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I want to use a 9.1V Zener diode for a ref voltage into a op amp comparator so that when the battery voltage (11-12V DC) drops below 9.5V DC it will cut the supply to the circuit via a P-channel MOSFET.

The supply voltage will go through a voltage divider dropping it by 10% so when it hits the magic level it will still be above the Zener voltage and cause the comparator to drive the gate of the MOSFET low and turn it off. Thus saving my batteries from harmful deep discharge.

Will this work or is there something I'm going to get bitten with?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Also, isn't 100kOhm in series with the zener a bit high? Don't zeners require something in the order of mA to bias them? \$\endgroup\$
    – Dzarda
    May 28, 2014 at 19:14
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    \$\begingroup\$ Comparators generally show open-collector outputs. In this case, in order to properly turn-off your high side P-channel mosfet, include a resistor between Gate and Source. \$\endgroup\$ May 28, 2014 at 19:15
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    \$\begingroup\$ Zeners are weak voltage references until you add a constant current source. Then they become just mediocre. \$\endgroup\$ May 28, 2014 at 19:27
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    \$\begingroup\$ One thing that can "bite you" is the common mode voltage range of the comparator. A common LM393 is outside the guaranteed range of Vdd - 1.5V (yet, unfortunately it will probably work, sort-of) even at 12V. I suggest you swap R1 with R2 and R3 with D1, then swap the inputs. Then everything happens down near the ground where single-supply comparators are happy. \$\endgroup\$ May 28, 2014 at 20:40
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    \$\begingroup\$ At the moment, you have the inputs to the opamp swapped. When the battery voltage is high, the opamp output will also be high, shutting off the P-channel MOSFET. \$\endgroup\$
    – Dave Tweed
    May 28, 2014 at 20:41

2 Answers 2

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Dzarda's comment is valid. You have about 29uA's of current flowing through there at 12V supply and even less as your battery voltage drops. The Zener voltage will depend on the current flow through it. The lower of a resistance you have supplying current, the easier it will be to keep the Zener voltage relatively constant/stable.

You'll definitely want to look into Dirceu's comment and make sure the output of the comparator is rail to rail and isn't open-collector or open drain. If it is, adding a resistor as he recommended will work.

I think the main reason this will likely work poorly is because there's no hysteresis in it. Depending on the current draw of the circuit, you'll end up with some voltage drop due to the internal battery resistance. As soon as this limiting circuit cuts off the current, the terminal battery voltage will jump back up because no current is flowing. That wall cause the circuit to turn back on, which in turn will cause this limiting circuit to shut it off, endlessly turning on and off the circuit. This may work for your purpose, but the main circuit may get quite confused and you'll generate a lot of noise. If you have a comparator with hysteresis built into it, or if you add hysteresis to this circuit, that will be avoided.

Your current resistors show that you'll cutoff at about 10.11 volts: 9.1*1000k/900k = 10.11 V.

Lastly, although Ignacio is correct that Zener's aren't great voltage references, you don't need anything terribly accurate for your application.

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    \$\begingroup\$ Yeah I know it'll cut out at 10.11V in this config but I was just tossing together a quick schematic for viewing purposes and grab wrong size. End circuit will be more like 50k/950k. Good catch on the Iz Dzarda. So the batteries are feeding a regulator network. Should be stable but if these LiPoly batts get down to sub 10V I'm okay with it causing problems as I would want to change them out by then. Ideally I'd like to do this differently but its a size thing and needs to be done yesterday. It's just a cheap safe guard. Just trying to confirm the basic function is "sound". \$\endgroup\$ May 29, 2014 at 3:20
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horta's answer is correct, but I think the best solution is to get rid of the zener. If you search for "micropower voltage reference" you'll find all sorts of better references. As an example, the Linear Technology LT1634 will probably meet your needs, and only requires 10 uA. You'd need to change your voltage divider ratio, of course, but that shouldn't be a problem. And going to a 4 volt reference will pretty much ensure that you don't have to worry about exceeding common mode levels at your comparator.

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