16 replies to this topic

[tadleysimon]

hi all,

i have a single speed plastic heater and id like to have a twin speed function like the mpi.
ive heard that the single speed heater are the same as the mpi type except the mpi ones have a resistor pack to make it twin speed.

so my question is.
how would i got about making a resistor pack to convert my single speed heater to twin speed heater?

if some body could measure the resistance across the slow speed on thier mpi heater for me that would be most helpful.

cheers everyone

[rasputin22]

Check out my reply in this topic. I have just done this. http://www.theminifo...x...15&start=15

[tadleysimon]

cheers mate, you say u used 2 1.8ohm resistors but its 3 speed. so id only need one to make it 2 speed right?

how have you wired the resistor in?

i was thinking of having 12 to the common of the switch,
one pole straight to the motor (full speed)
and the other pole through the resistor, then to the motor (half speed)

would that work?
does the large resistor put alot of strain on the cars wiring?

[mini13]

that sounds fine,

the resistors will not put extra strain on the wiring, they will actually reduce the strain.

[tadleysimon]

one more question. rasputin used a huge brass resistor but it was only rated at 1.8 omhs? this seems like a tiny resistance for such a large resistor.

what resistor should i use as all the 1.8 ohm ones i can find are the normal small type?

[rasputin22]

Check these out http://shop.ebay.co....o...r&_osacat=0

They are probably over kill, but ain't that expensive. The 2R2 or 2.2 Ohm resistors should be fine. A motor only has a low resistance anyway so therefore you only need to use a low resistance to make the motor speed different.

If you want i can draw you a little diagram on how to wire these.

[rasputin22]

Or be really flash http://cgi.ebay.co.u...=item27ae824596

[tadleysimon]

cheers mate, ill be fine wiring it up thanks, im an electritian so im fine with wiring but i never have to work with electronics so the little stuff, resistors n all so its a little alien to me

[mini13]

Dont use a small one!

the size is related to the power handling of the resistor

IIRC a heater motor consumes about 2 amps,

12v x 2 amps = 24 watts

24 watts / 2 squared = a heater motor restistance of 6 ohms., so adding about 2 ohs to that should give a significant drop in speed.

so assuming 6 ohms for the heater motor + 2 for the resistor = 8 ohms...


12v / 8 ohms = 1.5 v per ohm so there will be 3v across a 2 ohm resistor,

3v/ 2 ohm = 1.5 amps..

1.5amps squared x 2 ohms = 4.5 watts disipated in the resistor,

So I recomend a 5watt 2 ohm resistor. you can increase the resistance (say 2.2 ohm) without needing a higher wattage resistor, but if you go lower you may need to go larger.

if you want to check the sums....

http://www.the12volt.com/ohm/page2.asp

[tadleysimon]

Or be really flash http://cgi.ebay.co.u...=item27ae824596

that does look cool but id like to keep it looking like a mini heater. two speed is enough for me lol

[tadleysimon]

So I recomend a 5watt 2 ohm resistor. you can increase the resistance (say 2.2 ohm) without needing a higher wattage resistor, but if you go lower you may need to go larger.

so if i get this one http://cgi.ebay.co.u...=item20aef522a9

is it ok if the wattage is 50watts rather that the 5w that you recommended?

[mini13]

yes that will definately be fine.

[glyn]

with regards to wiring a resistor could someone please put a wiring diagram on here or send me a Pm thanks.


and would this work ok just connect the fan up to it and away i go?

http://cgi.ebay.co.u...=item27ae824596

thanks people

[glyn]

would this do the job?

http://cpc.farnell.c...=cpc/260055.xml

[tadleysimon]

with regards to wiring a resistor could someone please put a wiring diagram on here or send me a Pm thanks.


and would this work ok just connect the fan up to it and away i go?

http://cgi.ebay.co.u...=item27ae824596

thanks people

all you would need to do for this is it put the feed (after the switch) in one connector. and the connections to the motor in the other