Zodiac Tri Pool Chlorinator Repeated Zener Diode Failure

[Frankster]

Hello

I have a Zodiac Tri Chlorinator that had a zener diode (D25 in schematic) fail in it earlier this year (a common failure I believe.) I traced out and drew the main board circuitry using TinyCAD and is attached. At the time I replaced the shorted 0.5W 24V Zener with a 1W and thought that would be the last of it.... until last week that is. So a beefier Zener and still it failed short..

The way I see that section works is that D25 limits the voltage for deriving VCC for PFC/PWM IC U1 from 2 sources -
1. On initial power up from the parallel winding of L5 through voltage doubler C39, C40, D23, D24 so that U1 has VCC and can perform PFC even whilst charging tank cap C6.
2. Under steady state conditions from +380V bus through R46 and R47.

Steady state conditions put I zener at 380/200,000 = 1.9mA, thus P zener = 24 x 0.0019 = 0.046W. Surely should not trouble a 0.5W zener.

I am thinking that it is transient or initial power up conditions that are causing the zener fails. I bought a 5W zener, but before I soldered that in, I started thinking maybe adding a zener boosting transistor would be better.

I'm not sure so I thought I'd ask those with far more knowledge what they think?!

[coromonadalix]

I would check your 2x  100 k R46 R47 resistors, the c41 c39 c40 capacitors and the 2 In4148 diodes 


You could try an low power 500 ma 24vdc "3 legs" regulator, they can go up to 40volts ?? that would help, they have internals protections.

[Frankster]

Thanks for the reply. I have checked those components and could find no faults.
You suggest replacing zener with a 3 pin reg like a LM317?

[coromonadalix]

yep  maybe it could resist thoses spikes you talk ?

[jmelson]

Ugh, terrible circuit!  There's a 100 uF cap charging to some sizeable voltage, then dumps into the Zener through the opto-coupler.  It is surprising the opto has not also been blown.  I don't think upping the Zener is the answer.  I'd think maybe a series resistor somewhere should be added to limit the current when the opto turns on.

Jon

[coromonadalix]

yeah  c41 could be the culprit, when discharged  it may give a good kick to the zenner,  but / maybe  an 3 pin regulator  would be able to handle that,  since they have a couple of integrated solutions like short protection, thermal cut off  etc ...

Woth the try  no ?

[Frankster]

Ugh, terrible circuit!  There's a 100 uF cap charging to some sizeable voltage, then dumps into the Zener through the opto-coupler.  It is surprising the opto has not also been blown.  I don't think upping the Zener is the answer.  I'd think maybe a series resistor somewhere should be added to limit the current when the opto turns on.

Jon

Apologies for the less than crisp schematics as I think you may be referring to the incorrect zener D26. The one that has shorted is D25. Zoomed in pic attached.

[Frankster]

It has been suggested elsewhere that perhaps a TransZorb-type device like a P6KE24A may be the solution.

Any thoughts?

[Armadillo]

It would mean that the PFC is more spiky than usual causing more induction voltage across L5.

Take out C6 and check its capacitance and ESR.

Also check components associated with snubber circuit.

Edit: Also Take out C39 and C40 and check its capacitance and ESR. [Ensure there are not shorted].

[Other short cut solution would be to remove D23.]

[jmelson]

Apologies for the less than crisp schematics as I think you may be referring to the incorrect zener D26. The one that has shorted is D25. Zoomed in pic attached.

Yes, I got the wrong one!  OK, this is more complicated.  And, a FAR worse circuit!  There is NO resistance to limit the current on D26, when run from the extra winding on the PFC inductor!  Unbelievable!  I'm kind of amazed the Zener lasted more than a few seconds.  So, the inductor has 3xx V pulses across it, and pumps charge into the 100 uF cap via the two rectifier diodes.  (Maybe there is a ratio between the two windings there, not indicated on the drawing.)

There ought to be sufficient resistance somewhere in the circuit, or the 1 uF caps should be a smaller value, to limit Zener current.  There may be a tricky balance to allow enough current when the control is supplying loads and when it is idle, so the resistor needs to be chosen properly, by measuring Zener current under all modes of operation.

Jon

[Frankster]

Yes, I got the wrong one!  OK, this is more complicated.  And, a FAR worse circuit!  There is NO resistance to limit the current on D26, when run from the extra winding on the PFC inductor!  Unbelievable!  I'm kind of amazed the Zener lasted more than a few seconds.  So, the inductor has 3xx V pulses across it, and pumps charge into the 100 uF cap via the two rectifier diodes.  (Maybe there is a ratio between the two windings there, not indicated on the drawing.)

There ought to be sufficient resistance somewhere in the circuit, or the 1 uF caps should be a smaller value, to limit Zener current.  There may be a tricky balance to allow enough current when the control is supplying loads and when it is idle, so the resistor needs to be chosen properly, by measuring Zener current under all modes of operation.

