Designing a control circuit for a DC actuator - Page 1
Forum section: electronics / beginners

author: darkponay
author_rank: Newbie
timestamp: March 16, 2025, 01:04:39 pm
content: Hi everyone !I've been studying electronics at high school a long time ago, but I have to admit that this is too far gone now...I have a small electronic project for my car. It is a car from 2008 and I recently had a problem with the air intake flap (air recycling or recirculation) for the AC/vent system inside the car.It is actuated by a DC motor powered by 12V or -12V. It works well but, when the flap is in its start or end position, there is no feedback to stop supplying power to the motor. That's why the motor failed. It took few years for the motor to start failing but I would like to prevent this.The easiest solution to address this problem is making a intermediate circuit between the vent command module and the actuator itself.I see a way of doing it with a µC:We know:the command module sends +12V or -12V depending on the activation or not of the air recirculationthis +/-12V are supplied from the car engine start util the engine is stoppedthe actuator draws approx. 30mA when it turnsthe actuator draws approx. 280-300mA when it is blocked in start or end positionthere is a +12V power line next to where the circuit can be placedthis +12V power line is up only when car engine is runningFirst step:Providing power for µC and detecting the status of the commandFor the use of the microcontroller (i don't know which one) i need to transform the +/-12V into a logical level. Let's say we use an opto-coupler for this.A 12-to-5V DC-DC converter coming from the 12V power line, so when starting the engine, it will power-up the µC.With these 5V, i can power the µC and the optocoupler will feed (or not) 5V to a µC's digital input.Second step:ProcessingBased on the first step, when the engine start, the µC starts too.The start sequence could be something like this:StartDetect logical levelDepending logical level on the input, turn on or off a digital output for 5 secondsWait for a change on the inputThird step:ControlI have no idea how, from a digital output I could make +12V or -12V with enough power for the motor... I would like to avoid a relay, but a relay could easily passthough the initial +/-12V from the command module... I don't know...It looks easy but I'm lost with the third step and my electronic skills are now so old that I fail at finding the right components from suppliers websites, do the calculations, etc.What do you think of my project? Let me know if you need more explanations. Do you know where I could get some help for designing the schematics and calculating the values of the components? May I get help for this here?Thank you much in advance!

author: darkponay
author_rank: Newbie
timestamp: March 16, 2025, 05:49:59 pm
content: Hi again guysIt took me some time but i think i have now a diagram of what i am trying to achieve.

author: Benta
author_rank: Super Contributor
timestamp: March 16, 2025, 07:28:49 pm
content: It's very common to use small DC motors without end stops in cars.In the most simple applications, the driver circuit is simply current limited.More advanced circuits monitor the current and stops driving the motor when it suddenly increases (a kind of virtual end stop).

author: darkponay
author_rank: Newbie
timestamp: March 16, 2025, 07:39:43 pm
content: Hello, and thank you much for your replyIf i understand correctly, i should just leave it as it is, right?I did tests and the motor it powered up all the time when the car's engine is on. When the motor is turning, it draws 25-30mA under 12V, but when it's blocked, it draws almost 300mA, and I can also mesure a voltage drop from 12 to 9.8V.I tried to disassemble the dashboard command unit, there is nothing for stopping the motor when it is blocked, except that it is in series with a power resistor of 10 Ohm.Since the motor is becoming very very hot after some time (even if it is the right reference), do you think it could be a good idea to replace the resistor? By which value (20, 100, 470 Ohms, IDK) ?Thank you much in advance

author: SteveThackery
author_rank: Super Contributor
timestamp: March 16, 2025, 11:13:49 pm
content: I think your project, as drawn, should work. The only concern I have is that the optocoupler diode will, some of the time, have a reverse voltage of 12V* across it, depending on the output polarity from the car's control unit. Make sure it is rated to withstand that. You can protect it by putting another rectifier diode in series with it - a 1N4001 should do.* The nominal 12V can go up to 15V, or so, depending on the charging strategy used. Also, you might need to protect the microcontroller from the noise and voltage spikes which are prevalent in car electrics.

author: darkponay
author_rank: Newbie
timestamp: March 17, 2025, 07:56:13 am
content: Hi! And thank you! As I said in the first post, my electronic lessons are very far behind me, but it makes me very happy that I still have a few knowledge!That said, I don't know how to calulate the components values. I guess the first step would be the choice of the µC? I wans thinking about Arduino boards because I have a bit of experience with them, but the boards are a bit big. Do you have an idea?Thanks again!

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2025, 11:25:32 am
content: Pin 6 might need a pull-down resistor, to ensure it sees a logic low, otherwise, check you enable internal-pull down resistors when programming the microcontroller.The microcontroller's outout might not be able to source enough current for that relay. You'll probably need to add an external switching transistor.