Simple power supply overheating components, shorting zener

[Harry Dellamano]

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

Also add two more diodes and change your half wave rectifier to full wave
and replace the 2x 2uF with an X rated 275V 0.47uF cap. Change the series
resistor to a higher value energy rated puppy.
Schematic is left to the student.
Harry

 

[Paul E. Schoen]

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

It may be better to try an inductive line conditioner with a parallel
filter cap if the problem is caused by line spikes. I do have some 18 V
Transzorbs that might be able to handle the high current spikes. I have a
universal switcher with 12 VDC output in the unit, but I need all 8 pins on
the octal device for two SPDT relays and the input voltage sense lines.
This unit was designed to replace a simpler circuit that was unreliable and
inaccurate, in the same can.

When we redesign this test set, we will probably use a separate board,
where the power is drawn from the switcher. Actually, a switching supply
would also draw huge input current spikes because it has a FWB connected to
large electrolytic caps, although it usually has a line filter ahead of
that.

You have all given me some good ideas. I will make more measurements and
contact the customer later today.

Thanks,

Paul

 

[Paul]

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?

[snip]

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

Actually, the capacitors are 2.2 uF, 250 VDC, self-healing metallized
film. They are tested to 150% of that voltage, but that is just a
safety margin. With a reasonable sine wave 280 VAC input, the
capacitors should never see more than about 200 Volts peak. It may be
better to use a 1 uF 250 VAC rated polypropylene, but there is limited
room in the enclosure.

I checked the waveforms with a scope, at about 280 VAC input. The
resistor R1 has a jagged symmetrical waveform with peaks at about 3
volts, and no signs of spikes. The waveform on zener Z1 (and the filter
capacitor, which is actually 330 uF), is just about 12 VDC, with a
variable ripple about 200 mV above the 12 VDC, and sometimes as much as
800 mV below 12 VDC.

These readings were on a spare identical unit. The one that had failed
is inside the test set, and is difficult to get to. We ran the test set
at full rated current (1600 amperes) for about a half hour, and nothing
seemed to get hot.

We will open up the test set and look at the voltage relay again. Any
suggestions on how to test the capacitors to see if they may have been
damaged?

Thanks,

Paul

 

[Paul]

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.

It is not really that simple. For one thing, it is AC voltage, which I
am sampling with the PIC and averaging over 200 mSec periods without
rectification and filtering to get a faster response. The PIC flashes
an external LED through a solid state opto-relay to indicate if the
voltage is outside of either range, and another opto-relay enables a
contactor which configures a transformer for series or parallel input
depending on supply voltage of nominal 120 or 240 VAC. All of this on a
board about 1"x2".

Thanks for the alternate suggestions, however.

Paul

 

[[email protected]]

Actually, the capacitors are 2.2 uF, 250 VDC, self-healing metallized
film. They are tested to 150% of that voltage, but that is just a
safety margin. With a reasonable sine wave 280 VAC input, the
capacitors should never see more than about 200 Volts peak.

There's your problem, no where near good enough.

It may be

better to use a 1 uF 250 VAC rated polypropylene, but there is limited
room in the enclosure.

It sure would you should include a 250v varistor accross the line as
well.

We will open up the test set and look at the voltage relay again. Any
suggestions on how to test the capacitors to see if they may have been
damaged?

They may have lost some capacity.

 

[Eeyore]

There's your problem, no where near good enough.

He has got 2 in series ! An X-type might be better though.

Graham

 

[[email protected]]

He has got 2 in series ! An X-type might be better though.

Graham

I know, thats only 500v, not enough! and he has no charge balancing
resistors nor any transient suppressors. A supply destined to fail time
and time again.

 

[Ban]

I know, thats only 500v, not enough! and he has no charge balancing
resistors nor any transient suppressors. A supply destined to fail
time and time again.

Right, it is bound to fail, because of 2 violated max. ratings. He didn't
consider the inrush current. When you switch on in the wrong moment there is
over 20A running through the zener and 100u in parallel. The zener is
probably rated for 3 or 4A non repetitive surge. Also this violated the
dI/dt of the caps and it has already shorted one cap, because even with a
dead short acroo the O/P the current can not be higher than 70mA, but the OP
measured 100mA.
It is also a safty hazard to leave a 1u/400V charged capacitor open circuit
waiting to zap the technician even after days.

 

[Tony Williams]

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

Try this simple mod.

R2
+-------------/\/\/---------------------> To A/D circuit
| 100K 1W
| C1 C2 D2 R1, up to 100 ohms?
ACHI-+-)|---)|-----+--->|---+--/\/\--+----78L05--->
2uF 2uF | 1N4004 | | |
Film | | \_|_ |
D1--- | Z1 /_\ 12V |
1N4004 ^ 100uF=== | |
| | | |
| | | |
ACLO---------------+--------+--------+-------+----- GND

Zener diodes and big C's never sit well in parallel, and
a Zener always safer when it has some sort of source-R.

