I have a 29V Linear Actuator that I want to control - both direction and speed. For the direction I used this 4-channel relay module, but I only used 2 relays on it.

I was able to successfully control the direction of the actuator. However, upon activation of the relays, the actuator creates a strange sound or force. Hence, I thought of implementing a ramp up/down using PWM.

I found out that the relay module I am using has hardware limitations when it comes to PWM. After some research, I found a schematic below:

enter image description here

I have some questions regarding the circuit:

  1. First and foremost, is the circuit safe?
  2. What is the function of that 29VDC with a snubber diode that pulls the Normally Open signal of the relays up?
  3. What is the function of the BJT?

I appreciate any response that comes out. Thanks!

  • \$\begingroup\$ If MCU is open and switches in opposite directions , motor is ON. therefore move input 10k from 0V to +5V and ensure FET is logic Level gate drive rated for > 20x motor current rating since start surge is 10x rated current . Diode clamps switch off flyback voltage \$\endgroup\$ Jan 13, 2020 at 12:09
  • \$\begingroup\$ Where did you find the schematic? Why doesn't it list critical parts like the relays, the MOSFET and the BJT? \$\endgroup\$
    – Andy aka
    Jan 13, 2020 at 12:27
  • \$\begingroup\$ First off, you drew the coil of the relay as a resistor, which it is not. It is an inductor. Then, you need to flip the contacts of one of your relay outputs from normally closed to normally open. The way it is will not even put a voltage across the motor. \$\endgroup\$
    – user233390
    Jan 13, 2020 at 13:35

1 Answer 1

  1. First and foremost, is the circuit safe?

In principle it's safe for ordinary applications but those applications that are "medical" or "flight critical" or require special approval it takes more than a few words and a hand drawn circuit to establish it's safe. The relay device is this: -

enter image description here

And the relays are rated at 30 volts and 10 amps so, providing the current taken by the motor is below 7 amps (a de-rating imposed by me) then I'd say it was safe if built correctly. However, I have no idea if Sunfounder" are a reputable supplier/manufacturer. They are based in ShenZhen in China so, ultimately you have to decide whether they are reputable because "being safe" implies so much more these days than making sure it doesn't catch fire or electrocutes someone.

If you require it to be somewhat "fail safe" then you have to consider the problem of disconnecting the MCU - doing so will naturally activate the MOSFET and run the motor at full speed. See also my final paragraph.

  1. What is the function of that 29VDC with a snubber diode that pulls the Normally Open signal of the relays up?

The snubber diode and resistor ensure that inductive kick-backs from the motor are routed back to the positive rail. This prevents the situation of having an uncontrolled back emf destroying the MOSFET.

  1. What is the function of the BJT?

MOSFETs usually require a decent gate drive voltage to ensure that the MOSFET acts like a switch (ohmic region of operation) and many will work happily with 5 volt gate drive voltages so, if you were connecting the circuit to an MCU that only produces (say) a 3.3 volt logic level, the BJT buffers the signal to 5 volt to ensure better MOSFET operation.

However, if your application requires that the circuit is "off" when the connection to the MCU is removed, you need to reconsider how this circuit operates. Currently, if the MCU connection is removed, the MOSFET will be switched on and the actuator's motor will turn and might cause a problem.


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