Jon

I have no idea of the actual turns ratio for L5, but the reference circuit for the PFC/PWM chip (here - https://au.mouser.com/datasheet/2/308/FAN4803-1-1191942.pdf ) by Fairchild shows 102T/4T.
Interestingly too, the reference circuit does have a 10R resistor limiting current from inductor to VCC supply but the zener arrangement is different...

[drussell]

I would check your 2x  100 k R46 R47 resistors, the c41 c39 c40 capacitors and the 2 In4148 diodes

Thanks for the reply. I have checked those components and could find no faults.
You suggest replacing zener with a 3 pin reg like a LM317?

I would replace R46 and R47 with higher wattage parts to get higher voltage rating in case they are occasionally letting a blast of the 380 V through.  They likely used the two in series to increase the resistor voltage rating of the "compound" resistor formed by the two of them.  That is not an unheard-of fault.

You might even change all seven of those components just in case it has been blasted one way or another at some point.  I would use a 35V or higher capacitor for C41 if you do.  1 volt of headroom is pushing it a bit.

Also be sure to ensure that the board is totally clean everywhere.  Those things have a nasty habit of doing all sorts of strange things like growing corrosive, conductive fuzzies and such from exposure to chlorine vapours.

[jmelson]

I have no idea of the actual turns ratio for L5, but the reference circuit for the PFC/PWM chip (here - https://au.mouser.com/datasheet/2/308/FAN4803-1-1191942.pdf ) by Fairchild shows 102T/4T.
Interestingly too, the reference circuit does have a 10R resistor limiting current from inductor to VCC supply but the zener arrangement is different...

OK, the turns ratio is why this didn't blow up in an instant.  But, yes, the reference design sounds right.  What you have is a circuit with a fairly stiff source, and NO resistance to limit Zener current!  Incredibly bad design.  The only way this could work would be if they were depending on the resistance of the winding to limit current, but with an ~ 25:1 turns ratio, my guess is the 4 turn winding has a fraction of an Ohm resistance.  You'd have to figure out how much extra drive it has to know what resistor to provide.  The drive on the PFC extra winding is going to vary with line and load changes, such as when the chlorinator is actually dispensing.  But, you HAVE to add a resistor or the Zener will keep blowing.

Jon

[fzabkar]

I'm wondering whether D25 is really necessary. The application circuit in the datasheet has no regulation or external overvoltage protection on the Vcc pin (although there is an internal 16V zener clamp). In any case, Q9 and D26 circumvent any potential OV problem. Moreover, the run current for the IC is only 2mA, and almost all of that is supplied via R46 and R47.

[Frankster]

OK, the turns ratio is why this didn't blow up in an instant.  But, yes, the reference design sounds right.  What you have is a circuit with a fairly stiff source, and NO resistance to limit Zener current!  Incredibly bad design.  The only way this could work would be if they were depending on the resistance of the winding to limit current, but with an ~ 25:1 turns ratio, my guess is the 4 turn winding has a fraction of an Ohm resistance.  You'd have to figure out how much extra drive it has to know what resistor to provide.  The drive on the PFC extra winding is going to vary with line and load changes, such as when the chlorinator is actually dispensing.  But, you HAVE to add a resistor or the Zener will keep blowing.
Jon

I would check your 2x  100 k R46 R47 resistors, the c41 c39 c40 capacitors and the 2 In4148 diodes

I would replace R46 and R47 with higher wattage parts to get higher voltage rating in case they are occasionally letting a blast of the 380 V through.  They likely used the two in series to increase the resistor voltage rating of the "compound" resistor formed by the two of them.  That is not an unheard-of fault.

You might even change all seven of those components just in case it has been blasted one way or another at some point.  I would use a 35V or higher capacitor for C41 if you do.  1 volt of headroom is pushing it a bit.

Also be sure to ensure that the board is totally clean everywhere.  Those things have a nasty habit of doing all sorts of strange things like growing corrosive, conductive fuzzies and such from exposure to chlorine vapours.

I'm wondering whether D25 is really necessary. The application circuit in the datasheet has no regulation or external overvoltage protection on the Vcc pin (although there is an internal 16V zener clamp). In any case, Q9 and D26 circumvent any potential OV problem. Moreover, the run current for the IC is only 2mA, and almost all of that is supplied via R46 and R47.

Sorry I went missing for Christmas, but had lots of food to eat and beers to drink... Hope you all enjoyed your Christmas too!

Lots of food for thought here, thanks gents. I think there is a mistake in the Fairchild reference schematic where the cathode of 16V zener CR9 is meant to connect to VCC pin 7 and not I limit pin 6 as drawn, meaning the Zodiac circuit is more similar to reference than first thought, except for 24V zener instead of 16V zener, 10K R21 in shunt with zener and a 10R R31 current limiter from inductor winding in reference circuit.