The circuit is good for about a 10mAdc load and will
run cooler than the original.

 

[Ancient_Hacker]

R2
+-------------/\/\/---------------------> To A/D circuit
| 100K 1W
| C1 C2 D2 R1, up to 100 ohms?
ACHI-+-)|---)|-----+--->|---+--/\/\--+----78L05--->
2uF 2uF | 1N4004 | | |
Film | | \_|_ |
D1--- | Z1 /_\ 12V |
1N4004 ^ 100uF=== | |
| | | |
| | | |
ACLO---------------+--------+--------+-------+----- GND

Zener diodes and big C's never sit well in parallel, and
a Zener always safer when it has some sort of source-R.

The circuit is good for about a 10mAdc load and will
run cooler than the original.

That's an improvement, but still:

(1) The capacitors are under-rated voltage-wise. They should be at
least 600VDC rated each, at the very least, and preferably 120VAC
rated.

(2) Removing all resistance before the first 1N4004 is a bad idea.
Worst-case, you could be discharging 1uF at 700 volts across the first
1N4004 on power-up. Or with the opposite phase, discharging 700+ volts
across the second diode on power-up. Putting a 1 meg resistor across
the capacitors will help somewhat, but you'lll still get the jolt if
the guy switches the power off and on fast.



WOEFULLY under-rated. Every negative half-cycle

 

[Eeyore]

That's an improvement, but still:

This is utter nonsense !

There's no resistance in the way at all now !

What the heck are talking about ?

Graham

 

[Tony Williams]

R2
+-------------/\/\/---------------------> To A/D circuit
| 100K 1W
| C1 C2 D2 R1, up to 100 ohms?
ACHI-+-)|---)|-----+--->|---+--/\/\--+----78L05--->
2uF 2uF | 1N4004 | | |
Film | | \_|_ |
D1--- | Z1 /_\ 12V |
1N4004 ^ 100uF=== | |
| | | |
| | | |
ACLO---------------+--------+--------+-------+----- GND

That's an improvement, but still:[/QUOTE]

Yes, one step at a time though.

(1) The capacitors are under-rated voltage-wise. They should be
at least 600VDC rated each, at the very least, and preferably
120VAC rated.

(2) Removing all resistance before the first 1N4004 is a bad
idea. Worst-case, you could be discharging 1uF at 700 volts
across the first 1N4004 on power-up. Or with the opposite phase,
discharging 700+ volts across the second diode on power-up.
Putting a 1 meg resistor across the capacitors will help
somewhat, but you'lll still get the jolt if the guy switches the
power off and on fast.

C1 and C2 should be one capacitor with a series
flameproof resistor for current limiting.

WOEFULLY under-rated. Every negative half-cycle

That voltage doubler seems to look like a 14mAdc
cc source.

 

[Ancient_Hacker]

Not to beat a dead horse, but to summarize:

(1) The circuit has multiple problems. You have to fix all of them.
400V film caps can't take the voltage or current. The diodes can't
take the 1uF 700V switching on spikes. Putting a capacitor directly
across a zener is mostly pointless (the diode will keep the capacitor
from storing the charge you need for use during the diode off time--
you need a resistor before the zener). The resistor can't be
guaranteed to take the 700V voltage or current spikes.

(2) Just because you can't see spikes doesnt mean they're not there,
or won't be at the customer's site. Especially in an industrial
environment, there can be 1KV spikes every time some inductive load
gets switched on or off.

I would patch the design by using a tiny 240V transformer, available at
digi key for $12. About a cubic inch.

Or I'd start over with a quad comparator, about 7 resistors, five
1N4148 diodes, one zener and three capacitors should do the trick.
Rough description:

A microwatt power supply-- say a 100K 2Watt resistor, a 1N4007 diode,
10uf 50V capacitor, a 100K 1/2 Watt resistor, then a 25uf 15V capacitor
with a 12V zener across it.

A mickey-mouse logic voltage selector:

A five resistor voltage divider across the 50V capacitor, totaling
about 1 meg. The four taps go to four of the inputs of a 74C14
directly to the odd inputs, thru 1meg resistors for the even inputs.
Resistor values chosen so we got 4 VDC at each tap at the magic AC
voltages.

The outputs of the even numbered gates go to the cathode end of a
diode, anode ends go to the odd numbered inputs.

The odd numbered outputs go to cathode ends of two more diodes, the
anodes go together, up to +12 volts thru a 1 Meg resistor and signal
"voltage BAD". That point goes to the input of a fifth gate, the
output of that gate is a nice strong "voltage OK".

How it works: the voltage divider trips the gates in sequence at 90,
130, 200, and 270 VAC, at the lower schmitt trip voltage (about 4
volts).

