12V Motor controll help!

Vern

Hello everyone and thanks for taking the time to look over my problem.

Well I figured its time to start a project so I went ahead and ordered 3 peristaltic pumps that I hope to turn into a aquarium doser. I've been looking around for a way of interfacing them to the prop and have come up with tons of tutorials on servo motors, mosfets, relay boards and all kinds of motor control boards that are out of the price range for this project. So my question to you is....

What is the easiest way to safely control a 12V DC motor that has a max amperage draw of 85mA.

I have a PPDB but I think the max voltage rating for the motor controller on it is 7 or 8 volts iirc. Are mosfets the best way to go for something like this and how breadboard friendly are they? Keep in mind I'm still very new to all of this and this is going to be my first foray into an actual physical project and not just following along a tutorial.

Thanks again, I'll await the advice of the guru's here before I go crazy and start ordering stuff haha.

Ttailspin

The L293D half H driver on my PPDB is rated for 4.5 to 36 volts, and 600mA with a peak current of 1.2 Amp.

A 2N2222 transistor turning on a reed relay, turning on a small motor, would work, and is simple and cheap too...


What do you mean by "aquarium doser"?



-Tommy

T Chap

Does the motor only run in one direction? A single N channel mosfet would work fine on the low side of the motor, tie the +12V to the other side of the motor. Connect the gate to the prop via some small value resistor, 100 ohms would be fine to test. Are you trying to control speeds via PWM or is it just on/off? There are plenty of schematics online for n channel mosfet motor control. Very simple if only one direction.

JonnyMac

There is a little chip called the TC4427 from Microchip that I often use as a MOSFET driver but others have used it for direct drive of small DC motors. It has two channels so if you're only looking for speed control then you can control two motors with one chip. If you want to do speed and direction then this $1.50 chip will do it for you.

Vern

I'm building a machine that uses peristaltic pumps to accurately add nutrients and pH adjusters to a tank. Eventually I hope to add in the ability to read and monitor pH and EC allowing the doser to keep the tank at specified levels without any user intervention.

Ok so I would be safe to use the L293D half H driver on my PPDB then? I would like to order the parts to run all three motors as I plan on using perf board or getting a PCB made after the prototyping stage. I'll look into your suggested method of control, thanks a lot for the input any and all information is greatly appreciated.

Duane C. Johnson

Hi Vern:

What is the easiest way to safely control a 12V DC motor that has a max amperage draw of 85mA.

My standard recommendation is to use an IRF3708 low threshold MOSFET. The gate can be directly driven by the prop running on 3.6V. (OK, a 1K resister from the prop pin to the gate is prudent.)

Good design would assume the startup current may be as high as 3 times the running current.
So, one could use a 2N2222 NPN transistor with a 430 ohm resister from the prop pin to the base would be about right.

Duane J

Vern

Wow you guys are posting faster than I can respond.

@T Chap Thanks for the response I suppose I should have been more specific. I would like to have speed control on the motor in order to more accurately calibrate the amount of chemical being delivered. On off would work but when trying to regulate smaller doses it would advantageous to be able to slow the motor down.

@JonnyMac I think you just nailed it, I'll take a look at that chip and see what else I need to interface it with the prop. One chip takes care of two motors plus speed control, that is perfect for my needs and well within my price range.

Thank you for all your responses.

JonnyMac

If you just want speed control then any of the suggestions here will do, even down to using at 2N2222 transistor. The advantage of the TC4427 is that it would allow you speed _and_ direction control in a small package. Since you're just doing speed you could use the ULN2003A (which has seven circuits); it has an open-collector Darlington output and integrated input current limiters and pull-downs.

Vern wrote: »

@JonnyMac I think you just nailed it, I'll take a look at that chip and see what else I need to interface it with the prop. One chip takes care of two motors plus speed control, that is perfect for my needs and well within my price range.

Vern

Thank you all again for the responses, I'm going to have to go and look up the data sheets and for explanations on using the above pieces in a circut. I'm pretty sure you have all given me the nudge I needed in the right direction now I know where and how to focus my search but if anyone had a tutorial kicking around or an example of using 2n2222 transistor or any of the above I wouldn't turn my nose up at it. I'm off to google my way into further confusion lol.

@Duane The 2n2222 transistor and the Mosfet you mentioned are those two seperate options or do they need to be used in conjuction? Bare with me, I'm a bit slow

Ttailspin

Check out this page, it has the data sheet,
Have no fear, the L293D has thermal protection, and has shut down everytime I have "over driven" the poor thing,
At like 2.5 Amps of current, it gets very warm, and then stops working till it cools down...

Just to throw some more gas on the fire, directly below the L293D data sheet link, is another link, to a ULN2803A darlington array,

Sheesh, JonnyMac and Duane are on it, so you are in good hands here...


-Tommy

idbruce

Vern

I recommend reading this thread and using a ULN2803 Darlington array.
http://forums.parallax.com/showthread.php?132760-Using-the-propeller-to-trigger-24v-solenoids&highlight=ULN2803

One chip solution.

Bruce

Vern

Thanks again, ok so I'm going to use the ULN2800A I ordered some free samples from TI and picked up a ULN2803 while I was at it just in case.

@idbruce Thanks for the link to that thread thats exactly what I needed to wrap my brain around this stuff.

Thanks to the entire forum, I love this place.

idbruce

Vern

Your welcome.

I love this place.

Yea, besides the occassional bickering amongst the siblings, it is like one big happy family that helps one another

Vern

Show me a family that doesn't bicker from time to time lol.

I was so lost this morning when I made my first post and now I have a firm direction and general understanding of what I need to do in order to achieve my goal. Thumbs up!

Duane C. Johnson

Hi Vern;

Vern wrote: »

@Duane The 2N2222A transistor and the MOSFET you mentioned are those two separate options or do they need to be used in conjuction? Bare with me, I'm a bit slow

Separate methods.
The 2N2222A & 430 ohm resister is the smallest.

However, the IRF3708 is my favorite MOSFET that can be directly driven with a prop. This MOSFET is rated for 62A. Read the high current to mean "Low On Resistance" when driven with the relatively low gate drive voltage supplied by the prop.
This graph, which I measured, shows the gate voltage vs. resistance.

Clearly this would drive your motor. And fairly low cost at about $2.
I always have these in my junk box.

There are a few other MOSFETs but I haven't evaluated them enough yet.

Duane J

Vern

Thanks Duane ill keep that in mind im sure a mosfet that you can drive with prop would definitely come in handy.