Voltage tripping circuit

[ANTALIFE]

Gidday

I made the following circuit to try and monitor when the voltage of VLINE goes too low.



What I was hoping to achieve was: when VLINE goes below ~4V then the red LED would turn on.
What I got was: the LED would only turn on when VLINE was below ~8V
In the end I was hoping to use this on the output of a LM317 to display when the output has been shorted.

With all that said would any grey beards be able to offer some input as to what rookie mistakes I made? 

Cheers

[jeroen79]

Isn't the LED raising the mosfet a few volts?

[Simon]

You have a couple of problems here. Firstly you are relying on the gate threshold voltage which is probably going to be variable from batch to batch. Secondly that gate threshold voltage needs to appear between the source and the gate. But you have put a load in line with the source which means that the source does not sit on ground. Your input reference is to ground not to the source of the MOSFET. As your MOSFET starts to conduct the current through the load your LED and resistor causes a voltage drop on the load which pushes the source of the MOSFET of aboveground. Your input reference voltage is still referenced to ground so the MOSFET is now seeing less voltage so as soon as it tries to turn on it actually can't turn on and eventually it will take 8 V so that there is probably 4 V across the source to gate and for volts across your load. If you put your load on the train to the positive supply you will get rid of that problem but you are still dealing with a device that does not have a specific turn on voltage that you can rely on. The gate threshold voltage will vary from batch to batch it will vary with supply voltage current through the MOSFET temperature and pretty much which way the wind is blowing today. Your best solution is to use a comparator which will have a definite on and off state at a very specific threshold voltage that you set.

[ANTALIFE]

Isn't the LED raising the mosfet a few volts?

You have a couple of problems here. Firstly you are relying on the gate threshold voltage which is probably going to be variable from batch to batch. Secondly that gate threshold voltage needs to appear between the source and the gate. But you have put a load in line with the source which means that the source does not sit on ground. Your input reference is to ground not to the source of the MOSFET. As your MOSFET starts to conduct the current through the load your LED and resistor causes a voltage drop on the load which pushes the source of the MOSFET of aboveground. Your input reference voltage is still referenced to ground so the MOSFET is now seeing less voltage so as soon as it tries to turn on it actually can't turn on and eventually it will take 8 V so that there is probably 4 V across the source to gate and for volts across your load. If you put your load on the train to the positive supply you will get rid of that problem but you are still dealing with a device that does not have a specific turn on voltage that you can rely on. The gate threshold voltage will vary from batch to batch it will vary with supply voltage current through the MOSFET temperature and pretty much which way the wind is blowing today. Your best solution is to use a comparator which will have a definite on and off state at a very specific threshold voltage that you set.

Just to confirm one other thing, is this the correct way of controlling a load (R2 + LED) with a P-channel MOSFET? I though that you always had to know what the Source voltage is sitting at if you want to control the switch without a high-side driver.

[Simon]

Isn't the LED raising the mosfet a few volts?

You have a couple of problems here. Firstly you are relying on the gate threshold voltage which is probably going to be variable from batch to batch. Secondly that gate threshold voltage needs to appear between the source and the gate. But you have put a load in line with the source which means that the source does not sit on ground. Your input reference is to ground not to the source of the MOSFET. As your MOSFET starts to conduct the current through the load your LED and resistor causes a voltage drop on the load which pushes the source of the MOSFET of aboveground. Your input reference voltage is still referenced to ground so the MOSFET is now seeing less voltage so as soon as it tries to turn on it actually can't turn on and eventually it will take 8 V so that there is probably 4 V across the source to gate and for volts across your load. If you put your load on the train to the positive supply you will get rid of that problem but you are still dealing with a device that does not have a specific turn on voltage that you can rely on. The gate threshold voltage will vary from batch to batch it will vary with supply voltage current through the MOSFET temperature and pretty much which way the wind is blowing today. Your best solution is to use a comparator which will have a definite on and off state at a very specific threshold voltage that you set.

