Need help with Low voltage protection circuit

[flinx]

Re: Need help with Low voltage protection circuit

"The idea of hysteresis is to provide some feedback from pin 1 to pin 3 of the IC with a resistor, however you will need to add a resistor between the zener and pin 3 of the IC because it would otherwise have no way to increase the threshold. Looking at this, you will likely need to use the other amplifier in the LM358 to provide more consistent triggering, mostly because the voltage output of the LM358 will depend on the setpoint.."

+1

BTW, using compactor IC such as LM393 with open Collector output is more suitable than using OPAMP.

I have 10 lm393.

[eccerr0r]

Re: Need help with Low voltage protection circuit

Reproduction royalties to ... guess.

As far as I know this *should* work but no guarantees. Circuit on the left is the cutoff circuit, right is the original circuit. If you plan to hack the existing board, be careful there are subtile differences.

[flinx]

Re: Need help with Low voltage protection circuit

Reproduction royalties to ... guess.

As far as I know this *should* work but no guarantees. Circuit on the left is the cutoff circuit, right is the original circuit. If you plan to hack the existing board, be careful there are subtile differences.

I built the circuit except the only thing I do not have is a 15v zener. closest I have is 12v.

I could try scrounging one off a board if they had readable identification marks.

[petehall347]

Re: Need help with Low voltage protection circuit

add a 3v one to the end of it

[eccerr0r]

Re: Need help with Low voltage protection circuit

12V is fine, just needs to be less than 20V (Vgsmax) but above 5V (Vgsth). Circuit is poorly tuned and behavior/power consumption can be improved by changing component values. Should be able to get it less than 10mA draw, but at 10mA it's still pretty horrible. This also depends on voltage input too.

Note that the diode will limit current draw value, probably should omit that 1N4007 and short it out.

Again, no guarantees. It still has the problem of being setpoint dependent behavior though it should be somewhat linear, and because of this I did not do threshold value calculations.

Also the 1M resistor is probably superfluous, the zener itself should be sufficient to turn the mosfet off.

[flinx]

Re: Need help with Low voltage protection circuit

I used 3 5.1v zeners in series. I do not have the IRF9540n in the schematic and am using my IRF5305.

I have tested the circuit and since my power supply cannot go much higher than 19 volts (I have a better one on the way). I have the pot connected to In+, in-, and pin 3 of the lm358 set so that the transistor turns off at 17.26v and on at 16.3v.
The mosfet gate voltage is 6v when the transistor turns off, and 8.5v when it turns on.

The mosfet seems to be on all the time.

The second 10k pot seems to do nothing.

[eccerr0r]

Re: Need help with Low voltage protection circuit

I'd expect the second pot to do very little, it should change the voltage spread slightly.

Do not use three zener diodes in series. The circuit depends the single zener to discharge the gate - if wired correctly, should drop below 1V (with respect to source, of course.)

Also it should be mentioned that the voltage at pin 3 will change depending on the voltage at pin 1. What's unfortunate about this and the original circuit is that ideally the amplifier is running at fixed voltage instead of the varying voltage to ensure the behavior of the circuit is reliable.

Ensure the voltage at pin 1 will switch between IN- plus around ~0.2V to IN+ minus ~1.5V. If it doesn't, something is wrong.

[flinx]

Re: Need help with Low voltage protection circuit

I'd expect the second pot to do very little, it should change the voltage spread slightly.

Do not use three zener diodes in series. The circuit depends the single zener to discharge the gate - if wired correctly, should drop below 1V (with respect to source, of course.)

Also it should be mentioned that the voltage at pin 3 will change depending on the voltage at pin 1. What's unfortunate about this and the original circuit is that ideally the amplifier is running at fixed voltage instead of the varying voltage to ensure the behavior of the circuit is reliable.

Ensure the voltage at pin 1 will switch between IN- plus around ~0.2V to IN+ minus ~1.5V. If it doesn't, something is wrong.

I checked the workings of the lm358 part of the circuit with one of the battery alarms it is based on, and except for the hysteresis they both work pretty close to the same.

I then started over and rebuilt the design in your pdf from scratch again on my breadboard.
The differences between mine and yours are:
transistor is a 9014n I also tried a BC550 (accounted for pin differences) I do not have a pn2222a
mosfet is IRF5305
12v zener
removed 1n4007

Hysteresis is still a little over 1v which is good.
Nothing I do short of dropping the supply voltage below 10v shuts off the mosfet.

The behavior of the circuit is like this:
Changing the supply voltage causes 9014n to turn on at 17.44v and off again at about 18.55.
At the same time the voltage coming out of the transistor is 7.67vdc, AND at the same time voltage rises on the gate of the mosfet from about 8.4 to about 8.59 then drops back to 6.97 AND at the same time voltage on pin 1 of the lm358 goes from .076v to about 17.12v instantly.
It looks something like this when graphed:
edit graph removed because it is wrong.

just in case I did something dumb and don't see it here is pics:

[flinx]

Re: Need help with Low voltage protection circuit

here is the corrected graph I hacked together in excel that illustrates what is going on.

