A question for the electrical wizz kids on here (Car but not Stag related)

[Stagdad]

Just replaced the resistor pack on the blower motor of my Micra to fix the lower speeds that wouldnt work .I thought I would take a look inside the old one as I have two micras and the other one also has a broken resistor pack,at £50 I dont want to buy another one but would like to mend the old one.It really is very simple inside with just a flat track of risistor metal with 3 points of take off.
Would I be able to use 3 resistors from maplins to make up my own guts for this unit? and if so how do i work out the values of the resistors?IMG_2851.jpg
Also it had a heat pad over this track so do the cylindrical single resistors need heat shealds?



Cheers Steve

[V Mad]

Hi Steve
Yes you should be able to replace the resistor pack with high power resistors. I did this on a Vauxhall radiator fan a few years ago.

You will need to know the resistances (ohms) and wattage of each section. Can you get an ohmmeter in there to measure each section? You probably need resistors about 10 watts each or more.

[Stagdad]

Thats where I fall to pieces Chris,not really sure what im doing when trying to measure resistance although i have digital and machanical meters ( all the gear and no idea ).I can say for sur that the motor is protected with a 15A fuse so can that be used with 12V to calculate the wattage using ohms law?

Cheers Steve

[V Mad]

There is more than one way to find out what resistors you need, but if you want to calculate the right values you need to make some measurements. The quickest way is to measure resistance. Set the meter to ohms, and touch the end of the probes together, you should get a reading of about zero ohms. With the probes not touching it will read overrange. Then put the probes across one of the resistor tracks and it should read the resistance in ohms, so that's one value you need. To find out wattage, that is amps x amps x ohms. Assuming the motor takes 5 amps, and the resistor is 0.5 ohm, the power is 5x5x0.5=12.5 watts. If you only have 5 watt resistors you can use more than one to share the power but the ohms will be different and will need to be calculated depending which way you connect them.

If you can get some measurements I will work something our for you.

[Firept]

Steve.
As per Chris's reply / comments above
you need Ohms law:-
I=V/R
R=V/I
V=IxR
W=VxI
W=(IxI)xR (I squared R)
where:-
I =amps
V=voltage
R=resistance in Ohms.
W=Watts power.
First find out what the power and resistance of the motor is.
Connect up your meter in series with the motor set the meter to current 10 or 20 Amps range and then connect the motor free lead to the battery "-"or 12 Volt supply and the free lead of the meter to the "+" other pole of the battery. I am assuminmg that you have a digital meter that can read up to 20 Amps, if its analogue then make sure the leads are round the right way or it will read backwards, warning be prepared for the sudden torque of the motor these can really kick once the motor has settled to full speed read off the current probably 3 to 5 amps using the ohms rules above R=V/I assuming the supply to be 12 Volt give or take and the current to be 5 amps then 12/5= 2.4 Ohms, Power used is W=VxI 12x5= 60watts therefore from that result and keeping this all simples if you want proximately 1/2 speed then the motor will require 6 Volts across it (this very simples and takes no account of losses or load of the fan enclosure etc but near enough for Jazz), if you put a resistor of 2.4 Ohms in series with the motor then you will get 6 Volts across it. The power of the resistor will need to be 60 Watts (IsquaredxR ) thats a BIG resistor lots of heat hence why the original pack has burnt out no doubt !!
I would look for some NiChrome wire (old wire wound heater elements, toasters are a good source) or Maplin do sell "resistance wire" that is NiChrome and you can make your own you will need to crimp the connections on to the wire as normal solder will not take nor will it take the heat, silver solder is OK but unless you've got the kit stick with metal crimps(cut the red insulation off of a crimp butt splice).
Problem is now what do you do with this small radiant fire you've made? Most manufacturers fit them in the airflow of the fan or close to a good heat sink source large lump of aluminium or similar.
There is another way you can get some nice big power rectifier diodes and put a string of these in series with the motor each diode looses about 0.7 of a volt, not much you say but put 6 in series and you have just lost 4.2 volts they will need to be large such as http://www.rapidonline.com/Electroni...-Diode-47-5596.

Now if you want to get really smart you do it electronically and use a pulsed width modulated (PWM)supply and that will do the same job with minimal loss i.e less heat. have a look at (http://www.quasarelectronics.co.uk/c...1.5a-3a-6a.htm)
the only problem is that they are about the £35.00.

So it may well be one of those ..Oh well just pay up and replace at least it may last another 5 years. It gets very frustrating when you can see where the crap design is and its just not viable to rebuild it yourself some times fun trying though.
Dont burn your fingers!!

All the best
Paul.

[Stagdad]

Thanks Paul and Chris,well explaned ,i understand most of what you have said.I think on reflection with the heat issue and the fact that it has "do no repair" stamped on the unit and the fact that it is my daughters car I may for once not try and DIY this .Also the insurance could be at risk and if I create a fire under the dash I would never forgive myself .



Cheers Steve

[KOY 23]

Good call.