Wanderer Motorhome Page 13

Contents:

1.   ELECTROLUX RM4200-S Fridge Modification: 2. 555 Timer - Fridge Gas Flame ON Indicator

              Fitted in my Motorhome is the Electrolux [Dometic] RM4200-S fridge.

               A source of irritant was the inability to determine whether the fridge had been turned on.  When I say ON,  I mean,  had the voltage source supplies been connected?  

              Turning the switch on when travelling or when hooked up on site,  then the fridge is on.  Wrong,  During the initial phase of getting to know our Motorhome it took some time to realize the fridge did not turn on when we were travelling.  The fault was a fuse holder in the engine compartment melting, due to a loose terminal.  The fuse holder was not up to the job.    I decided to change this unsatisfactory arrangement.

              I have always been at a loss to understand why some sort of indication was not fitted to these fridges. I think the more recent ones have.

               This modification will show whether the current source from 12Volts or 250AC mains is connected.  I did this by fitting indicator lamps which would show if the voltages were connected and the fridge was operating.  I would anticipate this modification being carried out by others after the makers warranty had expired.

                The schematic drawing can be seen below and also photos of the fridge in different phases of carrying out this modification.   A green lamp was chosen for mains as it matched the green switch which turned the mains on.   A red lamp was chosen for the engine voltages as this matched the red switch which turned the engine 12volts on.

Carrying out the Modifications

First the top facial panel has to be removed, this is quite easy, unscrew the two screws on the panel, and pull off the variable knobs.  This can be done by using a pliers.  Place a piece of cloth on the knobs so as not to mark them when pulling.  You might find the door travel securing pin will have to be removed.

                  The fridge is secured to the frame with 4 screws.  These screws are covered with a set of covers just inside the door, remove then using a flat knife then unscrew the recessed screws.

Prior to removing the fridge from it's position. Take off the vents outside on the van wall. Then the black exhaust vent pipe should be removed by unscrewing the one screw holding it in position. 

Pull out the fridge, see the photo above,  just enough to work on the electrics which are situated on top of the fridge.   When refitting the fridge, check the gas vent at the back of the fridge,  as this will pull off and away from the front vent cover,  when removing the fridge.  It is just a push fit and can be accessed by removing the outer vent panel .

                  I produced thin metal brackets which supported the lamps.  This meant the fridge front panel could be taken off without disturbing the lamps in the same manner as the front controls are fitted.  The green lamp was fitted to the left of the panel.  The red one fitted to the right, as it was easier to see from the driving seat while travelling.

                   Below shows the brackets and lamps in place.

         The lamps were obtained from Maplins, a well known supplier to the Electronics DIY market.  One was a Neon 250 volts lamp coloured green as it matched the mains on/off switch,  and the other was a red 12volt LED lamp as this matched the 12volt red on/off switch. As these lamps do not have filaments they are ideal for a Motorhome environment and are long lasting.

The part numbers from Maplin's are:  
Red lamp for 12volt application; UF83E or UF85G
Green lamp for mains application; BK53H or MK79L

 Product Details   
 Description:  UF83E - Panel Mounting 12V LED with Plastic Bezel
A red or green LED in a red or green polycarbonate package with a round slightly domed front face. The package has an internal resistor so that it may be connected directly to 12V. The anode is denoted by a white spot, which is connected to 12Volts positive.
Dimensions 30.5mm long. Front face 8.5mm diameter. Mounting hole required 7mm diameter. Forward current approx. 15mA at 12V. 

 Description:   BK53H - Miniature Round Panel mounted Neon 2010 
Moulded body with built-in resistance for 230Vac use. Red, green or amber lens. Requires a 7mm mounting cut-out in panel.
Overall dimensions: 33mm long, 8mm diameter round lens. 

With regret the above lamps have not proved to be very reliable in service. When taking out the fridge for maintenance they have collapsed. So perhaps the alternative lamp, with vehicle type spade, should be used. Maplin Part number VW69A for mains use.

          The cables attached to the lamps were soldered on, for those not inclined to use a soldering iron, ordinary vehicle connectors from DIY shops will do just as well.

           The modification has been reliable during it's use now for 2 years, but for problems outlined above.

             Having seen my modification, a friend requested the same.  Before I could do this modification to his fridge, a check of the wiring had to be made, it is as below.  There was a slight difference,  the Live and Neutral on the 250vAC supply was switched and the Thermostat switched the neutral 250VAC line, and not the 250vAC line voltage as is usual!

              The Green lamp gives an indication when the hookup cable has been turn on,  The amber lamp gives an indication when the fridge has reached it's operating temperature. The red lamp indicates when the 12Volts from the engine is present.
             The amber lamp works on the principle where, when the thermostat turns off, the amber light then turns off.

Turning on the fridge, while using gas, then kneeling on the floor to monitor a gas flame and trying to hold the gas control in at the same time is not very convenient, to say the least. Also there have been times when the gas cylinder has run out. It can be some time before this is realised.

 I have come up with this idea, as seen below in the drawing. The Bead Thermistor will monitor the flame  and turn the LED lamp on. As soon as the gas flame expires the Thermistor will cool down and turn the LED lamp off. Even better, a buzzer could be used in addition as an audible warner. [ An on/off switch will be required!]

The diagram below shows how the basic Schmitt trigger using the 555 DIL Intergrated circuit can be adapted to control a high temperature application.  It is shown driving a relay, by wiring a BEAD THERMISTOR and variable resistor to pin 2 input terminal of the IC.

The Bead Thermistor must be a Negative-Temperature-Coefficient device. This thermistor must present a resistance in the range of 470ohms to 4,700ohms at the required turn on level. [4k7NTC]

Under Temperature control can also be obtained by using the connections as shown in the diagram at X-X.

The modern 555 device shown in the catalogues are: NE555N, TS555, ICM7555.

The bead thermistor is normally described as a BEAD THERMISTOR 15K.
It has a Resistance at:  25degreesC is 15K Ohms. 
                                    At 100degreesC is 2.77K Ohms. 
                                    At 0degreesC is 352.4K Ohms.
So it can be seen at the high temperature the THERMISTOR is within the required switching range as mentioned in the above text.

The thermistor would be attached to the body of the gas burner flame without being too close. The relay would then switch an LED lamp as an indicator.

I have experimented with this idea and found the circuit to be sound. There is a problem in that the thermister has to be placed to close to the flame to be effective and in turn destroying it. It's back to the drawing board!

I have left this page entry for others to come up with an idea for the flame monitor. Should anyone have a better idea please let me know. I would dearly love to monitor the gas flame on the fridge without getting on my knees.

2005. I have found that a 4k7 thermistor placed close to the flame has better control, than the 15k device.

          Another simple idea, It has been suggested to me perhaps an old electric iron adjustable thermostat could be used. Placed on a plate and fitted close to the flame would control a 12volt application.

For those who have problems with their fridge, then click on the link below for direct access to the Dometic site and their help advice pages.

Dometic UK  Ltd
Dometic House. 
The Brewery. 
Blandford St Mary
Dorset
DT11 9LS
 Tel. +44 (0) 844 626 0133

For spares see:

http://www.leisurespares.co.uk/stock/Dometic/4-Series-Fridges/RM4200-_-4201/62/