I'm trying to build a schematic which will turn on a relay in case motorbike battery voltage will reach some level. I've ended up with the following schematic.


simulate this circuit – Schematic created using CircuitLab

Input voltage can vary from 12 to 14.5 V. So the plan was to compare half of the input voltage from the 1:1 divider to the Zener voltage, which is always 6.8V. So when input voltage exceeds 13.6 volts, the transistor should energize relay coil. However there is a hysteresis in relay switching, from 10.1 to 12.8 volts. It switches on at 12.8, and then releases at 10.1 volts. What is the reason of that, and how can I adjust hysteresis loop to my needs?

  • \$\begingroup\$ Is that a typo in the title? an LM331 is a Voltage to Frequency Converter, not an opamp. \$\endgroup\$
    – brhans
    Commented Jun 14, 2018 at 20:24
  • \$\begingroup\$ How is your circuit constructed (breadboard, pcb, ...)? A photo of the physical construction would help. Is it possible there's some resistance in either the connection from the power to the relay, or in the ground return path from the relay back to the power source? \$\endgroup\$
    – The Photon
    Commented Jun 14, 2018 at 20:25
  • 1
    \$\begingroup\$ Also, LM311 has two output pins, while your schematic only shows 1. Can you edit the schematic to show the actual connections to the LM311? \$\endgroup\$
    – The Photon
    Commented Jun 14, 2018 at 20:26
  • 1
    \$\begingroup\$ The schematic you show simply won't work since you have not connected the LM311 Emitter at all. It should be connected to ground. \$\endgroup\$ Commented Jun 14, 2018 at 20:30
  • \$\begingroup\$ @The Photon, thanks to everyone. There was a typo in the title, fixed now. And my problem was that I didn't connected emitter! After proper connection relay works as supposed. But I have another problem, the relay transistor is getting very hot. \$\endgroup\$
    – Zhenek
    Commented Jun 14, 2018 at 22:25

1 Answer 1


12V relays to turn off a scooter from low voltage will create many new problems.

Most automotive power relays activate slowly “must switch” at 50% of rated voltage so 7V for a 12V relay rated for 14V nominal and Must switch off at 10% or 1.2V voltage but “may” switch off sooner.

All great relays have at least 50% hysteresis. Yours must have a weak spring.

The new problem is that batteries also have memory or hysteresis voltage effects with step load currents and this will force you to define acceptance criteria depending on current due to the ESR in batteries.

Motors also arc contacts badly until the motor stops or is switched off electronically forcing you to get a big snubber to absorb the motor/generator energy.

At the end of the day , you will be better off having a loud alarm buzzer that alerts the driver to stop. You could also sense high current and low voltage and logically AND them you can get by with a power calculated threshold to enable the alarm or some other soft limiting feature for power with the throttle control.

The alarm threshold might be like reserve fuel level , leaving enough time to drive to a charge port.


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