Dishwasher Control PCB Repair - Fuse Dilemma

[xyberlink]

Hi,

I'm attempting to repair the control PCB from a Bosch SPS59L12GB Dishwasher (no doubt common to a whole range of Bosch / Siemens dishwashers).

Initial symptoms were that it kept cutting out at random, as is the power had been cut, but after 30 seconds or so it would power itself up again and attempt to continue, before cutting out again. I have checked over numerous items and not spotted anything amiss. Heating element, pump, NTC sensor, and other items all appear to be ok. Then while attempting another test run, there was a bang.

The TNY264GN power chip on the control unit has blown (see photo 01). I've seen this mentioned on other forums and I've now replaced it with a new chip (a TNY276GN as that was more readily available).

However, I also have a burned out PCB trace (see photo 02), which appears to be on the output of a half bridge rectifier that feeds the main filter capacitor and also goes off to the tab dispensor solenoid (maybe other places also). I understand this trace is supposed to act like some sort of fuse, and no doubt the TNY264GN blowout caused a short that overloaded it. Can't say I'm impressed with this design, but we'll not dwell on that for now.

Not wanting to just bridge the tracks, I've installed a fuse (see photo 03). On powering up, the machine was alive again. I set a programme going and it ran for several minutes before cutting out again. Investigation shows the fuse has blown.

The problem, is that I have no idea what size fuse to install.

The fuse that blew is a T160mA. I figured a slow blow / time delay was wise to allow for the initial inrush on the main filter cap (100uF 400v) and I picked 160mA as the TNY264GN is rated at 5.5w, hence that plus a solenoid or two would mean a control load of maybe 20 - 30 watts max. 230v x 0.16A gives me a theoretical limit of ~37w (I know fuses are +/- a fair range).

The rectifier diode is an S2M, which I believe is rated at 2A, though presumably this would be well overspec'd, partly to allow for that initial inrush.

It's easy to install a bigger fuse, but it would be nice to understand what rating is appropriate - clearly a balance between sure operation and safety.

I've not found any schematic for this board, or much other info on a typical load for the control elements of a dishwasher.

Any advice much appreciated.

[TheMG]

Solenoids can actually require a fair bit of power, especially if they are designed to be strong and operated only intermittently. They are also an inductive load so will have a PF less than 1. The power supply itself without PFC will also draw more current than the actual power calculation would suggest.

I'd start off with a 1 or 2A fuse.

[Alti]

It's easy to install a bigger fuse, but it would be nice to understand what rating is appropriate - clearly a balance between sure operation and safety.

This fuse is installed on an non-isolated switcher so this is not safety related. It is already tied to mains at both sides.

If you want to protect some components with a fuse then there is a question: what these components are and what kind of protection you want to include. The fuse can be for example included to protect a varistor from thermal run-out. Or you could protect some component (like a supply cable) from long lasting but slight overload. Or from shorts that last split of a mains cycle and reach hundreds of amps.

[xyberlink]

Thanks for the replies....

It's easy to install a bigger fuse, but it would be nice to understand what rating is appropriate - clearly a balance between sure operation and safety.

This fuse is installed on an non-isolated switcher so this is not safety related. It is already tied to mains at both sides.

If you want to protect some components with a fuse then there is a question: what these components are and what kind of protection you want to include. The fuse can be for example included to protect a varistor from thermal run-out. Or you could protect some component (like a supply cable) from long lasting but slight overload. Or from shorts that last split of a mains cycle and reach hundreds of amps.

I'm not so much trying to add protection, as just restore the original protection (such as it was).

The burnt out track is hidden under an X2 cap, but I've added another picture showing similar traces on the PCB, where standard tracks narrow for a short distance.

Solenoids can actually require a fair bit of power, especially if they are designed to be strong and operated only intermittently. They are also an inductive load so will have a PF less than 1. The power supply itself without PFC will also draw more current than the actual power calculation would suggest.

I'd start off with a 1 or 2A fuse.

I've underestimated the power requirement of the solenoid(s). I had assumed we would be talking 5-10w, when in reality it's 30w+.

With that in mind, and having rechecked the board, I have installed a 500mA fuse (highest rating I had to hand), which should be good for ~120w. That has held through a quick wash cycle - hence it may be sorted.

I have no idea if it's pulling 50w or 115w, hence unsure how close the limit I'm sailing. Will have to wait and see if it remains good across multiple cycles.