I need to drop the voltage from 12 volt to around 9volt, Would a points style ballist resistor work? Anyone know how much of a voltage drop this type of resistor would be good for?

I need to drop the voltage from 12 volt to around 9volt, Would a points style ballist resistor work? Anyone know how much of a voltage drop this type of resistor would be good for?

A ballast resistor will drop the voltage, but the drop will be dependent on the current draw. If you use a 1.5 ohm ballast resistor, the drop across it will be 3 volts when you are drawing 2 amps into your load.

If you want to keep the drop fairly constant at 3 volts, use 4 silicon diodes in series. Be sure to select diodes that will handle the maximum current you draw.

What are you going to use the 9 volts for?

A voltage regulator is the best way to limit voltage. As onovakind said, the resistor approach is not straight forward. Also you trade the power loss for heat. A voltage regulator is much more efficient and precicely calibrated.

What are going to use the 9 volts for? :D

I converted my 69 RS headlight door to a headlight motor out of a 98 bird, They will work with a control module, but way to fast of a open & close rate. I use a reosta to dail the power back to 9v and it seemed to work fine. I'd rather have a resistor in line, than a reostat that could fail or get wet ect. I'm uesing two relays to power up and swap polarity from open to close rather simple. I need to place a resistor in the input power to limit the total current of around 9 volts to both legs open & shut. From what you posted, I'll try one ballist off a points set up to get a volt reading, if it works than great if not then I'll know what direction I need to go with more or less resistionce.

I'm not to cuncerned on load due to the limited use of the motors used only for a second to open or close. I've tried about 10 differant volt ranges and the 9volt seemed to have the right speed to mock the org vacume open rate. You could use the factory control module, But I found it a tad to quick, Hope know ones has fingers near the door when it is shuts at about 100mph. With the choice of 9volts I also won't have to deal with broken bent or lost parts. As for how I wired mine up simple two relaysmain power from batt, to fuse make a Y feed one to 85, 86 to switched gound, main ground to 87, 30 to motors on both.
2nd realy same as about except a swap on to poles 85 goes to switched ground 86 is now input hot. I used a ground switch in place of a hot switch cuz I'm lazy. With this realy set up You can go hot to open and the 2nd relay will be off and connected to dead ground, you can close and your open hot falls dead to ground and your ground goes to open hot, quick easy rev of poles. You can get a vett headlight switch and it had switched neg poles on them to complete the install I'm right at $125.00 out of a junk yard. If you need a few pics let me know

I use a reosta to dail the power back to 9v and it seemed to work fine.

How many ohms did it take to get the operating voltage down to 9v? Just find a fixed resistor of the right power rating to match your rheostat setting. To calculate the minimum power rating, measure the operating voltage across your rheostat, square it and divide by the measured resistance. P = E²/R

One hitch with motors and resistors to controll their speed is that a motor's current draw is not constant. It takes a fair amount of current to get them moving and once they are moving the current falls off drastically. Theoretically a motor will draw zero current once in motion, but friction demands a slight amount of current.

What that means is that a resistor may annoy the hell out of you long term. If you get it tweaked to run well today, once the bearings wear or the grease thickens up from the cold etc. they may not work.

If 9 volts seems to be the sweet spot, consider a 3-terminal voltage regulator. The LM7809 is a 9 volt regulator. It has a mounting tab which needs to be connected to something metal to dissipate heat. It takes 12 volts in, ground, and provides 9 volts out. Check out http://www.digikey.com for example. Radioshack may also have them. The end result is that you limit motor speed with voltage control, but the current demand can still be met to get things moving.

I wonder how many million heater motors work just fine with a resistor in series to control the speed? A resistor would limit the inrush current and provide a semi-soft start, which saves contact and motor life.
Let's say the motor draws 2 amps @ 9 volts and we've settled on a 1.5 ohm ballast resistor. The low initial resistance of the motor will raise the current substantially, (theoretically up to 8 amps) until the counter emf builds up, then it will draw the running current. The motor is never going to draw anywhere near zero current while it's attached to a load.
I don't think a 1.5 amp 78xx regulator is going to cut it. If the motor draws more than the rated amps the requlator will go into internal current limiting mode and your output voltage drops way off.

food for thought. if you have now a electronic dizzy. did you cut your resistor wire out going to your old points coil. that wire is to drop voltage down to around 8 or 9 volts. :D

I like your double relay setup. I designed a similar relay setup once when installing power windows and doorlocks in a 79 pickup. Then I got looking at the harness(out of an early 80 something chevy pu) and all taped up was a relay(1 relay) that did it all. I threw my mess to the side and put GM's deal in there:).

