PWM or Analogue - please help.

Thread Starter

Martin_Tris

Joined Jun 16, 2020
2
Hi all, I am a little new to all of this so please go easy.


I am looking in to using the simple circuit below to drive a 775 motor. I'd like to control the motor remotely so I will use two microbits. One will send a signal to the other using the radio function. The 'receiver microbit' will control the brightness of an output LED through PWM. I will then control the LDR on the MOSFET circuit using the LED (the LDR will be 10K max or the in line PD resistance will increase to run with the 1Mohm LDR).

I have chosen to do it this way for two reasons.

1. I do not want to interface directly with the microbit to prevent any potential damage.
2. This will be educational so I would like the kids to see the 'opto isolator' in action.

My question is should I be setting the circuit up so that when the LED is on the Gate is fully saturated, and then use the LED/LDR to control the PWM signal to the gate digitally through LED on/offs (will this even work due to speed of signal??!) or should I just vary the LED brightness through PWM to control the Gate voltage in an analogue fashion.

The motor will be pulling a lot of current I would imagine - something like 20Amps max on start up. Probably 5Amps continuous.

Any help would be appreciated.

Cheers.

Martin.
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MrChips

Joined Oct 2, 2009
21,302
Welcome to AAC!

5A @ 18V = 90W
That is a fairly hefty motor. Can you start off with a smaller motor?

What you have shown is a high-side driver which would be a P-MOSFET. It is more common to use an N-MOSFET on the low side.
You don't want to go analog because the MOSFET will run very hot. Use PWM right up to the gate of the MOSFET.

If you use an LDR as the photo sensor you need to keep the PWM frequency in the low range (<100Hz) since LDR has low frequency response. That is ok since the motor cannot take high frequencies anyway.
 

Thread Starter

Martin_Tris

Joined Jun 16, 2020
2
Wow, A response already! Thank you MrChips that is really helpful. I will go with the N-MOSFET and run the PWM to the gate as you suggested.

Thanks again.

Martin.
 

MrChips

Joined Oct 2, 2009
21,302
You are welcome.
Don't get me wrong. I meant that you still use your optical coupling idea if you wish but go straight digital.
 
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