The even numbered gates (that trip at the overvoltage for each range)
turn off the respective "voltage ok" gate for that range.

The output diodes negative logic OR together the results of low v ok
and high v ok.

You can probably lower the power dissipation even more by upping the
value of the first 100K resistor until the 12V barely regulates at
80VAC.

 

[Roger Hamlett]

Not to beat a dead horse, but to summarize:

(1) The circuit has multiple problems. You have to fix all of them.
400V film caps can't take the voltage or current. The diodes can't
take the 1uF 700V switching on spikes. Putting a capacitor directly
across a zener is mostly pointless (the diode will keep the capacitor
from storing the charge you need for use during the diode off time--
you need a resistor before the zener). The resistor can't be
guaranteed to take the 700V voltage or current spikes.

(2) Just because you can't see spikes doesnt mean they're not there,
or won't be at the customer's site. Especially in an industrial
environment, there can be 1KV spikes every time some inductive load
gets switched on or off.

I would patch the design by using a tiny 240V transformer, available at
digi key for $12. About a cubic inch.

Or I'd start over with a quad comparator, about 7 resistors, five
1N4148 diodes, one zener and three capacitors should do the trick.
Rough description:

A microwatt power supply-- say a 100K 2Watt resistor, a 1N4007 diode,
10uf 50V capacitor, a 100K 1/2 Watt resistor, then a 25uf 15V capacitor
with a 12V zener across it.

A mickey-mouse logic voltage selector:

A five resistor voltage divider across the 50V capacitor, totaling
about 1 meg. The four taps go to four of the inputs of a 74C14
directly to the odd inputs, thru 1meg resistors for the even inputs.
Resistor values chosen so we got 4 VDC at each tap at the magic AC
voltages.

The outputs of the even numbered gates go to the cathode end of a
diode, anode ends go to the odd numbered inputs.

The odd numbered outputs go to cathode ends of two more diodes, the
anodes go together, up to +12 volts thru a 1 Meg resistor and signal
"voltage BAD". That point goes to the input of a fifth gate, the
output of that gate is a nice strong "voltage OK".

How it works: the voltage divider trips the gates in sequence at 90,
130, 200, and 270 VAC, at the lower schmitt trip voltage (about 4
volts).

The even numbered gates (that trip at the overvoltage for each range)
turn off the respective "voltage ok" gate for that range.

The output diodes negative logic OR together the results of low v ok
and high v ok.

You can probably lower the power dissipation even more by upping the
value of the first 100K resistor until the 12V barely regulates at
80VAC.

Three other comments.
For many approval certifications, the circuit as given, would
automatically fail.
Being 'online', implies the need for extra money to be spent on insulation
on a lot of the parts. Going offline, with a small transformer is
generally both simpler and safer.
It might be worth looking at minature switch mode topologies as well (look
at the 'Power integrations' products). PI, also do the 'LinkSwitch-TN',
which for a non isolated supply, does this in an IC, with few extra parts.

Best Wishes

 

[Jeroen Belleman]

[/QUOTE]

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

 

[Harry Dellamano]

Right, it is bound to fail, because of 2 violated max. ratings. He didn't
consider the inrush current. When you switch on in the wrong moment there
is over 20A running through the zener and 100u in parallel. The zener is
probably rated for 3 or 4A non repetitive surge. Also this violated the
dI/dt of the caps and it has already shorted one cap, because even with a
dead short acroo the O/P the current can not be higher than 70mA, but the
OP measured 100mA.
It is also a safty hazard to leave a 1u/400V charged capacitor open
circuit waiting to zap the technician even after days.

Ban,
For some reason they are not buying changing the 1/2 wave to a full wave
rectifier. Just takes two more diodes and drops the coupling cap's value in
half. This drops the surge currents in half and allows one cap of 0.47uF.
The zener should be a transzorb.
cheers,
Harry

 

[Ancient_Hacker]

For some reason they are not buying changing the 1/2 wave to a full wave
rectifier.

Sometimes you need one side of the line (hopefully the neutral side)
running straight through. With a bridge your DC output is flopping up
and down with the AC line. Not cool sometimes.

 

[Ban]

Ban,
For some reason they are not buying changing the 1/2 wave to a full
wave rectifier. Just takes two more diodes and drops the coupling
cap's value in half. This drops the surge currents in half and allows
one cap of 0.47uF. The zener should be a transzorb.
cheers,
Harry

with differential amp he could sense the mains voltage via a divider
directly. His Pic can anyway only see half the wave, this way he could also
process the negative part.
How many times has this kind of supply been discussed here, with good advise
regarding galvanically not isolated mains supplies, and here all important
safety measures have been left out, delivered to customers?

 

[Paul]

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

 

[Jamie]

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

yeah how ever, if the zener was a voltage ref type with a temperature
compensation diode in it, that wouldn't work!