In another incredible coincidence, while my Zodiac Tri Chlorinator is out of action, I am using a friends borrowed spare Zodiac Tri that had exactly the same zener fail in it not long after mine failed originally.
I just happened to check it while it was running today and it seems to have failed again.  So for some reason, both these units operated for 7 -8 years with no trouble, and suddenly this zener failure seems to be a regular fault, which surely must be caused by the decline or fault of another component. I will be removing the components suggested and testing/replacing.

[soldar]

Things which would help:

- Bigger zenner (already done)

- Bigger C41 - I would start by doubling or even tripling the capacity

- Small value resistor or inductor in series with L5/2 - Difficult to determine best starting value but you can use a scope to see how the current behaves and have it smooth out

An alternative to the last one would be to try a resistor or cap in parallel with L5/2

[Frankster]

Things which would help:

- Bigger zenner (already done)

- Bigger C41 - I would start by doubling or even tripling the capacity

- Small value resistor or inductor in series with L5/2 - Difficult to determine best starting value but you can use a scope to see how the current behaves and have it smooth out

An alternative to the last one would be to try a resistor or cap in parallel with L5/2

Thanks heaps for reply soldar!

Bigger zener was my very first and second thought also. I NEEDED a working chlorinator TODAY so I hurriedly replaced zener with a 5W version and my unit is again functional (but I still have a friends spare dud unit with same fault to mess with.)

Bigger C41 hey!? Interesting that reference design has a 1000uF specified where Zodiac only use 100uF... (my plan was to check C41 ESR against a new one and replace it anyway..)

Small value resistor or inductor in series with L5/2. Like the 10R R31 in reference design? Also interesting that reference design also has a 10k resistor in parallel with zener where Zodiac doesn't...

[Circlotron]

Never had the problem but I did read the following:

Under certain conditions (low reverse current), some of these things exhibit a negative resistance, and can actually form a relaxation oscillator in the correct circuit configuration. Take a look at the expanded portion of Figure 5 on Page 19 of On Semi’s “TVS/Zener Theory and Design Considerations Handbook

http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF

So the zener might be oscillating and taking *big* bites of current from C41. Maybe.

[fzabkar]

I suspect that the normal operating voltage across the Zener is less than 24V. This is suggested by the low margin of the 25V capacitor. If the purpose of the Zener is to contain voltage spikes at power-on, then perhaps a 24V TVS diode might be a better fit.

[Frankster]

Never had the problem but I did read the following:

Under certain conditions (low reverse current), some of these things exhibit a negative resistance, and can actually form a relaxation oscillator in the correct circuit configuration. Take a look at the expanded portion of Figure 5 on Page 19 of On Semi’s “TVS/Zener Theory and Design Considerations Handbook

http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF

So the zener might be oscillating and taking *big* bites of current from C41. Maybe.

My tiny brain hurts after reading that. I had never heard of that phenomenon. How would I go about proving  this could be occurring and then rectifying it?

I suspect that the normal operating voltage across the Zener is less than 24V. This is suggested by the low margin of the 25V capacitor. If the purpose of the Zener is to contain voltage spikes at power-on, then perhaps a 24V TVS diode might be a better fit.

I suspect you are right about normal operating voltage being less than 24V, as chip VCC is internally clamped to 16 V. I need to go get and fit another zener in my mates dud spare and take some readings...
A TVS diode was been suggested before...

[fzabkar]

Sorry, I missed the earlier reference to TVS diodes.

[Frankster]

Sorry, I missed the earlier reference to TVS diodes.

Don't be sorry, glad you are the second to suggest it!

[MTBACHO]

Hello !! hi to everyone! I am new to this forum! but found this post very very helpful as I have been able to gather very useful information on how the power board of the Zodiac works.

I have repaired in other ocasions smps and this board is composed of several smps! this time I am struggling to find the IC u6 of my board, as it blew as it failed into a short circuit. Photo attached. I am missing part of the ref number. I have found some similarities to the schematic Frankster has uploaded!
I was wondering if anyone could help me out find the ref number of the U6 Ic driver.
Many thanks in advance!!
 

[Uunoctium]

The blown IC U5 is marked U1 in schematics above. But that's not all. In yor PCB resistor R17 looks like it is infinite and both are surely caused by failure of Q5

[MTBACHO]

Hii! Thanks Uunoctium! I have seen that R17 has had some wear and tear bur still tests a good 22k! Q5 and Q5 tests good also. What dic go short circuit was myD25 zener diode! As I see it the pcb that I uploaded has some differences with the schematics that has been uploaded! U2 on my board seem more likely to match U1 of schematics, With ref NCP1653A that drives the PFC mosfet. My U6 is for the low voltage power suppky that goes to the control board, a 15v line. The difference with the schematics is that it is generated differently. I have a dedicated smps for this, where the schematics show a 230 to 15v transformer and a bridge rectifier configuration. The U6 chip on my board is a DIP8 with 5 pin in use, as pin 3-4 are joined ans 6-7-8 also. as shown in the drawing that I have tried to trace back. Q2...VA is the only letters remaining and have not been enough of a clue to find a IC that does the job!  Anybody who has a chance to upload a photo would be of great help!
Best regards!