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I need to add a secondary means of cutting off power to a motor for safety reasons if a state change is detected on a 3.3V logic level signal while it is operating. The requirement I have is that it must be external to the existing system, fitting inline with the motor connector.

Currently, the brushed DC motor is driven bidirectionally with 12V (SLA battery, so realistically 11.5-13.5V) PWM using an H-Bridge onboard like the image below. This protection cutoff would essentially need to fit between the bridge and the motor, in the area of the red box

Right now I've considered a few solutions, but have the following concerns:

  • High Motor Current: Nominally about 5A, but if stalled could reach ~50A for a short period before FW detects the condition or our 20A slow fuse goes. I'm seeking parts rated to ~75A to provide some margin.
  • Bidirectional Drive: Whereas a high-side PFET may have been an easy solution if driven in one direction, bidirectional drive with the H-bridge makes a discrete solutions with FETs more complicated.
  • Flyback voltage: Since it is driven bidirectionally with the H-bridge, a simple flyback diode can't be placed across the terminals. I'm unsure with how to mitigate flyback voltage from this cutoff without interfering with normal bidirectional operation. I'm also concerned that flyback will damage my cutoff circuitry, particularly if I use a semiconductor device

Here are some solutions I'm considering so far:

  • MOSFET Discrete "E-Fuse"
  • Integrated E-Fuse, or Load switch, possibly with external FET(s)
  • Relays, Electromechanical or solid state
  • Some sort of SPDT switch, which could be used to simultaneously cutoff power and short the battery terminals to circulate flyback current (though I haven't found any sort of high power component for this...)

Any feedback on my considered solutions or suggestions would be great. My main difficulty is that finding parts that work bidirectionally and handle adequate current has not been easy, so I'm wondering if I'm looking at the wrong types of solutions.

H-Bridge

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    \$\begingroup\$ If you peruse various websites which cater to DIY electric vehicles, you will find they sell solenoids which could be used for this purpose. I would not use a semiconductor device. \$\endgroup\$
    – user57037
    Commented Dec 16, 2021 at 19:50
  • \$\begingroup\$ Your question is confusing: 1.) what an emergency cutoff has to do with your description? 2.) What's means bidirectionally driven? ... A motor shall be protected with appropriate fuse I^2 t rating., but you can also implement an electronic overcurrent detection., but all this has nothing to do with emergency stop. \$\endgroup\$ Commented Dec 16, 2021 at 19:56
  • \$\begingroup\$ Also: the protection shall not be as depicted in red box, rather on H-bridge supply. \$\endgroup\$ Commented Dec 16, 2021 at 19:59
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    \$\begingroup\$ Then you may be looking at a contactor of some sort, or possibly a solid state relay. \$\endgroup\$
    – user16324
    Commented Dec 16, 2021 at 20:44
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    \$\begingroup\$ Does it preclude operator intervention? With a 600 mV shunt at 40 amps a transistor could reset a latching relay. \$\endgroup\$ Commented Dec 16, 2021 at 23:02

4 Answers 4

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I see 3 solutions :

  1. If you are allowed to, simply cut the power supply of the H bridge : it is still independent from your main system, and it's far easier (a mosfet or a relay is enough, and you don't have to worry about voltage spikes or energy dissipation, as this should already be taken care of by the H bridge itself). EDIT : I have just seen the comment you added that you don't have access to anything except ground, the signal and the 2 wires going to the motor. If it's really so, then it won't work. But if for example you have access to the battery, then you can cut the current right at the output of the battery

  2. If you are not allowed solution 1), then you might consider using a relay. To avoid voltage spikes, you might put 2 zener diodes (with nominal voltage above the normal voltage of the supply) in serie (one in each direction) between the 2 sides of the motor: they will clamp the over-voltage. You can find zener diodes with very high current ratings. Just be carefull about power : if you have a lot of inertia, then the zener diodes are likely to overheat. If that's the case, you might use a relay with 2 position, and when you disconnect the motor from the H-bridge, you connect a power resistor between the 2 sides of the motor : the zener diodes will then only conduct for the short time until the relay finished to change position

  3. How unlikely is that emergency stopping. If it is only theoretical, then a simple relay on one of the motors wire is enough : you will get voltage spikes, it's not unlikely that you brake something, but the motor is no longer powered. If you expect the likelyhood of using this emergency higher than 1%, then forget about this solution

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  • \$\begingroup\$ Not sure why TVS style back to back Zeners didn't occur to me, I think that will pretty cleanly solve the flyback voltage issue if I can find a part that can withstand the current. Luckily this is just a safety backup, so it shouldn't have to regularly endure the shutoff condition. I'll have to look into using relays a bit more to determine if I can properly drive them with the signals I have available. \$\endgroup\$
    – Christian
    Commented Dec 16, 2021 at 21:10
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If your 3.3 V line is able to provide at least about 2 mA of current, do the following:

Run the line through a photovoltaic isolator providing ~9V isolated voltage. Get two fat NMOSFETs back to back, sources tied together. Gates tied together. Turn them on with 9 V from the isolator.

When the 3V signal goes low, the MOSFETs switch off slowly and current cannot pass through the motor.

Advantage over a relay is that it is easier to drive and that the transistors turn off slowly. Depending on the size of the motor, due to the slow turn off, you won't need TVS in parallel. For a large motor you will though.

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You could add a resistor (small value less than an ohm) to monitor the current of the bridge (between ground and the bridge). You will have to monitor this with an ADC or have a dedicated circuit to stop the drive to the H-bridge transistors in case of a high current.

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    \$\begingroup\$ I dont think adding a very small resistor provides any protection. \$\endgroup\$
    – Miss Mulan
    Commented Dec 16, 2021 at 20:01
  • \$\begingroup\$ The resistor is not the protection. It is a means to measure the current thru the bridge and act if a high current is detected. \$\endgroup\$
    – Rodo
    Commented Dec 16, 2021 at 20:56
  • \$\begingroup\$ @Rodo I appreciate the response, but please see my reply to Tony Stewart in the comments of my original post. We do have an integrated bridge driver that monitors current sense resistors, but we are looking for an external, redundant protection. \$\endgroup\$
    – Christian
    Commented Dec 16, 2021 at 21:24
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Why don't you connect a fuse in series with the motor? In order for the fuse to open the circuit at 3.3V divide that by the resistance of the motor Rm and buy a fuse of the same rating with the current through the motor at 3.3V.

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    \$\begingroup\$ The question asks about cutting off a high-current motor from an external stimulus, not cutting off a motor due to overcurrent. \$\endgroup\$
    – nanofarad
    Commented Dec 16, 2021 at 20:19
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    \$\begingroup\$ Not to mention that it is definitely NOT how a fuse is selected \$\endgroup\$
    – Maple
    Commented Dec 17, 2021 at 7:23
  • \$\begingroup\$ I missed that issue as well - this is dangerous advice and not how a fuse is selected. Please amend. \$\endgroup\$
    – nanofarad
    Commented Dec 17, 2021 at 21:42

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