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I am using a 4S LIPO-battery and a commercial constant current LED driver to drive a LED setup. Data Sheet for the driver

To avoid abusing the LIPO battery, I would like to build a low-battery cut-off using the driver's on-board dimming input. The driver has both PWM and analog dimming inputs.

As for analog dimming input, the signal voltages are as follows:

0 ... 0.2V -> the driver shuts off (battery voltage 13.0 or below)

0.2V ... 1.3V -> dimming in function (battery voltage 13.0 ... 13.5V)

1.3V ... 8.0V -> LED driver in 100 % (battery voltage 13.5V or more)

What kind of circuitry would solve my case here?

Thanks!

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  • \$\begingroup\$ Microcontroller. \$\endgroup\$ – Phil Frost Nov 25 '14 at 12:17
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I think a one or two channel op-amp and a precision voltage shunt would be your friend here. for the shutoff, simply make a voltage divider that at 13.0V, would output an equal voltage as your voltage shunt. Connect that voltage into the one input of the op-amp and the voltage regulated by the voltage shunt to the other. Connect the op-amp output to the PWM dimmer. That will take care of the low-voltage drop out.

The "work" on this circuit will be primarily in finding a particular op-amp and resistor combination to create a dimming profile that you like. (I.E. does it dim the right amount at 13.2V, 13.3V, etc.) To be clear, I am using the term "op-amp" loosely to include comparators and similar op-amp variants.

Additionally, if you want to ensure that everything is 100% on at >13.5V you can use a 2nd op-amp and a second voltage divider similar to above but with reverse the hookups to the op-amp so that it switches on at higher voltages and off at lower.

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  • \$\begingroup\$ Thanks for the help so far! Did you mean something like this? link \$\endgroup\$ – Marko Nov 26 '14 at 14:34
  • \$\begingroup\$ Pretty close. I like things like the lm4041 (or an op amp with a built-in reference voltage) rather than a zener, if accuracy is important. Not sure if you need that much feedback on your opamp, since hysteresis isn't needed much for this. At the end of the day, the main issue will be: do you like the dimming profile? You've got a good start, and post back if you need help tweaking it to your living. \$\endgroup\$ – SvdSinner Nov 26 '14 at 18:04
  • \$\begingroup\$ Thanks again! The Zener seems to be a little hard to adjust precisely. So, I figured out I might use a voltage regulator instead of Zener. Perhaps a MC7810 TO-220 would do the trick, don't you think? data sheet \$\endgroup\$ – Marko Nov 27 '14 at 8:46
  • \$\begingroup\$ It would be a lot more accurate than a Zener. It is probably not the best option if you are concerned about minimizing power use, but quite likely your LEDs consume enough that it won't matter.q \$\endgroup\$ – SvdSinner Nov 27 '14 at 11:44

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