Cheap way to lower motor speed

[DBCox]

Hello everyone,

I am trying to cut the speed of a 12 VDC motor by 3, or 4 if I can get it that low. The simple answer is, send it 3 vdc. But, the motor's max current is 20 amps, so I need to know a cheap and easy way to do this. Here is the setup:

Motor will be turned on using a microcontroller via a relay. It will run until a switch is triggered, and then cut the speed by 3-4 times (by lowering the voltage of course). Then, it will trigger another switch and stop. Finally, it will repeat the process in reverse to go back to its origional position. I need only 2 speeds, so a speed controller seems overkill and expensive. How can I drop the voltage down without spending over $50-60?

Thanks!

 

[Skogsgurra]

The standard way of doing it to use a transistor that is pulse-width modulated (PWM). Cheap, effient, proven in use. Any EE in your neighbourhood can help you doing ot.

Gunnar Englund

http://www.gke.org

 

[DBCox]

skogsgurra,

Thanks for the info. Can the transistor handle the 20 amp current?

 

[Skogsgurra]

It depends on what transistor you use. A power MOSFET can do that easily. But you have to use a heat sink. Bipolar transistors can also do the work. There are, in effekt, transistors that handle hundreds of amperes. But that would be overkill and quite expensive, in your application.

Gunnar Englund

http://www.gke.org

 

[DBCox]

Thanks again for the info. I found this MOSFET:

http://www.jameco.com/webapp/wcs/st...&catalogId=10001&productId=273279&pa=273279PS

Will 5V trigger it "on" though (PWM will be 5v from the microcontroller)? I am not familiar enough with electronics to be able to understand all of the technical terms in the documentation. Are there transistors that will switch on 12V but be triggered by 5V?

Also, how can I attach a heat sink to this thing? It seems pretty small...

Thanks!

 

[denison]

I think i would change the gears myself or ratio, so to speak. Just an idea.

 

[BobM3]

If you don't care about power consumption you could throw a large resistor in series with the motor. Simple but wasteful.

 

[Skogsgurra]

blackrapter,

Good hunting! You found a transistor that will do the work.

Yes, it will turn on at 5 V TTL levels. The gate threshold voltage is typically 1.6 V and guaranteed 2.5 V. The gate voltage can be up to 15 V without damage. So, you are safe there.

The transistor can switch 100 V - no probs either. You have 12 V, I think.

The heat dissipation will be current squared times channel resistance. Channel resistance is 0.09 - 0.12 ohms. Current squared is 20*20 = 400. So dissipation is 400*0.12 = 48 W maximum.

If you clamp the transistor to a good heat sink (there are heat sinks and springy clamps available) and add some thermal grease, then you will have a thermal resistance (junction-case and case-ambient) of about 2 K/W. So the temperature rise will be rather high; 48*2 = 96 K. At an ambient temperature of 40 C, you will have a junction temperature equal to 40 + 96 = 136 degrees celsius.

It is high, but the transistor can take 175 C. So it should be OK.

But I would invest in a somewhat larger piece with lower Rds(on). Like 0.05 ohms.

Gunnar Englund

http://www.gke.org

 

[dewone]

BobM3 has my vote. A resistor will give the desired results with a min of wiring. He already has the switching taking place. Just switch in the reduce voltage via the resistance and done. Wire wound varible will give enough speed control to adjust to his likeing.

 

[DBCox]

skogsgurra,

Thank you again for your help. Where did you find that information about the channel resistance? I cannot find it in the documentation for the transistor I posted or the other 2 Jameco has that are rated at 20 amps or higher. Another question about heat though, those leads seem pretty small. Isn't 20 amps a lot to send through them? Should I use 3-4 in parallel to lower current? Finally, are there any other components neccessary to hook the transistor up, such as resistors or capacitors? (I told you I am not good with this, but I have noticed many ICs require external components, and this one doesn't have a diagram).

