Help with a wire resistor

[church's pew]

The fan for the heater blower in my car has never worked on any setting other than the highest. The highest fan setting is operated via the batteries full 12 volts, whereas lower settings bypass the direct battery connection and pass through a resistor - the purpose of which to dissipate some voltage to reduce the speed of the fan..

Becoming increasingly annoyed with having to demist the windscreen on full setting, I've removed the resistor unit and carefully chipped the cement material that was covering the wire coils to discover the source of the problem was a corroded wire that has separated over time.

I intend to either rejoin the broken coil or make a new one with a piece of Nichrome wire; unfortunately I won't be able to replace the cement. Firstly what is the purpose of the cement, is it simply an insulator or a heat sink? Can anyone foresee any problems with the idea..?

Edited by church's pew on Wednesday 5th February 03:24

Edited by church's pew on Wednesday 5th February 03:26

[dudleybloke]

i cant help you but i admire your enthusiasm.

would it not be a lot easyer to replace the unit though?

[jet_noise]

Dear cp,

I'm with Dudley.

The ceramic is there primarily as a holder. Those bits of wire won't last long if not supported. Vibration. Hardening. Wire breaks.

If you must DIY then I'd attempt to measure the wire's resistance and fit some individual resitors of appropriate value & power rating,

regards,
Jet

[annodomini2]

Unless you have to do it as a project buy a new unit.

[anonymous-user]

As an aside, Ebay would provide you with a number of cheap, relatively high current PWM modules suitable for variable speed control of this 12v DC fan.

for example:

15A pwm controller

[Pete102]

Ohms Law, V=IR, as you alter R i.e. R becomes bigger you increase the voltage (and the fan speeds up). Reduce R the multiplication factor becomes smaller resulting in a smaller voltage and the fan slows down.

If your really that bothered about making this a home project, measure the resistances of the other coils and select a value based on those. It might take some trial and error associated with wire diameter and number of coils but you'll get there.

[hairykrishna]

Why on earth have they used coils of resistance wire for that application? Anyone know?

A normal resistor (probably a chunky power rating with heat sink) or a cheapo PWM would make a more sane replacement. If you're going to go with wire, you could always embed it in potting compound?

[CubanPete]

hairykrishna said:

Why on earth have they used coils of resistance wire for that application? Anyone know?

A normal resistor (probably a chunky power rating with heat sink) or a cheapo PWM would make a more sane replacement. If you're going to go with wire, you could always embed it in potting compound?

Probably to get the power rating up...

I wouldn't use potting compound as it won't be very heat resistant

[church's pew]

I've since re-joined and soldered the wire and it's working as intended; so it was a rather cheap fix (free). See how it goes.

[jet_noise]

Dear hk,

hairykrishna said:

Why on earth have they used coils of resistance wire for that application? Anyone know?

A normal resistor (probably a chunky power rating with heat sink) or a cheapo PWM would make a more sane replacement. If you're going to go with wire, you could always embed it in potting compound?

It'll be cost. At the volume, power and in the automotive electrical & envirnmental, er, environment of those parts it would have been the cheapest way. Power wirewound resistors are still cheap & robust compared to other technologies,

Oh, and well done cp

regards,
Jet