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JOURNEYMAN
24-08-13, 09:17
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

charles_reed
24-08-13, 11:55
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.

vyv_cox
24-08-13, 13:48
Mine shown below. I agree with Charles, anything that gives six volts at sufficient power should be OK.
http://i77.photobucket.com/albums/j62/vyvcox/Our%20boats/April020.jpg

nigelmercier
24-08-13, 14:14
A few of these in series would probably be OK:

http://cpc.farnell.com/on-semiconductor/mur1560g/diode-ultra-fast-15a-600v/dp/SC07384

It says they drop 1.5V, but I would start with about 6 - 8.

vyv_cox
24-08-13, 18:57
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?

charles_reed
25-08-13, 07:23
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?

Ah! the questing scepticism of the true engineer!!!

nigelmercier
25-08-13, 10:07
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?

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
25-08-13, 10:18
Thanks for the explanation. Seems to us that the term 'forward voltage' is highly misleading in this instance.

AngusMcDoon
25-08-13, 11:15
Thanks for the explanation. Seems to us that the term 'forward voltage' is highly misleading in this instance.

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.

nigelmercier
25-08-13, 11:52
... Bit of a kludge, but cheap.

Oh absolutely :)

nimbusgb
25-08-13, 19:23
Oh absolutely :)

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!

AngusMcDoon
25-08-13, 19:39
Yup, cheap and cheerful solution ........... Replacing the pump wont be though!

Warrabout this then?

http://www.ebay.com/itm/DC-DC-Converter-12V-24V-to-6V-100W-Power-Supply-DC-Voltage-Regulators-Stabilizer-/221270045087

nimbusgb
25-08-13, 19:53
Warrabout this then?

http://www.ebay.com/itm/DC-DC-Converter-12V-24V-to-6V-100W-Power-Supply-DC-Voltage-Regulators-Stabilizer-/221270045087

I've got a couple of their 12v stabilised supplies and i'm very happy with their performance.

AngusMcDoon
25-08-13, 19:56
I've got a couple of their 12v stabilised supplies and i'm very happy with their performance.

I don't know if that one could cope with the start-up inrush current requirement of the pump's motor or if it would have a paddy and just shut down.

Pete7
25-08-13, 20:08
Why not fit a 12v pump?

Pete

William_H
26-08-13, 03:53
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

nigelmercier
26-08-13, 07:07
Warrabout this then?

http://www.ebay.com/itm/DC-DC-Converter-12V-24V-to-6V-100W-Power-Supply-DC-Voltage-Regulators-Stabilizer-/221270045087

Much better idea. Didn't realise they could be so cheap.

The diodes would probably be fine, but would need some experimentation. A resistor has all the disadvantages, and some more of its own.

vyv_cox
26-08-13, 08:19
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.

JOURNEYMAN
27-08-13, 09:36
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

owen-cox
27-08-13, 12:22
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.

owen-cox
27-08-13, 12:27
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
When you say wired to the thermostat do you mean to one of the Danfoss compressor control boxes? as this is where it should be connected. if the pump is too powerful for the control box installing the LED indicator as per the manual will give you a fault code from the danfoss control box ( ir of you have the isotherm control with LEDS on it this will also indicate the fault) If the pump draws too much current you can install a 12v relay to start the pump directly from the battery supply and switch the relay from the fan output on the Danfoss compressor control box. it may be that the pump is tight and drawing too much current and that is why the voltage dropper failed?

vyv_cox
27-08-13, 17:27
When Owen built my unit for me it used a four port secondhand Shurflo pump, very large for the purpose. I decided to replace it with the Par Max1 shown above to save power, as the fridge runs continuously all summer. However, in the meantime I had installed a voltmeter/ammeter and found that the current draw of both pumps was very similar and very small, less than 0.5A. A domestic water system pump draws around 3A thanks to the pressure generated but these open-ended fridge systems are miserly by comparison.