Get yourself a power door lock switch and relay.:thumbsup:

I already cut my resistor wire from the bulkhead connector upgrading to MSD 6al wish I still had that little bit of wire, anyone have any?. I used a ballist resistor today worked great for one motor but when I loaded both motors nothing, I think I have a 10 ohm resistor I'm going back to get a 5 ohm which should put me back up to 10 ohm at load on both motors. I wish I could find a weather proof reostat which would allow me to set it & forget it, but if I had a problem I could advance the reostat to take up and slack. Lets see if I have this correct. If I had a 10ohm resistor and one motor worked perfect then I should drop to a 5 ohm to load two motors to 9volts I'm thinking that one ballist is dropping volts to 9volts and with 2 motors in line I get more like a drop to around 6 or even less volts across the circuit. I guess it's a process of elimnations to get the sweet spot. I need to post a PIC of the relay boards I have made up I've relayed everything from main power to headlights, A/C, fuel system ect. All in all I'm at 7 realys as of now, cool part is they are all hidden but in servicable reach. I'll post if the 5 ohm balsist works if not back to the diagrams and more scratching my head lol Thanks for all the great input, look forward to the problem being solved.

I already cut my resistor wire from the bulkhead connector upgrading to MSD 6al wish I still had that little bit of wire, anyone have any?. I used a ballist resistor today worked great for one motor but when I loaded both motors nothing, I think I have a 10 ohm resistor I'm going back to get a 5 ohm which should put me back up to 10 ohm at load on both motors. I wish I could find a weather proof reostat which would allow me to set it & forget it, but if I had a problem I could advance the reostat to take up and slack. Lets see if I have this correct. If I had a 10ohm resistor and one motor worked perfect then I should drop to a 5 ohm to load two motors to 9volts I'm thinking that one ballist is dropping volts to 9volts and with 2 motors in line I get more like a drop to around 6 or even less volts across the circuit. I guess it's a process of elimnations to get the sweet spot. I need to post a PIC of the relay boards I have made up I've relayed everything from main power to headlights, A/C, fuel system ect. All in all I'm at 7 realys as of now, cool part is they are all hidden but in servicable reach. I'll post if the 5 ohm balsist works if not back to the diagrams and more scratching my head lol Thanks for all the great input, look forward to the problem being solved.


Why not just put one resistor in series with each motor?

Because I'm running both motors on the same circuit, so they both move at the same time. Both motors are tied together with a harness then I jump the relays to a T in both lines, the resistor or resistors will go in the power feed into the relays to drop the power before it enters the relays so in other words my relays are just switching points, that is under control. I thought about adding a resistor in line before each motor but the power has to pass the first motor to get to the sencond unit, I'm thinking that a in line resistor will not work right, But I've got enough parts to try that path, I'll keep ya posted.

I wonder how many million heater motors work just fine with a resistor in series to control the speed? A resistor would limit the inrush current and provide a semi-soft start, which saves contact and motor life.
Let's say the motor draws 2 amps @ 9 volts and we've settled on a 1.5 ohm ballast resistor. The low initial resistance of the motor will raise the current substantially, (theoretically up to 8 amps) until the counter emf builds up, then it will draw the running current. The motor is never going to draw anywhere near zero current while it's attached to a load.
I don't think a 1.5 amp 78xx regulator is going to cut it. If the motor draws more than the rated amps the requlator will go into internal current limiting mode and your output voltage drops way off.


You raise a good point about the limited current capability of a 78xx regulator. However there are ways to work around that and perhaps a better option is to choose a regulator with a higher working current.

Although blowers and headlamp doors are both operated by a motor, I'm not sure your comparison is apples-to-apples. The load on a blower motor is fairly low when it's standing still. Only enough power to overcome inertia of the blower is required until the motor spins fast enough to develop a wind load. Headlamp doors are subject to gears and levers doused in grease and will have a very different start up characteristic than a blower will.

Pay Dirt I got two 1 Ohm 10 watt 10% tolerance wirewound resistors. I ran them back to back and got a steady 9volt @ 3amp load start to stop. I'm useing them before the relays, and when you hit the limit of the swing they shut power to nothing, kind of like a built in load cut off. All of .99 for the pair at radio shack I'd call that pay dirt. I'm going to try to post a few shots as well as a clip of the open rate & close rate. If I add everything up less the late nights pulling hair, I'm right at $140.00 done finished and perfect, and the vette switch is great no hidden switches or spliceing.

The load on a blower motor is fairly low when it's standing still.

When any electric motor is standing still, the current is at maximum. It's called locked rotor current.

When any electric motor is standing still, the current is at maximum. It's called locked rotor current.

True, but a fan that isn't moving air isn't loading the motor.

Your turn.