Pagetslady
Well-Known Member
I want to make my fridge over ride the thermostat when the engine starts, I assume I need a relay that switches on the fridge when the voltage is around 13.5 volts any ideas will help
Mike
Mike
Voltage controled relay
- Thread starter Pagetslady
- Start date
Rigger Mortice
N/A
More commonly known as a Voltage Sensitive Relay. Lots available on E-bay and elsewhere.
I'm not sure why you want it to override your thermostat though. Surely you would want it to override the on/off switch so that the fridge runs when the engine is running? If it overrides the thermostat the fridge will just keep running all the time the engine is running and eventually it could ice up (assuming it is switched on. If it is off then nothing will happen whether the thermostat is overridden or not?)
I'm not sure why you want it to override your thermostat though. Surely you would want it to override the on/off switch so that the fridge runs when the engine is running? If it overrides the thermostat the fridge will just keep running all the time the engine is running and eventually it could ice up (assuming it is switched on. If it is off then nothing will happen whether the thermostat is overridden or not?)
VicMallows
Well-Known Member
I can well understand why one would want to override the thermostat .... so as to cool down as much as possible while the engine is running. I do exactly this with my (absorption) fridge. Yes, all you need is a relay to short the thermostat contacts when the alternator is charging. Ideally, I suppose, you could use a voltage-sensitive-relay (VSR) to be sure that the alternator was actually charging adequately ......but I would simply use a feed of the 'ignition' switch and assume the alternator was probably working!
Pagetslady
Well-Known Member
My fridge came with a device which did this but it has failed twice so I binned it an have used the fridge with just a thermostat ( a Chinese one with digit read out it is great) but no it would be good to control the fridge to automatically come on when the engine is running, I thought about running a wire from the ignition switch but then thought rather than start messing with the wiring on my new Beta control board, it doesn't have a ignition switch as such when you press the heater button that switches on the panel etc any way thanks for the help.
Mike.
Mike.
nimbusgb
Well-Known Member
Just a voltage comparator driving a small relay in parallel with the thermostat contact. Voltage above about 13 then contact is closed.
Google Lm311. A few components, vero board and an hour at the bench.
I'd be tempted to modify this to run off a regulated supply or use a zener diode for a more stable reference point rather than a ratiometric one.
The more complicated diagram introduces a bit of hysteresis to prevent relay chatter, there is also flyback protection.
View attachment 40234
Google Lm311. A few components, vero board and an hour at the bench.
I'd be tempted to modify this to run off a regulated supply or use a zener diode for a more stable reference point rather than a ratiometric one.
The more complicated diagram introduces a bit of hysteresis to prevent relay chatter, there is also flyback protection.
View attachment 40234
Last edited:
William_H
Well-Known Member
There is an even easier way to build a VSR using a little chip called TL431. This is described as a precision adjustable zener diode.
http://www.alldatasheet.com/datasheet-pdf/pdf/57216/CALOGIC/TL431.html
essentially it has internally all the circuitry described above.
You connnect a vartiable resistor to the control pin centre to control top to +ve supply bottom to negative. Connect a small current realy coil between the cathode and +ve supply. It will operate when voltage exceeds the preset point.
Just one trap. This device emulates a zener diode which is always connected +ve to cathode because they "break down" (start conducting) with reverse polarity at the zener voltage. Hence the description of cathode on the circuit diagram which actually has +ve applied. (I got caught).
A great little device I ended up with 20 for just a few dollars on ebay. good luck olewill
http://www.alldatasheet.com/datasheet-pdf/pdf/57216/CALOGIC/TL431.html
essentially it has internally all the circuitry described above.
You connnect a vartiable resistor to the control pin centre to control top to +ve supply bottom to negative. Connect a small current realy coil between the cathode and +ve supply. It will operate when voltage exceeds the preset point.
Just one trap. This device emulates a zener diode which is always connected +ve to cathode because they "break down" (start conducting) with reverse polarity at the zener voltage. Hence the description of cathode on the circuit diagram which actually has +ve applied. (I got caught).
A great little device I ended up with 20 for just a few dollars on ebay. good luck olewill
paulrae
Well-Known Member
Fridge Mate
You could do the job cheaply - it is PBO, after all. I took the quick, easy and probably expensive option and bought a Fridge Mate:
http://www.mailspeedmarine.com/fridge-mate/refrigeration.bhtml
You'll be able to pick it up cheaper elsewhere, I'm sure.
It brings the fridge on when the engine is running and has a convenient pushbutton on top to over-ride the voltage-sensitve switch for an hour or two.
I find it a reasonable price to pay for the convenience, no doubt others won't.
You pays your money .........
You could do the job cheaply - it is PBO, after all. I took the quick, easy and probably expensive option and bought a Fridge Mate:
http://www.mailspeedmarine.com/fridge-mate/refrigeration.bhtml
You'll be able to pick it up cheaper elsewhere, I'm sure.
It brings the fridge on when the engine is running and has a convenient pushbutton on top to over-ride the voltage-sensitve switch for an hour or two.
I find it a reasonable price to pay for the convenience, no doubt others won't.
You pays your money .........
nigelmercier
RIP
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Why not just bypass the thermostat with a relay when the ignition is on?
sailvayu
New Member
How about a oil pressure relay so that the frig runs when the engine is running. This way the compressor will not be kicking in when cranking the engine. Some nice ideas in this thread never thought about doing this but I am going to now.
Capt. Wayne
Capt. Wayne
2copplane
Well-Known Member
Vsr look quite pricey. I recall wiring aN automotive relay (circa £3.00) to my ignition warning light connection on the back of the alternator. When the engine ran this goes to 12v and can trigger a relay. Worked for several years as a cheap way of split charging, but should be okay for this requirement and you don't need to mess With the Beta control panel.
Alternatively reroute the engine exhaust through the fridge therefore when the engine runs it would heat the thermocouple and fool the thermostat into running continuously, could double as an anti siphon as well. Assumes engine outputs enough hot water to beat the fridge cooling.
Alternatively reroute the engine exhaust through the fridge therefore when the engine runs it would heat the thermocouple and fool the thermostat into running continuously, could double as an anti siphon as well. Assumes engine outputs enough hot water to beat the fridge cooling.
nimbusgb
Well-Known Member
Simple but a comparator also works if you have shore power or solar charge available.
For the ultimate you need an intelligent thermostat otherwise your fridge becomes a freezer.
A dixel thermostat with digital display and a comparator added will regulate to two setpoints and cost the same as a fridgemate.