Gold and silver resistors

[Avulon]

I've had one gold resistor 'explode' - the radiator and fan survived .

The reason for the explosion is simple - water ingress - when super heated inside the resistor body turns to pressurised steam causing it to blow the core out of the body.

The current resistor on mine has the ends sealed with high temperature silicone sealant to prevent water ingress: and so far, over about a year and 15k miles, has been fine. A 100W resistor is well up to the job. The standard 50W silver resistor isn't, the designers (or penny pinchers) de-rated the component on the basis of it getting airflow to aid cooling - they did their sums wrong: resulting in a shorter life for the part. If you are going to go to the trouble of multiple resistors in a network and relocate them then you may as well look at designing a PWM circuit... The gold 100W resistor is fine when correctly fitted and sealed.

[T-Cut]

Originally Posted by Avulon

The standard 50W silver resistor isn't, the designers (or penny pinchers) de-rated the component on the basis of it getting airflow to aid cooling

The 50w resistor was fitted in the early 2-speed system and was later uprated to a 75w. I believe it was also designed to fail rapidly (fast blow) if overstressed. There was an internal thermal fuse or something of that nature integrated with the resistor coil. Many other cars with resistor controlled fans have thermal fuses to minimise fire risk.

If you read the manufacturers' datasheets for the 100w resistors (see below), you'll see they specify mounting it on a heat sink of considerable area. The normal maximum operating temperature is quoted as below125°C. The lack of space will preclude DIY attempts at this, but by installing the resistor in the air flow it's assumed to be good enough. In most cases it is, but with no thermal fuse (as in the OEM version), the wiring is very vulnerable.

Farnell Datasheet (see HSC100 range): http://www.farnell.com/datasheets/1498243.pdf

TC

[SD1too]

Originally Posted by T-Cut

T... and was later uprated to a 75w. I believe it was also designed to fail rapidly (fast blow) if overstressed.

Where did this information come from please T-Cut?

Simon

[T-Cut]

Originally Posted by SD1too

Where did this information come from please T-Cut?

Jules. He cut one open and explained what he found. It'll be archived somewhere

TC

[HarryM1BYT]

Originally Posted by 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 .

The heat generated by the resistor is due to wattage dissipated by the resistor. All of which is waste heat. All it needs is a resistor which is designed to cope with that wattage, the original was not. Resistors can be bought in a wide range of sizes, so no reason to double up to produce a higher dissipation version. From memory I fitted a 0.47 Ohm 100w(?) gold resistor.

The 'gold' has no particular meaning, other than all of these high dissipation resistors happen to be gold coloured.

A much better way to do it, would be by the means of PWM (Pulse Width Modulated) system. Basically the 12v to the fan motor would be rapidly switched on and off, such that the on, versus off period would be 50/50 for half power.

A PWM system would generate very little waste heat, so its cooling requirements would be minimal. The cars climate system uses PWM to control the climate interior fan speed. PWM systems are quite simple to design, especially a fixed speed system.

[ceedy]

I didn't use the usual plastic ties , but made a alum plate and screwed it all on , so the Resistor has a bit of a extra heatsink.. done years ago.. never had any probs... also used an RS resistor

C.

[Avulon]

Originally Posted by T-Cut

The 50w resistor was fitted in the early 2-speed system and was later uprated to a 75w. I believe it was also designed to fail rapidly (fast blow) if overstressed. There was an internal thermal fuse or something of that nature integrated with the resistor coil. Many other cars with resistor controlled fans have thermal fuses to minimise fire risk.

If you read the manufacturers' datasheets for the 100w resistors (see below), you'll see they specify mounting it on a heat sink of considerable area. The normal maximum operating temperature is quoted as below125°C. The lack of space will preclude DIY attempts at this, but by installing the resistor in the air flow it's assumed to be good enough. In most cases it is, but with no thermal fuse (as in the OEM version), the wiring is very vulnerable.

Farnell Datasheet (see HSC100 range): http://www.farnell.com/datasheets/1498243.pdf

TC

Yes quite right - I should have said that even the 100W resistors being used this way are an uprating for the component - the airflow replacing the larger heatsink that would otherwise be required to meet the 100W rating. With the 100W resistor you do have a second option of locating it elsewhere if it's firmly in contact with metal to act as that heatsink e.g. bolted to the slam panel would work and protect it from the elements somewhat.


I've never seen a resistor specified as fast blow or anything like it (i.e. a resistor in double role as a fuse as well). The OEM resistors fail because the cooling effect of the fan at standstill was overestimated by the designers - resulting in easy overheating and early failure.

[SD1too]

Originally Posted by Avulon

I've never seen a resistor specified as fast blow or anything like it (i.e. a resistor in double role as a fuse as well).

No, neither have I.

Simon

[T-Cut]

Originally Posted by Avulon

I've never seen a resistor specified as fast blow or anything like it (i.e. a resistor in double role as a fuse as well).

Originally Posted by SD1too

No, neither have I.

I said I believed this was the case and was simply my expression/interpretation of what I remember from some work done years ago by Jules. He opened up a failed OEM resistor and noted something about the way the wire coil inside had failed. I recall it as an unexpected feature. I may be completely wrong since it's a while ago. The relevant thread will be archived on the forum, so will be searchable. I'll have a look when I have time. Or maybe Jules will see this and respond himself. It's no big deal.

TC

[MarkS]

Originally Posted by MarkS

Can I recommend that everybody (that has had a resistor failure) check that the main fan fuse FL4
1) Has a 80 amp fuse fitted.
2) Remove FL4 and check that somebody hasn't bypassed the fuse with a piece of wire ( as has been known to happen to alternator fuses et al).
After all, we could be being led up a blind alley by idiots bypassing fuses!!!

Nudge nudge !