Vacuum cleaner Miele

Miele Electronic S4241

Description of failure

Does not turn on.

Troubleshooting

When having a closer look at the power PCB, it can be seen that one thermistor is missing. This device has desoldered itself. After replacing this device, the vacuum cleaner works fine!

Schematic investigation

To limit the motors inrush-current, two 7 Ohm NTC-thermistors TH1 and TH2 (negative temperature coefficient) are placed in series. After the triac D2 has turned on, the current would be too high without any current limiting elements. The thermistors limit the current, while producing heat. Due to the negative temperature coefficient, the resistance gets lower after a short time while the temperature rises. So, the motor can operate with full-power, after the final motor speed is reached. Thermistor EPCOS/TDK order number B57237S0709M000.

Some additional comments to the schematic:

• C1 works as an X-capacitor, C2 and C3 work as Y-capacitors for EMC filtering.

• Once the motor M1 blocks or a too high current flows trough the motor, a significant voltage drop accours over L1 so that the warning-lamp turns on.

• to turn on the triac D2, a diac D1 together with the capacitor C4 is used to provide the necessary current to fire the triac D2.

• The potentiometer R1 represents the slider to control the vacuum cleaners power.

Motor failure

Another common fault in vacuum cleaners is the motor itself. Replacement motors can often be purchased from the manufacturer or on ebay.

Open the housing

May this figure helps to detect screw holes to open the housing.