Just to confirm one other thing, is this the correct way of controlling a load (R2 + LED) with a P-channel MOSFET? I though that you always had to know what the Source voltage is sitting at if you want to control the switch without a high-side driver.

Oh I see it a P-channel I always get MOSFETs mixed up. It could be down to the gate threshold voltage then. I think your zenner might be backwards. This circuit is never going to be exact either way you should be using a comparator. If you want to use a single transistor then a BJ T would be more reliable as 0.7 V conduction point is fairly constant.

[ANTALIFE]

Isn't the LED raising the mosfet a few volts?

You have a couple of problems here. Firstly you are relying on the gate threshold voltage which is probably going to be variable from batch to batch. Secondly that gate threshold voltage needs to appear between the source and the gate. But you have put a load in line with the source which means that the source does not sit on ground. Your input reference is to ground not to the source of the MOSFET. As your MOSFET starts to conduct the current through the load your LED and resistor causes a voltage drop on the load which pushes the source of the MOSFET of aboveground. Your input reference voltage is still referenced to ground so the MOSFET is now seeing less voltage so as soon as it tries to turn on it actually can't turn on and eventually it will take 8 V so that there is probably 4 V across the source to gate and for volts across your load. If you put your load on the train to the positive supply you will get rid of that problem but you are still dealing with a device that does not have a specific turn on voltage that you can rely on. The gate threshold voltage will vary from batch to batch it will vary with supply voltage current through the MOSFET temperature and pretty much which way the wind is blowing today. Your best solution is to use a comparator which will have a definite on and off state at a very specific threshold voltage that you set.

Just to confirm one other thing, is this the correct way of controlling a load (R2 + LED) with a P-channel MOSFET? I though that you always had to know what the Source voltage is sitting at if you want to control the switch without a high-side driver.

Oh I see it a P-channel I always get MOSFETs mixed up. It could be down to the gate threshold voltage then. I think your zenner might be backwards. This circuit is never going to be exact either way you should be using a comparator. If you want to use a single transistor then a BJ T would be more reliable as 0.7 V conduction point is fairly constant.

Roger that, will give it a go with a BJT but in the end will probably end up using a comparator instead. Thanks for the help

[Gyro]

If you want reasonable accuracy then this looks like a good application for a TL431. Probably cheaper than the mosfet.

[Audioguru]

Oh I see it a P-channel I always get MOSFETs mixed up. It could be down to the gate threshold voltage then. I think your zenner might be backwards.

1) Please study Mosfet symbols.
2) Please learn that the "threshold voltage" of a mosfet is when it is practically turned off (a current of only 0.25mA) and not when it is turned on.
3) Please correctly spell "zener" diode and learn about how it works.

[fubar.gr]

What I was hoping to achieve was: when VLINE goes below ~4V then the red LED would turn on.
What I got was: the LED would only turn on when VLINE was below ~8V
In the end I was hoping to use this on the output of a LM317 to display when the output has been shorted.

With all that said would any grey beards be able to offer some input as to what rookie mistakes I made? 

Cheers

The zener drops the input voltage by 3 volts. So the voltage at the mosfet gate will be Vline - 3 Volts.

P -type mosfets will be turned off as the gate voltage increases up to Source voltage.

Your source voltage is 6.6 Volts, so when Vline = 9.6 Volts, the voltage at the gate will be 6.6 Volts. Therefore at Vline= 9.6 volts the mosfet is fully turned off.

Don't increase the Vline voltage too much above 9.6 volts, because then the gate gets reverse biased and this might damage the mosfet. See in the datasheet how much reverse voltage the gate of that particular mosfet can tolerate.

[Zero999]

A similar thing was discussed a couple of weeks ago. The TL431 and a transistor will do the job.

https://www.eevblog.com/forum/beginners/voltage-level-indicator/msg875244/#msg875244

The values for R1 & R2 can be calculated using the site linked below:
http://solderer.tv/tl431-calculator/