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  #1  
Old 01-25-2008, 09:31 PM
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Rpm's of blower; see resistor enclosed

I recently did a R&R of the W108. Unfortunately, I did not inspect or test the existing 0.75 ohm & 2.5 ohm resistors inside the heater box.

The blower fan is running at high Rpm's only. The blower motor is 1.5 ohms.

Using Ohms Law, 12v x 25A (fuse at #6) divided by 1.5 = 8A, or 100watts.

So, I will install this Ohmite resistor, in series, after the motor, for a total of 1.5 + 0.7 ohms. This results in 65 watts. I am hoping the result will be a blower motor that runs at 2/3 maximum rpm's, single speed only.

The advise I been given is that the Ohmite resistor will be as warm as a 65 watt light bulb.

Any Electrical guys out there?

http://ourworld.compuserve.com/homepages/Bill_Bowden/ohmslaw.htm

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Rpm's of blower; see resistor enclosed-ohmite_0.5_rated.png  
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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old
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  #2  
Old 01-26-2008, 02:06 AM
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< >>

Your resulting values are correct, but the 25A fuse has nothing to do with the calculations..that is just the value of the fuse and is not a draw value for Ohm Law in your circuit

The calculation is :
Amps= V/R
..and then use A to get:
P [watts] = V x A
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  #3  
Old 01-26-2008, 10:11 AM
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I think you will end up with the equivilent of a blower set to "2".
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  #4  
Old 01-26-2008, 06:20 PM
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Thanks; worried about temperature of resistor.

The Ohmite resistor is conductive, so I was going to clad the resistor in fiberglass.

The resistor is 0.7 ohms (measured).
I am not certain how warm it will get (it will be wired after the blower motor).

Any thoughts?
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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old
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  #5  
Old 01-29-2008, 10:24 PM
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Using another method

I didn't like the idea of a single speed blower, so I purchased this pulse-width-modulation DC motor controller.

http://www.electronic-light.com/advan.html

Price Each: $16.95 Kit
Assembled and Tested: $34.95

Specifications:

Supply/Motor Voltage: 6 - 15 VDC
Output Current: 0 - 20 amperes continuous
PWM switching frequency: 1250 Hz - 120,000 Hz (continuously adjustable)
Physical Dimensions:

Approximately 2.7" x 2.2" x 1.6" with heatsink, throttle potentimeter, and terminal block attached.
Kit includes:
(1) Printed circuit board
(1) 5 kΩ potentiometer
(1) 100 kΩ potentiometer
(2) Integrated circuits
(2) IC sockets
(3) Transistors
(1) Power MOSFET
(9) 1/4 W Resistors
(8) Capacitors
(1) Diode
(1) 4 Position Terminal Block Connector
(1) 1 Heatsink
(1) MOSFET Mounting Hardware (#M-3 machine screw, lock washer, shoulder washer, and TO-220 insulator).
(1) Complete circuit schematic and assembly/soldering instructions.
__________________
1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old
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  #6  
Old 01-30-2008, 10:29 AM
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that is going to work great! Now I am jealous, I want a infinitely variable blower fan. So are you going to rig that baby under the dash, or behind the dash?

Post photo's when you do the install.

Good luck

Last edited by Blue 72 250; 01-30-2008 at 03:55 PM.
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  #7  
Old 01-30-2008, 10:28 PM
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I will simply install.......

the unit in the 'old' A/C potentiometer location. (No A/C on this car) Even the 'old' A/C knob should work.

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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old
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