dishwasher powersupply/controller board - Page 1
Forum section: electronics / repair

author: mikec
author_rank: Contributor
timestamp: March 19, 2015, 01:48:42 pm
content: HiI am repairing a dishwasher and i have removed the controller board for this. There appears to be an issue with the power supply and possibly a 74HC595 chip on board.I have removed the 74HC and I wanted to try and power up the board without this mounted but I have hit a wall with the power supply.I was tracing the tracks as it appeared some had fused (some of the PCB tracks are deliberately setup to fuse) but I noticed that the connection to the Live/Hot terminal of the transformer is connected to the output of a 5Volt regulator on the secondary low voltage side of the transformer. I am absolutely boggled by this and I although it must have worked like this for years i am hesitant to power up the board. This seems incredible to me and I have no idea why this would need to be done or how it would work.At at guess I would assume it has something to do with floating voltages and this is some cheap trick?I also think this may have caused a problem while repairing the dishwasher, one of the sensor inputs touched of the chasis of the dishwasher. This would have been one of the low-voltage inputs that goes to an ATMEGA controller. This managed to blow the input fuse on the primary side of the transformer (and I guess fused the pcb tracks as well), this seemed a bit strange to me at the time as from experience stuff happening on the secondary side (low currents and voltages) doesn't normally blow a fuse on the primary side. It also left a nice burn mark where they touched and before I took out the board I had assumed from this that the sensors are supplied with 240V and converted down on the board to suit a controller because this incident had all the hallmarks of a high-voltage short... Instead after studying the board there is very little in terms of components between the sensors and controller (isolation, basic resistance etc). which suggests they are low-voltage.Appreciate any thoughts, what is going on with the live/hot been tied to the output of the 5volt regulator?Regards,Mike

author: SeanB
author_rank: Super Contributor
timestamp: March 19, 2015, 07:25:53 pm
content: They have the whole board connected to one side of the mains so that they do not need any isolation to drive the triacs that power the pumps, interlock heater, valving and the main motor. This then just means that the control panel has to be insulated to mains standards, which is very easy if you are going for a totally sealed and waterproof front control panel. The sensors themselves are designed in any case for mains connection, and in any case the mostly plastic construction means it will be safe with mains applied. Sensors would be a door switch and a few temperature sensors for the built in heater, along with level sensors, which is a LC oscillator that is read by the microcontroller.Testing the micro is simple, do not plug the unit in to the mains, and apply 10VDC to the filter capacitor of the power supply. If the microprocessor is ok it will beep to tell you there is no mains clock input that it uses to drive the triacs. If not check it has a clock and reset is correct, but if you shorted a sensor lead to ground you probably blew it to pieces internally. Then you need a new board.

author: mikec
author_rank: Contributor
timestamp: March 20, 2015, 11:48:44 pm
content: Thanks SeanBYea tried powering it up but the 5Volt regulator appears to be getting pulled down by another short or damage on the board. I will try and locate this, hopefully it isn't the controller. No beep anyway.Just so that I understand the Triac is controlled by the ucontroller by sending a DC pulse to the Triac gate at the right part of the AC cylce. In order to know where the cycle is the ucontroller I assume from reading there is a zero-crossing circuit on the board which feeds a signal into the ucontroller. How does connecting the 240VAC live to the 5Volt regulator output achieve this?Mike

author: SeanB
author_rank: Super Contributor
timestamp: March 21, 2015, 06:09:54 am
content: One pin of the microcontroller is fed with a sample of the AC from the transformer secondary, using either a diode in the power supply or just a connection to the one side of the winding, with a resistor in series with the line and a 4V7 zener diode on the MCU side of the resistor, and possibly a 1n capacitor. Internally this is used to either drive an interrupt controller ( set to trigger on a rising edge and a falling edge) which runs a loop that flips the output pins.