BobM3, where might I find a resistor that large? The resistors I am used to seeing cannot handle 20 amps. Although, I am leaning toward the transistor if I can make it work, that way I can control speed if something turns out to be too fast.

Denison, I agree with you about changing gears. That was the initial indea, unfortunatly, space is too limited.

Thank all of you for all of your help, I really appreciate it!

 

[Skogsgurra]

The problem with the resistor approach is the the speed will be very load dependent. If you adjust the resistor to have the right speed in one direction and with one load, it will probably be too low or too high in the other direction or with another load. Also, at 20 A and dropping 8 V will dissipate 160 watts. And that is if you use a series resistor. If you want better performance and use a voltage divider, you will have even more power dissipation.

No, the resistor approach is not a good one. That's why it isn't used very often.

Gunnar Englund

http://www.gke.org

 

[Skogsgurra]

blackrapter,

We cross-posted. Didn't see your questions.

Detailed data about this transistor is available from the data sheet. After you have clicked the link in your posting, click "data sheet (pdf)" a bit down the page.

You will see a pdf document. Go to Features on pg 2. Read RDS(ON) 0.09 ohms typical.

Next page (3) gives more detailed information: Static Drain-Source On Resistance 0.09 typical 0.12 maximum.

Regarding wire dimensions. Yes, they are thin. But good for 20 A. The reason you object to this is that normal electric wires have an insulation that shall be kept at low temperature. No such problems here. But, as you see, the 20 A is the Absolute Maximum Rating and not something you should operate the transistors at continously. That's why I recommended you to find a larger one. And, yes, you can parallel MOSFETS. They have a positive channek resistance, so current will be shared fairly well.

Controlling a MOSFET means pumping charge in and out of the gate. You will need to put a buffer/driver between the micro and the transistor. I do not think that this is the place to go into such detail. You will probably have to talk to someone that helps you out with this.

Gunnar Englund

http://www.gke.org

 

[DBCox]

Skogsgurra,

Thank you again for all of your help. They have 2 other MOSFETs rated for 27 and 75 amps that are actually a LOT cheaper. I will see what I can find for their thermal properties. One of those may be my best bet. I will get in touch with the controller people I know to properly size the buffer.

Thanks again, you've been very helpful!

 

[LionelHutz]

I would think about something along the lines of;

IRF3710 - 100V, 57A, 23mohm MOSFET
IR2121 - low side gate driver

You can look these parts up at irf.com. There are a lot of application notes for their driver IC's there too. You'll need at least 10V to run the gate but the IC will take your 5V input. There are also dual output drivers such as the IR4426 you could use to parallel MOSFETs just to save on the number of seperate IC's in the circuit. For a practical recommendation keep the driver IC very close to the MOSFET with short leads and it will be much happier.

There are also some HP/Avigo? drivers such as the HCPL-3120 which could provide opto-isolation between your processor and the power circuit which is always a good thing. Once you see how the IR parts hook up these would be used very similarily. You'd need an isolated 10-15V power supply for true isolation between logic and power circuits.

I have also found Digikey to be another decent source of parts.

 

[sreid]

If the FET is placed in the return lead, the 12 V supply can be used to power the gate driver.

Also, don't forget the free wheeling diode. This must be rated for at least 20 amps. The free wheeling diode keeps the current flowing in the motor when the FET is off (and also clamps the inductive kick from the motor inductance).

 

[Skogsgurra]

Comments to the last posts.

The integrated (in the transistor) diode can take 20 A continuosly and 80 A pulsed. So there is no need for an external diode.

The normal configuration in non-bridge applications like this is common source. I.e. source tied to ground.

That makes the available 12 V a perfect source for gate and motor. No extra power supply needed.

Yes, there are issues like ground noise and such things. But, as said before, if blackrapter turns to an EE, he will get the design done. We do not design circuits here - just giving advise and tips.

Gunnar Englund

http://www.gke.org