse in one of the AC connections is mandatory for fire safety!
Jeroen Belleman
Thanks to all for the many good observations and comments. I will be
getting some 250 to 305 VAC X2 rated capacitors (1 uF). The larger 220R
carbon resistor might be a good idea, but it would need to be 3 watts,
or else 82R 1W. I need at least 1 uF (which provides about 110 mA at
280 V, so that I can get enough current at the low end (about 75 VAC)
to drive two solid state relays at 10 mA each.
The customer called back and said they use this device on a portable
generator, and sometimes they have it connected and turned on before
they start it up. There are probably lots of noise spikes as a
generator comes up to speed.
The circuit is protected by two 5 amp fuses. Possibly an in-line noise
filter might reduce the high frequency spikes which are probably
causing the failures. Unfortunately we cannot accurately simulate this
condition, and I don't think the customer has the necessary power
quality analysis equipment.
I agree that this is not an ideal design. We started with a
commercially available octal voltage relay, but it had only one
setpoint, and did not provide the versatility of the new PIC based
design. We needed a plug-in replacement, so we were constrained by the
octal relay can. Our new design will use a separate power supply, which
should eliminate the weaknesses in this design.
We will need to evaluate the best way to make the existing units safe
and reliable. The ideal solution would be a replacement octal can that
the customer could replace in the field. This is inside a test set that
weighs over 120 lb, so it is difficult and expensive to ship. We
sometimes go to the customers' sites for calibration, so we could add
external line filtering, larger resistors, or even rewire the unit with
a PC board powered from the existing universal 12 VDC supply.
Thanks,
Paul