# OpAmp/Zener Low Voltage Cut-off Circuit

Discussion in 'The Projects Forum' started by somegeek, Jan 15, 2013.

1. ### somegeek Thread Starter New Member

Nov 15, 2012
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OpAmp/Zener Low Voltage Cut-off Circuit

I haven't worked with opamp components in years(since college which I vaguely recall) however it was suggested to me to use a zener diode and an opamp to build a circuit for my kids' led boxes I built them. I'd like to build a cut-off circuit so that when the battery pack(4.8V NiCd) drops to 4V, the power to the light circuits is shut off and they know when they really need to charge them.

I did some reading last night and watched some youtube videos but this is still lost on me. It was suggested to use a zener diode for the reference voltage as one input and the source voltage as the second input and not use Rf. I was using circuitlab.com to build / test circuits to no avail.

Also saw where folks used mosfets as a switch as well vs an OpAmp.
Can someone please point me in the right direction? Appreciate any input.

Thanks,
somegeek

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2. ### Ron H AAC Fanatic!

Apr 14, 2005
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How much current does the LED box draw?

3. ### somegeek Thread Starter New Member

Nov 15, 2012
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(19) 3.2V 20mA LEDs with 100Ω resistors, when all are turned on, 380mA calculated.

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4. ### ErnieM AAC Fanatic!

Apr 24, 2011
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I like the box, looks fun.

What turns everything on and off? Is there one master transistor switch inside? (If not, you'll need one).

Here's a direction to look at:

I did not look up D3 but you want to pick a voltage under the 4V min... so this voltage stays constant while the battery is dying. The other input to the amp comes from a pot: the pot is adjusted to give you the other input's voltage when the battery hits the 4V wall.

When the battery dips under 4V the op amp (which is connected as a comparator) will go high: for low just swap the inputs.

You'll also need a single supply op amp for this. MCP6231-E/P is the cheapest I could find at .38 a piece at digikey.

I expect this to be a discussion thread, so come on back with some questions.

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5. ### Ron H AAC Fanatic!

Apr 14, 2005
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Do you want to switch the low (ground) side of the load, or the high side?

6. ### somegeek Thread Starter New Member

Nov 15, 2012
15
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I really appreciate the help here!

Zener diode I'm using in this setup = D3 = 3.9V V_z

Currently everything in these boxes is wired as simple as it could be. Each LED has it's own individual 100Ω resistor in line. There's five switches controlling power to five sets of LEDs connected to a battery pack(no transistor). I'd like to shut power off(with a MOSFET?), between the charge port and the LED negative bus/rail. My Hakko 888 should be arriving tomorrow and I'll need to order components and prototype board material to make up these two boards for each of my son's boxes. Done putting time in on my \$7 pencil.

I played with this setup a bit, flipped the inputs to the OpAmp and with the 500k pot tap fraction set to .97 I see the following:

Battery at 3.9V, OpAmp output is low.
Battery at 4.0V, OpAmp output is high.

Sweet!!

For my own edification... at 4.8V on my pack, I see 3.9kV out from the OpAmp. Is this output connected straight to the base of a transistor or MOSFET or somehow stepped down? Out of curiosity, I threw in the transistor with rails and saw that voltage go high at around 2.7V.

I was thinking of switching the low side on, however that's only due to having seen the load position in other schematics. Which do you guys suggest?

Thanks!
somegeek

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Last edited: Jan 16, 2013

Nov 15, 2012
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8. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Good, always nice to see the sim work. Nice video, glad to hear your kids like it too.

Have you considered how you are going to turn the power on and off for low battery? That's going to influence how you detect the low voltage. A low gate voltage (logic level) MOSFET should do nice.

One show stopper I just though of is once your low battery circuit trips and turns the power off... the battery voltage will promptly rise and turn things back on, and it will keep doing that for a long time. So the comparator will need some hysteresis to do this: it means there is some feedback from the amp output back to the reference so it pulls it down some when engaged.

See if Google helps you, or we can find a link for that too.

9. ### wayneh Expert

Sep 9, 2010
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I wonder if a latch - that would only reset except on a toggle of the manual switch - wouldn't be better. Ok, that's just extreme hysteresis I guess.

10. ### somegeek Thread Starter New Member

Nov 15, 2012
15
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Is there a MOSFET part number you can recommend for this application?

I have my homework now. I will be able to dig into this tomorrow evening.

Thanks,
somegeek

11. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Well since you asked so nicely... a STP27N3LH5 may work well here... I'm not 100% sure as the data sheet for 4V operation is only "typical" and not worst case, and it's crammed into the corner of the graph (figure 6 of the data sheet). But I would try that one for a buck.

(When I get stuff from Digi Key I pick the cheapest USPS shipping, 2.50 or so, still comes in 2-3 days to NY.)

12. ### somegeek Thread Starter New Member

Nov 15, 2012
15
0
Thanks for this and noted. I do like the cheap shipping too. Beats gas \$ driving locally or the \$7 minimum shipping some vendors have for stuff.

somegeek

13. ### somegeek Thread Starter New Member

Nov 15, 2012
15
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I watched this video on youtube and think I'll need a 200Ω Rf to create .4V of hysteresis. I calculated this using the 4.0V point. Is this correct?

Thanks,
somegeek

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14. ### somegeek Thread Starter New Member

Nov 15, 2012
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Any suggestions on the pot type? Are the single turn pots difficult to tune?

Thanks,
somegeek

15. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
197
Neat project.
But, with the low ohm resistors a LOT of current will be unnecessarily wasted through the zener diode and output. The circuit will still work the same with all resistors multiplied by a factor of 100-200, and draw much less current.

EDIT: Something else occurred to me. Swap the input signals with hysteresis to + and maybe eliminate the pot, because the zener sets the trip at 3.9v. If that is close enough then just connect the input through a resistor to the battery. (hysteresis is positive feedback, not negative feedback)

Last edited: Jan 19, 2013

Nov 3, 2012
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17. ### Ron H AAC Fanatic!

Apr 14, 2005
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I would do it something like that. A 3.9V zener is a really poor reference. It has a soft knee and relatively high dynamic resistance. LM385 can operate as low as 10uA, and has a 1Ω dynamic impedance.
I have been working on a similar circuit. Here it is.

• ###### LED box low battery switch sch.png
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18. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Those boxes are getting pretty fancy!

Pretty soon you'll be adding a uC and wireless to them.

19. ### Ron H AAC Fanatic!

Apr 14, 2005
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Fancy?
In the sim, that LED box was a 12Ω resistor.

20. ### phototron New Member

Nov 22, 2013
22
1
Hi folks, this is my first time on the forum. I know this thread is aging, but I have a need for a circuit that performs the same function. I was wondering, did the original poster get a circuit to work?

I will be using 4 NiMH cells in series @ 4.8v to power a resistor to produce around 1.5 - 2 watts of heat. This is only to prevent a hummingbird feeder from freezing on nights when the temperature falls below zero C. (I live in Victoria, BC, and despite the sugar the water will freeze at -1°).

Naturally I want to protect the NiMH cells from falling below about .9v, and so I am trying to learn how to make a low voltage cut-off circuit, which lead me through the tangled inter web to this thread and forum!

I do intend to enter college soon for and electrical engineering program, however, in the mien time we have cold hummingbirds.

I can buy the components, and I have tools and multimeters and soldering irons, but I am unsure as to how to adjust the resistance values in the example circuits to achieve the desired cut-off point.

Guidance would be appreciated! Thanks in advance.