[eccerr0r]

Re: Need help with Low voltage protection circuit

oh fiddlesticks this is what happens when I don't play with discrete P-channel devices and assume it works like PNP. You're right, there's a problem there, really need to have simulated it and got my current directions wrong. I'm still not going to simulate since there's no warranty expressed or implied :-P

Now I was hesitant on doing the lower left no-BJT version in the first place, though it may well just work. Do not use a "logic level" MOSFET here, IRF9540 and your IRF5305 should work fine. If you you have to use a logic level or the no-BJT version doesn't work, try the upper left version with three BJTs.

[flinx]

Re: Need help with Low voltage protection circuit

IT VERKS! IT'S VERKING!
hysteresis is about 1.09v second pot has barely any measurable effect. maybe .04 volts?
But it VERKS!

now if my power supply would ever get here from china I could set the voltages proper.

To easyeda!

Can the lm393 be substituted for the lm358? pinouts are the same.

[eccerr0r]

Re: Need help with Low voltage protection circuit

Reduce that 100K resistor in line to 47K or 68K to increase hysteresis, and you need the LM358, the LM393 will not work as drop in.

[budm]

Re: Need help with Low voltage protection circuit

So you are using the lower left circuit utilizing both opamp?
What is the DCV at output pin 1 and pin 7 when it is not trip and when it trips?

[eccerr0r]

Re: Need help with Low voltage protection circuit

For LM358, when not tripped pin 1 should be around the same voltage as pin 8, minus 1.5 volts or so. This was my main concern for direct driving in the first place.
Pin 7 should be no more than 0.2V.

When tripped, swap the voltages around.

Dang if this really doesn't exist anywhere I should productize this. But this is a ridiculously simple circuit and those chinese mass producers will beat me to the punch. Grr.

[flinx]

Re: Need help with Low voltage protection circuit

Reduce that 100K resistor in line to 47K or 68K to increase hysteresis, and you need the LM358, the LM393 will not work as drop in.

47k gives 1.7 to 2.3 volt spread. excellent

[budm]

Re: Need help with Low voltage protection circuit

I look look at your first circuit:
When battery Voltage is good, the pot is set so the + INPUT (3) is >5.1Vref on the -INPUT (2) so Output (1) swings high (max out = VCC - 1.5V per spec) which turns on the 2N2222 and P-CH MOSFET since now the Gate is toward ground providing > 3V Vg-s.
When battery goes low to the point that - INPUT (3) is < 5.1V, the Output (1) will switch low (0.2V) which will cause the 2N2222 and MOSFET to be off.
Am I missing something?

I think the error is in OP post #46:
I have the pot connected to In+, in-, and pin 3 of the lm358 set so that the transistor turns off at 17.26v and on at 16.3v.

[eccerr0r]

Re: Need help with Low voltage protection circuit

should make it clear that this will not work at 5V. The minimum operating voltage is probably around 8V minimum in order to bias everything properly.

[flinx]

Re: Need help with Low voltage protection circuit

circuit board

with 1oz traces it can handle about 8 amps, with 2oz it can handle 13, and with a 12 gauge bare copper wire soldered to the bottom traces for b- to out- and b+ to source it should be able to handle at least 20 amps.

The mosfet would be laid down, soldered to the board and the output wire taken directly from the tab.

board is 2.6"x1.4" I'm sure it could be made much smaller with SMD but that's not my bag.

also I would most likely not be using this with anything below 14 volts.

[budm]

Re: Need help with Low voltage protection circuit

You should put down Thermal Vias for the MOSFET.
Is the copper plane as heatsink for the MOSFET large enough for it to handle 20A or are you going to use heatsink to keep MOSFET junction at safe temperature?

Also consider VIA Stitching for high current.
https://www.microtype.io/high-power-...n-tips-tricks/

[eccerr0r]

Re: Need help with Low voltage protection circuit

I look look at your first circuit:
When battery Voltage is good, the pot is set so the + INPUT (3) is >5.1Vref on the -INPUT (2) so Output (1) swings high (max out = VCC - 1.5V per spec) which turns on the 2N2222 and P-CH MOSFET since now the Gate is toward ground providing > 3V Vg-s.
When battery goes low to the point that - INPUT (3) is < 5.1V, the Output (1) will switch low (0.2V) which will cause the 2N2222 and MOSFET to be off.
Am I missing something?

I think the error is in OP post #46:
I have the pot connected to In+, in-, and pin 3 of the lm358 set so that the transistor turns off at 17.26v and on at 16.3v.

Yeah I thought the initial version should work, but variations in the components used (specifically, leakage in the S9014/PN2222/... BJT is suspect) in the breadboard seems to make it fail. The later versions with active drive should compensate for non-ideal components.