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Need Help with simple 9V circuit design.

RobH2

Aug 6, 2020
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That was it. From the original schematic, I had no idea which lead was the middle pin given my lack of knowledge. Once I changed that, all is working without overheating. Like Bertus says, "You don't have to know everything, if you know where to find it." So thanks for tolerating my "non-conventionality." I'm sure if you were to do some of the 3d animation I do, you'd break some conventions and I'd be glad to overlook them in an effort to assist you. Thanks for this, you got me working and made my client smile.

Any thoughts about inserting that pot?
 

PETERDECO

Dec 19, 2019
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Thank you for tolerating mine! When I started working for our company they were using a wirewound 100 ohm pot to control the speed of a small hobby motor. It had its limitations. You had to start it on high speed and slowly work it down in speed. A lot of customers didn't understand that and left it in slow speed only to find it wouldn't turn at all the next time they used it. That is one of the limitations using a pot. I take it you're using a gear motor?
 

RobH2

Aug 6, 2020
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Ha, I don't know what of your's I'm tolerating...lol...

Hmmm, not sure about the motor. It's from a circuit I didn't design. My client had a prototype of a motor driven by 9V, a physical slide switch and a pot. I disassembled that and am inserting a touch switch as you know. I didn't source the parts so I don't know it it's a gear motor. I've attached a photo. Circuit 1_Smokes.jpg
 

Audioguru

Sep 24, 2016
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The datasheet for the LP2950 voltage regulator shows the pinout and shows important input and output capacitors that you are missing.
 

RobH2

Aug 6, 2020
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@PETERDECO I'm taking a crash electronics course to boost my knowledge and I've figured out what's going on in this simple circuit. I'm now a little more knowledgeable about the circuit you guys helped me setup. Again, my thanks.

The LP2950 is regulating my voltage to the Touch Switch, the MOSFET is allowing current to flow through the motor when energized. I believe the 10uF Cap is setting the output V of the LP2950. I don't know the theory well enough to know what the 10K resistor does but no problem.

What I can't figure out is what the diode does. Why does it even need to be there? It's blocking current from the battery to the other side of the motor but if it was just gone, current could't take that route anyway.
 

Harald Kapp

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Nov 17, 2011
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I believe the 10uF Cap is setting the output V of the LP2950.
This capacitor does not "set" the voltage. You'd have to use an additional resistor circuit to achieve this, see datasheet of the LP2950.
This capacitor filters the regulated DC output from the LP2950 to make it smooth. It also supplies very short current peaks (e.g. due to switching of components attached to this voltage) to minimize the drop in the output voltage from the regulator.
I don't know the theory well enough to know what the 10K resistor does but no problem.
Why no problem? It is always good to know why a circuit looks as it does. This 10 k resistor ensures that the gate-source voltage of the MOSFET is 0 V if the output of the touch switch is not connected or not yet in a defined state. Thus with VGS=0V the MOSFET will be turned off in a defined way.
This resistor may or may not be required, depending on the output circuit of the touch sensor. If you don't know what the sensor's output looks like, it is good practice to include such a resistor here.
What I can't figure out is what the diode does. Why does it even need to be there?
This is a so called "flyback diode" to prevent damage to the MOSFET when the motor is turned off. When a motor or any inductive load is turned off, the energy stored in that inductance is released (it has to go anywhere, doesn't it?) which manifest in a very high negative voltage (negative with respect to the previous operating voltage) across the inductor's pins. Thus the formerly 0V on the lower pin of the motor (lower with reference to your schematic) will become positive and reach a hig voltage level. This high voltage can and will damage the MOSFET. The diode acts as a short circuit to prevent this overvoltage. More details in the link at the beginning of this paragraph.
 

RobH2

Aug 6, 2020
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@Harald Kapp Outstanding explanations and it adds to my knowledge set. Thanks for straightening out my thinking. I'm just diving into seriously learning electronics and realize it's going to take years to get good at it. I appreciate your time explaining this to me.
 

RobH2

Aug 6, 2020
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I have this circuit working nicely. The "Touch" switch works perfectly as an On/Off switch. How would I wire an pot into the circuit so I can vary the speed?
 

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Alec_t

Jul 7, 2015
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It will take more than a pot for speed control. I suggest you google for some tutorials on 'PWM speed control'.
 

RobH2

Aug 6, 2020
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I think I figured it out. I'm guessing that the current between IRLZ44N and the Motor is a good spot to put a pot. I placed a 1K one there and it seems to function just fine. The low end is a little funky but it works well enough for the prototype I'm making.
 

Audioguru

Sep 24, 2016
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No, the speed of the motor should be controlled with a Pulse Width Modulation circuit (PWM).
Your pot will get hot and probably soon burn out and if the speed is set low then the motor probably will not start running. If the load on the motor increases then it will slow down and maybe stop.
 

RobH2

Aug 6, 2020
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OK, I'll explore PWM. I'm rather "nooby" so I may come back if I have problems. You guys are very helpful. Feel free to ask me for expert advice on 3d modeling or animation and I can pay back the generosity...lol...
 

ratstar

Aug 20, 2018
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You can run the motor off the potentiometer, but it has to go before the base of the transistor where the switch is, then its not seeing the high current.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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You can run the motor off the potentiometer, but it has to go before the base of the transistor where the switch is, then its not seeing the high current.
BS! The transistor is a MOSFET, acting as an on/off switch in series with the motor and the 9V battery. It does not have a "base," it has a "gate" controlled by TTL (Transistor Transistor Logic) logic levels from the touch switch. If you want to control the speed of the output shaft (this IS a gear-motor BTW), you can place a rheostat (a potentiometer wired as a variable resistor) in series with the 9V battery supply and the motor. Just disconnect the power from the battery to the motor and insert the potentiometer-wired-as-a-rheostat there.

From the size of the potentiometer you show in post #23, along with the size of your gear-motor and the 9V "smoke alarm" sized battery, I don't think you will have any problems with the "speed control" overheating. What you may have problems with is being able to afford to replace the battery when it expires... which will be in a matter of hours if run continuously. You may want to consider substituting a cheap "wall-wart" DC power supply for the battery. Or you can purchase a six-cell battery holder that allows you to insert six 1.5V AAA-, AA-, C-, or D-sized cells connected in series to create a 9V battery pack with greater energy capacity. And maybe use a cheap Asian pulse-width-modulated speed control module for better speed control. Not that you actually NEED these "bells and whistles" since you said in the beginning that the original circuit worked just fine for whatever purpose your friend had in mind.

As others have mentioned, a variable resistor inserted in series with the motor is a really poor way to control a permanent magnet DC motor, but it is (in effect) exactly what you started with, before substituting the touch-switch for the original mechanical-switch. The only hiccup with doing this was the incompatibility of the touch-switch module with the 9V battery used to supply power to the motor. Plus, the touch-switch is not capable of providing power directly to the motor... hence the need for the MOSFET switch. And then you also needed a voltage regulator to "step down" the 9V battery voltage to the 5V maximum voltage that the touch switch accommodates.
 
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