# OpAmp/Zener Low Voltage Cut-off Circuit

#### somegeek

Joined Nov 15, 2012
15
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

#### Attachments

• 18.3 KB Views: 203

#### Ron H

Joined Apr 14, 2005
7,012
How much current does the LED box draw?

#### somegeek

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

#### Attachments

• 75.2 KB Views: 80

#### ErnieM

Joined Apr 24, 2011
7,994
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.

#### Attachments

• 4.1 KB Views: 475

#### Ron H

Joined Apr 14, 2005
7,012
Do you want to switch the low (ground) side of the load, or the high side?

#### somegeek

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

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

somegeek

#### somegeek

Joined Nov 15, 2012
15
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

#### Attachments

• 11.8 KB Views: 78

#### somegeek

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

Thanks,
somegeek

#### tubeguy

Joined Nov 3, 2012
1,157
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:

#### Ron H

Joined Apr 14, 2005
7,012
Or use a pot and a smaller voltage zener or 1.25-2.5v reference:
View attachment 51056
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.

#### Attachments

• 50.4 KB Views: 119
Last edited:

#### thatoneguy

Joined Feb 19, 2009
6,349
Those boxes are getting pretty fancy!

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

#### Ron H

Joined Apr 14, 2005
7,012
Those boxes are getting pretty fancy!

Pretty soon you'll be adding a uC and wireless to them.
Fancy?
In the sim, that LED box was a 12Ω resistor.

#### phototron

Joined Nov 22, 2013
22
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.