Electronics > Repair
Zodiac Tri Pool Chlorinator Repeated Zener Diode Failure
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Frankster:

--- Quote from: jmelson on December 20, 2018, 09:51:08 pm ---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

--- End quote ---

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:

--- Quote from: coromonadalix on December 18, 2018, 11:52:23 am ---I would check your 2x  100 k R46 R47 resistors, the c41 c39 c40 capacitors and the 2 In4148 diodes
--- End quote ---


--- Quote from: Frankster on December 18, 2018, 07:58:15 pm ---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?

--- End quote ---

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:

--- Quote from: Frankster on December 22, 2018, 07:58:12 am ---

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...

--- End quote ---
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:

--- Quote from: jmelson on December 23, 2018, 02:37:28 am ---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

--- End quote ---


--- Quote from: drussell on December 22, 2018, 12:46:01 pm ---
--- Quote from: coromonadalix on December 18, 2018, 11:52:23 am ---I would check your 2x  100 k R46 R47 resistors, the c41 c39 c40 capacitors and the 2 In4148 diodes
--- End quote ---

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.

--- End quote ---


--- Quote from: fzabkar on December 23, 2018, 06:09:18 am ---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.

--- End quote ---

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.  |O 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.
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