istherm voltage reducer

J

JOURNEYMAN

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hi,

boat has a isotherm fridge with a voltage reducer from 12 to 5.9v to the water pump. this is caput and I am looking for a replacement. best I can find is Euro 238 !!!!(I am in Greece). have looked on e bay and there is an array from £ 20 upwards. problem is I don't know which types will or won't work on our fridge system ... any advice welcome

thanks J
 
I've quite frequently had to replace the Danfoss unit which converts 12v dc to 220v ac for the compressor unit, fortunately mine is a standard air-cooled Isotherm unit. This has always occurred as the result of flooding.

There is a tabbed take-off on that for the fan, but I'm pretty sure that's a 12v output. I'd have thought that any solid state voltage unit that gives a 12.5 - 13.5 input to a 5.9 volt output would do the job for you. You'll obviously have to use commonsense to wire it up yourself.
 
Mine shown below. I agree with Charles, anything that gives six volts at sufficient power should be OK.
April020.jpg
 
vyv_cox said:
I am totally ignorant about these things. I assumed that 'forward voltage VF max 1.5 V' meant that it would only deliver 1.5 volts. How does putting a few in series deliver 6 volts?
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It means that each diode drops up to 1.5V when passing high currents. Putting one in series with the supply will drop up to 1.5V (in practice about half that), so the more you add, the less the resulting voltage will be. Not stabilised, but for this application it doesn't matter, just add enough so when running the pump sees about 6V.
 
vyv_cox said:
Thanks for the explanation. Seems to us that the term 'forward voltage' is highly misleading in this instance.
Click to expand...

A diode has a forward voltage drop - the difference between the out volts and the in volts when current is flowing in the direction it is allowed to, i.e. forwards. Opposite this is the reverse voltage breakdown, i.e. how many volts it needs on the downstream side for the diode to start conducting in the wrong direction.

Think of a non-return valve for fluid - it will have a forward pressure drop - how much the pressure drops across the device when flowing in the intended direction, and a reverse pressure capability - what pressure it can withstand from the normally downstream side before it breaks and allows flow the wrong way.

What Nigel is suggesting would be like putting a series of non-return valves in a fluid line to drop the pressure at the end of the series from the beginning - a small drop across each valve. Bit of a kludge, but cheap.
 
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nigelmercier said:
Oh absolutely :)
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Calculated at a nominal 12v say 4 1.5v vFwd diodes gives you 6v out.

But running off batteries with the supply down to say 11.0 v your pump will be getting 5v. With the engine running and the alternator pushing out 13.8v your pump is getting nigh on 8v. If you have a 3 or 4 stage battery charger you could put nearly 9v into it. Now its a dc motor so it will 'probably' survive.

Yup, cheap and cheerful solution ........... Replacing the pump wont be though!
 
Yes it is odd that the pump runs on 6volts. The voltage can be reduced by a resistor like about 1.5 ohm for 4 amp drain but dissipating 25 watts. A big resistor, which will waste as much power as the pump uses and the resultant voltage at the pump will vary with supply voltage. A variation is the linear regulator which varies its resistance to produce an accurate 6v output regardless of current drain or supply voltage. This also will get hot disipating as much power as the pump.
The best regulator is a switch mode type which in effect transforms the 12v supply to 6v. Although there are losses it will essentially draw 4 amps from 12v supply to provide 8amps at 6v. Acually about 5amps at 12v due to inefficiencies. The link given by Angus seems like this kind of regulator and idealfor the job. However as said much depends on the current demand of the pump especially at start up. The regulator should have a current overload so shut down but this will then stop the start up unless the regulator can supply the start up current. In practice you would have to buy the regulator and see if it works. Or try to monitor current drain on start up when running it fropm a 6v battery.
In any case a small resistor in series with the pump may limit the start up current. Typically .1 ohm resistor will waste .4 volt at 4 amp running current. But will limit the start current to perhaps around 10 amps by dropping the voltage by a full volt. (just a guess). good luck olewill
 
The pump in my system, shown above, is a 12 volt one. It runs on 6 volts because the system designer believed the fridge would be overcooled by using it with 12 volts. The flow from the PAR Max1 shown, and the far bigger Shurflo that preceded it, is perfectly adequate. The pump will last pretty much indefinitely used this way. The 12/6 volt converter has a button by which 12 volts can be given for priming purposes, on which the pump is intrusively noisy and sounds as if it is working hard.
 
thanks all. Mine is like yours vyv. I am running it at 12v but it wont work wired to the thermostat so I am turning it on and off manually. Mine has 2 compressors (big freezer) so I think the issue is the three things cant get enough start up power with the pump at 12v. I will get the e bay converter recommended. I just don't understand why the isotherm unit is > £200 when you can get something that does the job for $20??? I know marine proprietry bits are expensive but 10*???

anyway, I'll order the bit and see how we go - thanks again all
 
The other Option I have used in the past is use a 24v pump. The reason they run the pump at 6V is to keep it quiet and it works very well. the button on the isotherm unit bypasses the 6v section and provides 12V for priming the pump however I have found this is not necessary if the pump is installed below the waterline or at least not far above it. Running a 24V pump from the original danfoss box cuts out some more parts to go wrong and sometimes a used 24V pump can be found cheaper.
 
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