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[SOLVED] Driving 24v Vacuum Pump - DC Motor

Xenobius

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Hi,

As the title implies I have a 24v (500mA) DC motor for a vacuum pump and a power supply of 30v. I do not need an H Bridge or complex drivers, I only need to turn it on or off using a microcontroller and preferably without relays. How would you go about it?

What I was thinking of is using a TL2575 ADJ which is a switching regulator that can take my 30v down to 24v and also have the enable pin which can interface directly with a microcontroller. Do you see an issue with this approach? Would you do it differently?

Thanks
 

KlausST

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Hi,

you say "not need an H Bridge or complex drivers"
but then use a switch mode regulator. Isn´t this complex, too?

If you don´t want to waste the (30V-24V) * 0.5A = 3W then the next simple solution is a Mosfet and a free wheeling diode.

Almost any microcontroller has a PWM ouptut. So code is very simple: just a couple of lines for PWM setup ...
Enabling/disabling the PWM will be as simple as switching a port pin ON/OFF. (one simple line of code)
Try 75% duty cycle.

Klaus

Added:
This does not mean that the switch mode solution is bad.

Btw: there are dedicated simple DC motor control ICs, which include the free wheeling diode and also include features like soft start, overcurrent protection, overheat protection....
 

stenzer

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Hi,

which DC motor are you using? Please provide a datasheet.
For which condition are the 500 mA stated, NO LOAD, MAXIMUM EFFICIENCY or MAXIMUM POWER?

You have to keep in mind that the inrush current when starting your mortor may be much higher.

greets
 

Xenobius

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Hi and thanks for your input,

@Klaus: You make a very good point and I can simply control it with PWM from MCU however my choice of that IC was because I wanted to regulate voltage more than because I needed PWM. I will google a bit to understand how a free wheeling diode can help reduce voltage.

@stenzer: this is the vacuum pump https://www.aliexpress.com/item/32862878409.html?spm=a2g0s.9042311.0.0.43ea4c4dN69YtX

- - - Updated - - -

I don't quite get how it will work - I thought its needed for inductive loads to stop back emf from harming the circuit. I still need to provide 24v no? What did I miss?
 

KlausST

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Hi,

I will google a bit to understand how a free wheeling diode can help reduce voltage.
No. The diode is not to reduce voltage. It is to protect the MOSFET from high voltage spikes caused by the inductivity of the motor (O.K. it does reduce voltage (spikes) in this case)

Klaus
 

Xenobius

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Right ok so we are on the same page. I will try to do more research
 

stenzer

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Hi,

the diode is placed in parallel to your motor, where the cathode is connected to your positive supply (30 V) and the anode is connected to the drain of the MOSFET. When the MOSFET is switched off, the diode "short-circuits" the inductor (motor) and the energy stored in the motor (magnetic field) is dissipated.

As suggested by Klaus, control the MOSFET by a PWM. By doing so, you can control the average current through your motor. This limits the power dissipated by your motor when operated at a higher voltage.

greets
 

Xenobius

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I didn't know that I could drive a motor with higher voltage so long as the PWM duty cycle doesn't go beyond a certain limit.
Specifically I calculated that on the arduino you can specify a 100% duty cycle with 255 so if my supply voltage is 30 and the motor is rated at 24 I can set any value up to 200.
I will obviously have to test it out but this is really great! Thanks all for your help
 

FvM

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Although the free wheeling diode of a pwm motor driver surely protects the switching transistor against overvoltage, the circuit can be also analyzed as step down converter. The energy stored in the motor inductance isn't dissipated in the first place, it's driving the motor shaft.
 

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