# Circuit Considerations

Note: Can skip all images except last, which is a schematic created using the editor.

I am at a point in the design process where I have created a schematic of the circuit that I want to create. I am using a buck converter to step down a 24V DC battery to 5V and using that battery to power an arduino. This arduino is acting as a speed controller, using PWM and getting input from external switch to set the speed. The input from this switch will be programatically read to set the speed output. The 5V output signal from the arduino then gets run through an op-amp configured as a non-inverting amplifier and drives a motor. The schematic follows: []1

This is the first real circuit I have created, and I don't know where to go from here. It is more than likely that I have made a mistake somewhere or forgotten a part or some heat/power consideration, but I don't have the slightest idea (1) how I could check my circuit for those things or (2) how I would remedy them.

I was wondering how I could go about doing these things and improving my knowledge of electronics in this way. I apologize if this question is somewhat vague; however, I really respect this community and the years of experience you have in the field, and I was hoping that your insights would be valuable in this question, as I don't really know how I could make it more specific. Thanks.

EDIT 1: Ok, after suggestion I am replacing the op-amp with an n-channel MOSFET as follows (Edit 2: added a 10K pull-down resistor on the gate of the MOSFET):

EDIT 3: Added pull down resistor to schematic and flipped MOSFET:

EDIT 4: Adding a full schematic (Edit 5: switched NMOS and Motor position):

simulate this circuit – Schematic created using CircuitLab

• Is this a bad idea? I thought the effective result would be the same as an NPN transistor, but I am just more comfortable with op-amps. Commented Jun 27, 2016 at 19:53
• My biggest doubt thus far about your circuit is whether or not the OP-Amp can actually source enough current to drive the motor you're using. You would be better off using a power MOSFET here. Place the motor between the drain and Vdd, ground the source, and send the PWM signal to the gate, keep the flyback diode. You'll need to find a power MOSFET with a low enough gate threshold to use the PWM signal directly from the Arduino. Commented Jun 27, 2016 at 19:54
• Also, Be aware that you have a small oscillator going with the OP-Amp and the 3.6k resistor. Nothing insane, but it will vary the motor's performance quite a bit.
– user86234
Commented Jun 27, 2016 at 19:57
• I see... I was thinking that the MOSFET wouldn't work because it only worked on or off, but I completely forgot about the PWM of the arduino! Will modify and update, thanks! Will the MOSFET be able to switch on and off quickly enough? Commented Jun 27, 2016 at 20:02
• @louiemcconnell: I tweaked the CircuitLabs schematic a little to compress the width. SE reduces it to a 640 pixel-wide image which makes large schematics difficult to read. Please check that I didn't introduce any errors. +1 for a good detailed question. Please add some text in under the schematic explaining what S1 and the resistors are supposed to be doing. It looks unusual so you may be misunderstanding something. Commented Jun 27, 2016 at 23:10

Your power supply concept makes sense (using a buck converter to make 5 V from 24 V), but your motor driver needs work.

The voltage follower you show in your second schematic won't deliver much voltage to the motor. The FET will likely need a few volts G-S to turn on enough to allow useful motor current. With the gate at 5 V, that leaves very little to the motor. It also uses the 24 V supply very inefficiently. Most of the power will go into heating the FET, not running the motor.

Your third schematic makes no sense at all. A flipped N channel MOSFET has a internal diode from source to drain, so the motor will always be on with over 23 V on it, and regardless of what you do with the gate.

I would use a N channel MOSFET as a low side switch. Connect the motor between its drain and the 24 V supply, and don't forget the Schottky flyback catch diode in reverse across the motor. Use PWM to set the effective motor voltage. For example, if the motor is supposed to run from 6 V, then use 25% duty cycle. This is also much more efficient.

Something like the IRLML0030 can be driven directly from the 5 V PWM output of the microcontroller. At 40 mΩ, it will only dissipate 40 mW with 1 A thru it.

• Thank you for your help! I made the changes you mentioned. Will the new schematic work? Commented Jun 27, 2016 at 21:03
• @louie: The schematic seems clear enough, so I'd say it works. That does not mean, of course, that the circuit will work. Commented Jun 28, 2016 at 10:27