Switching +25VDC using micro controller - Page 1
Forum section: electronics / beginners

author: rrooggiieerr
author_rank: Newbie
timestamp: October 09, 2025, 12:48:29 pm
content: I've got this wired remote that controls 2 motors that I'm trying to add to my home automaton system. The wired remote got 4 buttons for up/down each motor, 2 additional buttons for up/down both motors, and a 6 way connector containing +25VDC, 0V en 4 wires, two wires for each motor direction. The buttons send +25V to the corresponding wire to make to motor move in that direction.The motor housing contains a relay board that takes the +25VDC signal and switches 240VAC to run the motors. I didn't dive to much into the 240VAC side of the thing, I don't think it's relevant. Thus the +25VDC signal from the wired remote switches the relays (inductive load) in the motor housing.My goal is to create a drop in replacement for the PCB of the wired remote with one that contains an ESP micro controller. The wired remote should still work as designed and additionally it should be able to be controlled by the home automation system.I'm using an off the shelf buck converter module to create +3.3VDC from the +25VDC for the ESP. The buck converter shares the 0V, it's not isolated. I've got this working together with the button inputs to the ESP and +3.3VDC output outputs from the ESP.To my understanding switching is easier on the low side, but since I can't control the low side due to lack of those individual paths I need to switch using the high side.Now my question: What is the best way to switch the +25VDC inductive load using a +3.3VDC signal?I could use some (miniature) relays to do the switching, but switching relays using relays doesn't feel right and overkill.I read about P channel mosfets but that you can't use a common ground when using these. Is there maybe some IC that would do all the work, takes the +3.3V input signal and switches the +25VDC lines?

author: JustMeHere
author_rank: Frequent Contributor
timestamp: October 09, 2025, 02:04:08 pm
content: You could try this:https://www.littelfuse.com/assetdocs/littelfuse-integrated-circuits-cpc1907b-datasheet?assetguid=26e79d12-7fc1-482a-9ccb-227163caa3acBut you need to understand more about how the motor controller works."Switching is easier on the low side" because of how NPN mosfets work. Plenty of info on that around the net.

author: rrooggiieerr
author_rank: Newbie
timestamp: October 09, 2025, 02:15:10 pm
content: Thanks! I've been googling and watching YouTube for the last couple of evening and couldn't find (or understand) a solution that fits my case.I do understand why why low side switching is easier for PNP mosfets, but unfortunately low side switching is not possible because I don't have access to the low side of the relays.Which answers do I need to understand better how the motor controller works? It's a +24VDC signal that switches the relay in the motor controller.

author: rrooggiieerr
author_rank: Newbie
timestamp: October 09, 2025, 03:26:13 pm
content: I just opened the motor housing, and this is the relay board.It uses 4 Phoenix Mecano BS-115cs relays which are rated for 12VDC(?). The board itself also seems to be developed by Phoenix Mecano

author: Benta
author_rank: Super Contributor
timestamp: October 09, 2025, 05:26:54 pm
content: Simplest would be to use small 3.3 V reed relays to drive the larger relays.

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2025, 05:40:24 pm
content: There is no such thing as an NPN or PNP MOSFET. NPN and PNP relate to BJTs, not MOSFETs. MOSFETs don't use PN junctions to pass current, under normal circumstances. The current flows through the channel. There are N-channel and P-channel MOSFETs.How is the microcontroller circuit powered?According to the photographs of the circuit board, there are four diodes close to one another and a diode for each relay. The diode near each relay will be a freewheeling diode, which absorbs the high voltage back-EMF and the remaining four diodes will be a full wave bridge rectifier, which suggest it's powered off AC. The is no smoothing capacitor, which will affect the voltage reading taken with a basic multimeter.I've found a pin-out for the relay, which will help you reverse engineer the board.

author: rrooggiieerr
author_rank: Newbie
timestamp: October 09, 2025, 07:22:30 pm
content: Thanks, I think I was confused and meant P-channel mosfet.Actually the relay board is powered by a +24VDC power supply which does the rectifying and smoothing. In hind sight the motors are also driven by 24VDC and not 240VAC like I mentioned in my opening post. Yes, the 4 diodes near each relays seem to be the freewheeling (fly back) diodes.The other 4 diodes are in line with the 4 signal wires coming from the remote.The microcontroller circuit is powered using a buck converter which brings the +24VDC to +3.3VDC.The original PCB of the remote doesn't have a microcontroller, only switches, some diodes and a LED resistor combo.

author: xvr
author_rank: Frequent Contributor
timestamp: October 09, 2025, 08:21:39 pm
content: P channel MOSFET will work out.

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2025, 09:37:27 pm
content: Quote from: xvr on October 09, 2025, 08:21:39 pmR2 is far too low. Did you mean 200k?Move the base resistor to the emitter. Now the BJT acts as a current sink and the voltage drop across the collector resistor will be limited by the collector current.P-MOSFET driver 3V3 24V.asc(1.19 kB - downloaded 4 times.)I would just change the P-channel for a PNP.

author: tooki
author_rank: Super Contributor
timestamp: October 10, 2025, 05:31:29 pm
content: Some other options: a small solid state relay (like JustMeHere suggested), or an integrated load switch IC.You could also considerreplacingthe 12V electromechanical relays altogether, for example with solid state relays.

author: rrooggiieerr
author_rank: Newbie
timestamp: October 12, 2025, 12:32:36 am
content: My plan is to keep the relay board as is and replace the PCB in the wired remote. Considering the space constraints I think a solid state relay is more efficient and easier to implement than a bunch of transistors, mosfets and resistors.I found the datasheet of the relays which states that the coil resistance at 24VDC is 1600 Ω and a nominal current of 15 mAI think this solid state relay should do the job:https://www.lcsc.com/product-detail/C425151.html

author: BennoG
author_rank: Frequent Contributor
timestamp: October 12, 2025, 09:59:40 am
content: I use the BTS724G for this kind of high side switching directly from a 3.3V or 5V CPUand JLC has them in stock so you use them on your PCB if you like.https://jlcpcb.com/partdetail/InfineonTechnologies-BTS724G/C82656Benno