DC motor

You didn't give any voltage or current ratings. If it is a low power DC motor, I drew up something that would probably work for you. Check it out at

Brian

Are you sure it's a simple DC motor? Most small fans are not, and cannot be reversed.

If, as you said this is a motor for an RC race car. A potentiometer will not be a reasonable way to control speed. Such motors are wound with as few as ten turns per phase and draw a LOT of current. A solid state controller for that type of motor may have as many as 6 MOSFET's to switch a PWM for speed control. Reversing is just a mater of reversing the leads. However it will cause additional wear on the Brushes.

I would recommend getting a more suitable motor rated for a somewhat higher voltage (say 24VDC) surplus, and working with a lower current requirement and a slower speed due to an already reduced voltage vs. rating.

-"You didn't give any voltage or current ratings. If it is a low power DC

I'm also wondering about the parts in your dual powersource drawing. You have a NPN and a PNP transistor. Will any do or do they need to be specific? What do I use for the amplifier? I apologize for my ignorance. I'm a newbie who builds junkbots, if I don't have the part number, it's hard for me. This is for a small underwater ROV. The motor will control the verticle movement. Raul

Yes, by switching the leads the motor will run either clockwise or counter clockwise. So I guess I just need a switch to swap the voltage. That's good. I can have the servo connected to the switch and the speed control at the same time. That might be the easiest way. Thanks, Raul

------ This depends on how the motor is wound. If it has a wound field- then you need to get into it to separate the field and armature windings. Then you can use a reversing switch to reverse either the field or armature. This requires surgery on the motor. If it has a permanent magnet field then reversing the connections should do it. Note that often a motor will rn better in one direction than the other. A center tapped potentiometer could do it, with the right connections but this is horribly wasteful of energy. Better to use a reversing switch along with your speed control which could be electronic rather than a potentiometer.

--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer

The parts aren't that fussy. The op-amp could be something like a LM741. What transistor to use, depends on how much current your motor takes to run. I would use a transistor that could handle the stall current of the motor. To find the right transistor, I would suggest you go to

and use their search engine. Their search engine is very good. After you select "transistors", it will give you some parameters to choose from. Based on that, it will give you a list of transistors to choose from. Brian

The op-amp should work under the voltages that you are using and supply enough drive current for the transistors. The transistors need to work with the voltages being used (I like to have them work with 4 times the max. voltages required). The transistors must handle the max. current and have enough current gain for the drive current supplied.

The 741 op-amp supplies 0.02 amps of drive current for the transistors. If the transistors have to supply maybe 0.8 amps to the motor, then the current gain (Beta) of the transistor has to be 0.8 / 0.02 = 40. So if you are using a +/- 6 volt supply, under normal working conditions each transitor will need to handle 12 volts, so choose a transistor that can handle 48 volts, with a minimum Beta of at least 40 or more.

I got all the parts but it doesn't work yet. I'm sure it's some stupid thing I'm doing wrong. Here is a shot of the bread board setup. What do you think the problem is?

Thanks, Raul p.s. I must add I can't believe how great the response has been for my problem. If I could only get people that actually work for me to be this enthusiatic about stuff.

Your schematic should show pin numbers on the op-amp, and should also show the power supply connections for the op-amp.

I don't see any power supply connections on the breadboard - without that, it is hard to say what is wrong.

I don't see the feedback connection on the op-amp (output to -input), and one lead of the pot is not connected.

One of the transistors is inserted in the breadboard incorrectly, I think. The flat sides should both face the same way, not towards each other.

--
Peter Bennett, VE7CEI  
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that's your main problem solved. some motors only run one-way whatever the polarity attached (automobile starter motors, for example).

you want a power mosfet in to drve the motor with a PWM signal; that's most efficeint. there are some simple and really cheap circuits out there that you can use. just do a search on the web.

--

Fat, sugar, salt, beer: the four essentials for a healthy diet.

You don't identify the op-amp, but whatever it is, you seem to have several pins unconnected. In fact it looks to me as if you only have pins 5 and 6 connected! In particular, you have not supplied power to the opamp. If that's a 741 or similar, +V should go to pin 7, and 0V (ground, or battery negative) to pin 4. And what is pin 6 connected to via the red and yellow links? Looks like they just go to strips 18 and

19 of your stripboard, but then nowhere! And what transistors are those, and what pins are which?

Why not *draw* the schematic, i.e. show what wires connect what pins etc.

--
Terry Pinnell
Hobbyist, West Sussex, UK

Ok, I re-did the bread board but still no dice. Here are pics of the current setup.

If I swap the transistors the motor runs in one direction but one of the transistors gets really hot.

Thanks for all the help. Raul

Ok, I re-did the board but still no dice. Here are pics of how it sits now.

If I swap the transistors the motor runs but one of the transistors gets really hot.

Now what am I doing wrong? Thanks for the help. Raul

--- Probably nothing, since if the motor runs, one of the transistors _has_ to get hot! The transistors just look like variable resistors, and since you're dropping some of the supply voltage across and allowing current to flow through the one that's running the motor, the amount of current flowing through the resistor multipled by the voltage dropped across the resistor will be equal to the power it's wasting in order to slow the motor down.

Unfortunately, there's not a lot you can do with the motor even if you do get it to run back and forth, since at low speeds you'll get very little torque out of it and there'll be a pretty wide "dead band" around the center of the pot's range where the motor won't do anything except sit there.

The best way to do what you want to do is to pulse-width modulate the motor, which means that if you want it to run slow you turn it all the way on but only for a short time every once in a while, and if you want it to run faster you leave the switch on a little longer every once in a while. The slick thing about doing it that way is that since the switches are only ever either all the way on or all the way off they don't get very hot. I'm working on a simple circuit to do that, and I'll post it when I'm done if you're interested.

-- John Fields

You have the negative sides of the two batteries connected together, so you have two positive supplies, not one positive,m and one negative.

Please show pin numbers for the op-amp on your schematic. As far as I can tell, the op-amp is not connected corretly - it appears that you misunderstand how the pins are numbered, but without pin numbers on the schematic, it is hard to say how you think the pins are numbered.

The dimple on top of the IC is close to pin 1, and the pin numbers run counterclockwise from there - just as shown on the pinout drawing.

You appear to have the output (pin 6) connected to V- (pin 4), and the wiper of the pot connected to V+ (pin 7)

You still don't have any power supply on the op-amp, so it won't work.

--
Peter Bennett VE7CEI 
email: peterbb4 (at) interchange.ubc.ca        
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Several things, including my suggestion that you *draw* the schematic so that we can be clear what you are doing. It doesn't have to be a work of art. Just identify all connections clearly, and photograph

*that* instead of your breadboard.

Also, if that's 3V you're using for the 741, it's too low a voltage.

--
Terry Pinnell
Hobbyist, West Sussex, UK

You are still only using 2 connections on the op-amp chip.

It needs at least 4 connections, input, output, plus power and minus power.

The power connections are not shown in some schematics, but the op-amp always need power to work.

One wire from the pot is not connected to anything..?

Your pictures are very good, and they make it easy to help you. It is easy to see how the circuit actually is connected.

You need to check all connections of all components on the schematic and in the real breadboarded circuit.

--
Roger J.