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Dave Puttock
Experienced User
Username: ariel

Post Number: 40
Registered: 5-2010
Posted on Saturday, 31 May, 2014 - 11:22 pm:   Edit PostDelete PostView Post/Check IP

My Shadow 1 heater fans use three heavy duty wire resistors on round ceramic discs to control the voltage on the motors. When dismantling I did not realise that they were of different values.
By measurement they appear to be 0.7ohm, 1.0ohm and 1.7ohm. Obviously at slowest speed all three are in series and at higher speeds 2,1 or no resistors are in use.
Problem, what order should they be in for correct speed stepping?
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John Kilkenny
Prolific User
Username: john_kilkenny

Post Number: 167
Registered: 6-2005
Posted on Sunday, 01 June, 2014 - 12:55 pm:   Edit PostDelete PostView Post/Check IP

Early Shadow 1's used two resistors A and B and the switch selected A or B or A+B to reduce the motor speed.
Later a simplified circuit used three resistors progressively switched in series. This would also allow lower wattage resistors to be used though I don't know if this was a reason.
To drop the motor speed the series resistors should be added in value order, that is the lowest value first.
So the 0.7 ohm resistor will provide 20% of the total voltage drop, 0.7 + 1.0 will provide 50% and 0.7 +1.0 +1.7 will give 100%.
This type of speed reduction is very inefficient and Pulse Width Modulation is now generally used to control DC motor speed (as amusingly referred to in another thread)

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