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Dc drying motor running too fast
Posted by: roger wilson (---)
Date: January 19, 2021 03:53PM

With respect to the post made by Joe, about using a DC gear motor and a 3-24 volt power supply to power the motor.

Joe made the comment that, even with the power supply turned down all the way, the dryer was still turning too fast.

I was doing some motor work today, using the same 3-24 volt power supply and wished for the resting rpm to be 0.

Basically, I made an addition such that I drop 3 volts across a series of power diodes to then transform the power supply to a 0-20 volt supply.

Then, when the voltage knob is turned down, the voltage drops to 0 and the motor stops. Then, raise the voltage to bring the rotating rpm of the motor to any desired speed.

Following items:
An example of a 1-24 volt adjustable power supply:

Here is the same supply - except that this one is rated for 3-12 volts.


Now, if one wishes to reduce the starting voltage of the supply to 0 volts, and also drop the top voltage by the same amount, insert a string of small power diodes hooked in series to drop the voltage as much as you need.
The typical silicon power diode will have a voltage drop of .6 to .7 volts. Simply continue to add diodes to the string to get the low voltage to as close to zero as you wish.
In my case - today - I used 4-power diodes to drop the voltage to essentially 0 volts.

However, don't use Schottky diodes because there is only a 0.3 volt drop across this type of diodes. You can use them and they will work well, but to get the same voltage drop you would have to double the number of diodes for the same voltage drop as a standard silicon power diode.

Example of a power diode. Normally, the motors that we mention draw less than 1 amp of current. So one could use either 1,2 or 4 amp diodes and they would all work fine.


This is a lot of 125 pieces so you will have about 120 left over. Trouble is, for many folks there are no Radio Shack or other similar shops close to home to pick up a half dozen diodes.

When you solder the diodes together, you sill connect a barred end to a non barred end of the diode. Typically folks will use 4 diodes in series.
If you want the diodes to conduct and pass current, you will put the + voltage power lead on the non barred end of the diode, and then connect the barred end of the 4 diodes connected in series to the + terminal of the motor.

Note: a nice video:

When working with diodes, as shown in the previous video, the end of the diode that has the bar on the end is called the cathode, and the non barred end of the diode is called the anode.

If you want the motor to run in the opposite direction, just put the barred end of the diode that is connected to the + terminal of the power supply and connect the - terminal from the power supply to the other motor terminal or what had been the + terminal of the motor.

If you just put the 4 diodes in series with your voltage supply you will then have a dryer motor that will slow down to 0 rpm, and adjust upward to the maximum rpm that is available with the 4 volt lower dc voltage.

Take care

Edited 1 time(s). Last edit at 01/20/2021 11:41AM by roger wilson.

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Re: Dc drying motor running too fast
Posted by: Mark Talmo (---)
Date: January 19, 2021 09:20PM

Roger and all viewers,
You are undeniably the electronic Guru of this site and your gift of giving is greatly appreciated by all. The electronic circuitry you designed to my specifications for my wrapper upgrade performs flawlessly and I am happier than a pig in slop. As you, my Dad is a retired EE but I absorbed a very small percentage of his vast knowledge. That is exactly why I employed you to help me with my upgrade.
Most of us “commoners” regard electronics an elusive field, “out there” with rocket science and atom-splitting; couldn’t tell the difference between a diode and a dog bone. LOL. That is why we commoners rely on Gurus like you. If anyone reading this is ever in the need electronic wizardry (to us), rely on Roger to design the circuitry (rudimentary for him); you’ll be happier than a pig in slop!!!

Mark Talmo

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Re: Dc drying motor running too fast
Posted by: Joe Vanfossen (---.neo.res.rr.com)
Date: January 19, 2021 09:30PM

Roger, that is some good stuff, and a means of controlling voltage that never even crossed my mind. I'm pretty sure my electronic instrumentation professor back in the day would beat me over the head.

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