Gold and silver resistors

[DerekS]

I see threads recently about problems with the fan resistors . ....overheating to the extent that they fail and sometimes with disastrous consequences .
The resistor is there to reduce the voltage supplied to the fan motor, and it has to be able to carry quite a high current . The heat generated by this resistance in a small component must be dissipated or the resistor will fail due to overheating .
I am wondering .... why not use two resistors in parallel, to reduce the individual current in each component , thus halving the heat each resistor has to dissipate ? The total heat will be the same , but spread over two instead of one , so reducing the temperature the resistor has to stand . Smaller current , higher value resistors should be cheaper than the
single heavy duty one required .
With so many high class, knowledgeable members , i wonder if this has been considered . I would be interested in their ideas .

[T-Cut]

I've certainly considered doing that myself, but haven't gone any further. For a double unit, you'd need two 1ohm resistors, preferably rated 50watts minimum. Whether that would prevent resistor 'exploding' through water penetration or whatever the cause, is debatable. The main problem is fitting two resistors onto the fan guard. You'd have to organise a second mounting point which might be easier said than done. Of course anything's possible with enough effort. Realistically, the chance of anyone's fan exploding is pretty small, around (0.3%) according to Jules who's supplied more than anyone. My view is try to minimise the damage IF it happens.

So far I've simply changed the 80amp fuse to a 40amp to reduce the possibility of frying the harness if the motor is seized or something stops the fan from turning.

TC

[oswestryalex]

YOu need to be careful with the 'effective resistance' of what you end up with, of course - so two in parallel will halve the value, thus that needs to be taken into account. Perhaps a set of 4 two in series, paralleled (if that makes sense?) might do it - but the mounting would be the main bugbear.

[MarkS]

I am going to go for 2x 100W resistors in parallel, and a 40 amp fuse :

http://www.the75andztclub.co.uk/foru...2&postcount=34

(I don't fancy doing a Zafira!)
(Or should that be a Zafire ?)

[T-Cut]

Originally Posted by MarkS

I am going to go for 2x 100W resistors in parallel, and a 40 amp fuse

According to the theory as to why they 'explode', the twins in parallel approach is no less likely to blow than the standard system. They'll both get above 100C internally. The 40amp fuse is a wise move IMO. Let us know how you get along with the fitting. It'll be worth taking plenty of photos for a HowTo, so please do come back on this. They need to be 1ohm each of course.

TC

[DerekS]

Well thats stirred up a bit of interest . Thank you all for your comments .
Can anyone answer MarkS question (elsewhere ) about the specification of the normal silver and gold resistors? ( amps and max temperature ) .
I havent had the 'pleasure '..yet... of fitting a resistor to the fan housing , but would it be possible to use a small metal plate to mount it on, and then fasten it to the housing ? .
I am just trying to reduce the temperature of the resistor, by the plate, and halving the current through each parallel resistor .
And best of luck to the experimentors . Be careful and get the calculations right .

[Mike Trident]

if you could bolt the thing to a piece of sheet alloy to work as a heat sink it would help, but where could you put it?

Mine blew due to my fault. Like T-Cut says, some may blow from time to time, just like some tyres have punctures now and again. Like it has been said, the odds of one blowing is small. It's the possible damage to the wiring or even the whole car that is more of an issue. I would rather pop a 40 amp fuse now and again than watch my car go up in flames. At least the fuse is in a very easy place to get at, you don't need to remove the glovebox lol.

[MarkS]

Originally Posted by T-Cut

According to the theory as to why they 'explode', the twins in parallel approach is no less likely to blow than the standard system. They'll both get above 100C internally. The 40amp fuse is a wise move IMO. Let us know how you get along with the fitting. It'll be worth taking plenty of photos for a HowTo, so please do come back on this. They need to be 1ohm each of course.

TC

Just as a little aside, you recommend fitting a 40amp fuse. I notice the R75 fusebox only have 50, 60, 80 and 100amp fuses fitted. Where would/did you get the 40amp fuse from ?

[SD1too]

Originally Posted by MarkS

Just as a little aside, you recommend fitting a 40amp fuse.

I think some explanation and caution would be wise here.

KV6 and diesel engines are fitted with a 450 watt motor which, when running at full speed (according to the simple electrical equation) draws about 33 amps.

T-Cut, who said he had fitted a 40 amp fuse, has the 1.8 engine with, most likely, a 350 watt motor which draws about 30 amps at full speed.

In both cases, should the resistor become open circuit, there will be a considerable surge when the motor starts at full speed. In these circumstances, there isn't much margin for error in the case of the 450 watt motor; the fuse is at risk of blowing when a fault is not present and resulting in overheating of the KV6 engine.

Fuses provide only short circuit protection and they work extremely quickly, probably a matter of milliseconds. They are not designed to detect a relatively slow increase in current flow resulting from an under-rated resistor becoming too hot and partially disintegrating. When this happens, the resistor itself behaves like a thermal switch.

To put this in perspective, excluding the initial non-resistor design which is not susceptible, we have lived with failing resistors for fifteen years now. I do not remember a single case of a catastrophic fire being caused, or a burned out loom for that matter, but no doubt someone will now come forward with just such a story!

As you might have seen from a recent rather passionate debate, I am in favour of improving upon manufacturer's recommendations when justified. In this particular case though, I'd stick with the 80 amp fuse, just for "peace of mind" as they say.

Simon

[HarryM1BYT]

Originally Posted by MarkS

Just as a little aside, you recommend fitting a 40amp fuse. I notice the R75 fusebox only have 50, 60, 80 and 100amp fuses fitted. Where would/did you get the 40amp fuse from ?

Any motor factors should stock them, or Ebay...

http://www.ebay.co.uk/itm/MIDI-STRIP...9Z8OIsmYstDCmA