I am looking to control multiple DC motors off of a single H bridge. I only need one active at any given time I need direction but not speed control. I'm not looking for a product recommendation, but I guess asking is it possible to use an analog mux or shift register on the output of the H bridge to act as a switch? I am a software guy just getting into hardware so if I'm missing something obvious please forgive me!
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\$\begingroup\$ Maybe some automotive stuff would come to play, as Infineon BTS switches: infineon.com/dgdl/… \$\endgroup\$– Marko BuršičCommented Feb 14, 2020 at 16:06
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\$\begingroup\$ How many motors are we talking about? 2? or 12? \$\endgroup\$– DKNguyenCommented Feb 14, 2020 at 16:27
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\$\begingroup\$ 8 motors! Either 12 or 24v but both run below 1A. \$\endgroup\$– WilliamBCommented Feb 14, 2020 at 16:31
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\$\begingroup\$ @DKNguyen so I couldn't use an H bridge IC/ motor driver chip and have a multichannel switch of somekind on the output? My original solution was a DPDT relay for direction and a relay per motor for on/off plus the octocouplers and supporting circuitry to isolate my logic chip. So as far as hardware I have a decent margin for less hardware \$\endgroup\$– WilliamBCommented Feb 14, 2020 at 16:38
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\$\begingroup\$ Welcome to EE.SE. EEs are into schematics. Could you try to depict what you want to do with those motors and black boxes? And if possible more infos about those motors? \$\endgroup\$– AriserCommented Feb 14, 2020 at 17:01
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2 Answers
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Right off the bat, your typical analog mux won't work nor will digital logic components such as shift registers or digital muxes. They are meant for low power signals, not spikey, hazardous, high level motor currents. The bidirectional current requirement immediately rules out digital logic.
The thing is...an H-bridge is four switches and you will need four semiconductor switches per motor anyways for the power mux. Two per motor wire since they only pass current one way or block current one way depending on the switch, except for triacs where you can use one per motor wire. Along with gate drive circuitry, you might be saving much much less hardware than you think.
Building a Solid state bidirectional switch is much more complicated than relays, unless you feel like using triacs or anti-parallel thyristors (only pass current one way) but both those are lossy and still more complicated than relays. Those latch on and cannot disconnect/interrupt current themselves once latched. Your H bridge will have to kill the current for these devices to stop latching on.
Relays are far and away the simplest muxing solution. But you are best off just using multiple h-bridge chips since your motors are so low power. Will be smaller than a relay and cheaper. You can mux the control signals instead to save MCU pins.
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\$\begingroup\$ so I couldn't use an H bridge IC/ motor driver chip and have a multichannel switch of somekind on the output? My original solution was a DPDT relay for direction and a relay per motor for on/off plus the octocouplers and supporting circuitry to isolate my logic chip. So as far as hardware I have a decent margin for less hardware \$\endgroup\$– WilliamBCommented Feb 14, 2020 at 16:39
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1\$\begingroup\$ @WilliamB you are better of with multiple h bridge chips which will be smaller than a relay. you can mux the control signals to save pins. I know it seems inelegant but that is because you are not aware of what goes into a bidirectional semiconductor switch. it is almost as much work as an h-bridgem sometimes more..Unless you use triacs or thyristors but they are unsuitable in other ways and will take low-moderate effort but you can make it work if you are realy in love with the idea. \$\endgroup\$– DKNguyenCommented Feb 14, 2020 at 16:51
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\$\begingroup\$ so something like this? (not part reccomendation more so confirmation of understanding) mouser.com/ProductDetail/Toshiba/… \$\endgroup\$– WilliamBCommented Feb 14, 2020 at 16:54
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\$\begingroup\$ Thank you for the help and being understanding! \$\endgroup\$– WilliamBCommented Feb 14, 2020 at 17:00
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No, that won’t be suitable for at least three reasons.
- The mux or shift register will not be able to handle motor currents
- A H bridge is used to reverse motor direction and this means reversing polarity and I’ve never come across any semi complex silicon product that would survive something like this.
- Back emf from the motor are handled by the H bridge rather nicely using diodes or MOSFETS with diodes but a mux or shift register will just probably die.
answered Feb 14, 2020 at 16:08
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\$\begingroup\$ He never mentions 'silicon product' all he wants is an analogue mux. So a set of relays would work : -) \$\endgroup\$– OldfartCommented Feb 14, 2020 at 16:11
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\$\begingroup\$ They are small motors that operate at less 1A (still deciding on motor but all are less than one amp). I was planning on a diode at the motor connection anyways. I was actually trying to move away from a relay solution for size and if possible cost reasons. \$\endgroup\$– WilliamBCommented Feb 14, 2020 at 16:12