Need help please

[Phillips3236]

Hi all, could someone help me with this circuit board. I have a tankless water heater and this board keeps going bad. I have replaced it 8 times and it's getting expensive. I was wanting to build a new board with components that will last. Thanks

 

[Phillips3236]

Country is USA, voltage is 240v@60hz, heater actually has 2 240v inputs system is rated at 18kw I have 2 240v cables protected by a 40a breaker on each branch

 

[Phillips3236]

https://s1274.photobucket.com/user/jophillips350/library/

 

[Phillips3236]

https://s1274.photobucket.com/user/jophillips350/library/

https://i1274.photobucket.com/album...obile Uploads/20140415_110323_zpsq3wbvi9g.jpg

 

[Phillips3236]

https://i1274.photobucket.com/albums/y424/jophillips350/Mobile Uploads/20140415_110323_zpsq3wbvi9g.jpg

https://i1274.photobucket.com/album...obile Uploads/20140415_110233_zpsts2eom1s.jpg

 

[skimask87]

How long did it last with each new board?

 

[KeepItSimpleStupid]

What's the failure mode?

I see two POSSIBLE difficiencies, which need some investigating.

a) Draw out the circuit connected to the power pins of the LM334 Op. Amp. At least until the part were power enters the board. It may not have any regulation and/or clamping circuits.

We had a thermocouple scanner at work that had the entire regulated power supply not fitted. Added clamps and no problems.
They were operating near a 1000 W halide lamp that was blinking due to a temperature controller. With mo regulation on the CMOS ICs, the scanner died.

RFI through the power lines is yet another issue especially if you have a newer furnace with a variable speed motor. You fix that at the furnace side.

So, let's see what the power supply looks like?

b) Which I totally doubt has to do with dv/dt limiting on the triac/scr, but your not mentioning any failures.

 

[Phillips3236]

How long did it last with each new board?

About a day. Or so

 

[KeepItSimpleStupid]

I'd spend some time looking for loose connections everywhere including power in and out. Go all the way back to the main house panel.

Within a day, it "MAY" qualify for free replacement although beware of "Electrical parts not returnable".

 

[skimask87]

So, a board burns out after day, you replace the board, it burns out again, you replace it, and so on.
Sure sounds to me like you can keep replacing that board for the next fifty years and not fix the problem.
You don't think maybe the problem is somewhere OTHER than that board?

 

[Misterbenn]

Sounds like either the board isn't the right one for the job, perhaps overstressed in the application. Or there is a problem somewhere else in your setup.

Is there any visible damage to the board?