Hi Everyone,
I have an application where my resistive load and battery share a common ground, and wondered if the circuit below has merit using a N-Channel Mosfet as a low side switch.

 

Hi Everyone,
I have an application where my resistive load and battery share a common ground, and wondered if the circuit below has merit using a N-Channel Mosfet as a low side switch.

Can you explain the complete application?
I ask because this could make a difference.

Also, that zener diode may not be biased correctly to start with. That is probably not the best way to do this.

 

Hi Everyone,
I have an application where my resistive load and battery share a common ground, and wondered if the circuit below has merit using a N-Channel Mosfet as a low side switch.

Maybe not. The circuit you have drawn is a high-side switch and not a very good one.
1. The output voltage will be lower than the supply voltage by Vgs.
2. You will be disappointed by the accuracy of that zener diode.

 

As noted, that's a high-side switch, not low side.
Exactly what is this circuit supposed to do?

 

Can you explain the complete application?
I ask because this could make a difference.

Also, that zener diode may not be biased correctly to start with. That is probably not the best way to do this.

Hi MrAI,
This is for a Laboratory Heater for heating analysis samples, (there is a push to break switch on pin 4 of the rightmost 555 to ground not shown). The new heaters share a ground on the load meaning the loads can no longer be connected high side, and I wished to ask whether it would be possible to switch on the low side as apposed redesign if at all possible.

 

Herewith the original circuit which cannot be used as the new heaters have a grounded terminal;

 

This is a very simple and easy way of Driving an Isolated NFET.
It doesn't care about "Hi-Side" or "Low-Side".
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This is a very simple and easy way of Driving an Isolated NFET.
It doesn't care about "Hi-Side" or "Low-Side".
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View attachment 307525
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Thank you LowQCab ... Let me try track down one of these elusive FDA215's

 

There are also a couple of other manufacturers that make
Photovoltaic-Isolated Gate-Drivers.

DigiKey has ~50 in stock.
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You need a high current P Channel mosfet if switching the high side. I don't see how the FDA215 is going to improve much in this application.

 

That is incorrect.
Any N-Channel-FET can be used with a Photovoltaic-Isolator.
The Isolator actually generates a Voltage for the FET-Gate.

A P-Channel-FET is not required.

The only drawback is that they are not particularly fast,
and therefore may not be practical for High-Speed-Switching applications.
But they can be paralleled for increased Switching-Speed.
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Very clever as I see how it's connected now.

 

Since your load is grounded I would use a P-channel MOSFET so that nearly the whole power supply is applied to the load.

 

Since your load is grounded I would use a P-channel MOSFET so that nearly the whole power supply is applied to the load.

That's what I suggested also but after studying the FDA215 it appears one could connect both sections in series to generate 11 volts across the Gate - Source since the IRFZ44N is not a logic level mosfet. Comments?

 

At 0.1-Ohms, and ~12-Volts, your Heater will draw roughly ~120-Amps.
You will need a very stout N-Channel-FET and a large-Heat-Sink to deal with that much Current.
Not only that,
You will need ~4-gauge Automotive-Starter-Cables between the Battery, and the FET, and the Load.
An IRLZ44N is not going to survive this abuse. It will pop instantly.

I would recommend something like an IXTN660N04T4 , DigiKey has over 1,000 in stock, ~$31.oo each.

What is the Load exactly ???
What is the factory rated Wattage of the Load ???
An average Automotive-Lead-Acid-Battery will only last a few minutes at ~120-Amps of Load.
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If it is only switching a opto speed, then how about an Albright contactor? That would have no trouble with 120A and doesn't need a heatsink!
Use the magnetically latched type, and it doesn't even need continuous drive current.

 

At 0.1-Ohms, and ~12-Volts, your Heater will draw roughly ~120-Amps.
You will need a very stout N-Channel-FET and a large-Heat-Sink to deal with that much Current.
Not only that,
You will need ~4-gauge Automotive-Starter-Cables between the Battery, and the FET, and the Load.
An IRLZ44N is not going to survive this abuse. It will pop instantly.

I would recommend something like an IXTN660N04T4 , DigiKey has over 1,000 in stock, ~$31.oo each.

What is the Load exactly ???
What is the factory rated Wattage of the Load ???
An average Automotive-Lead-Acid-Battery will only last a few minutes at ~120-Amps of Load.
.
.
.

The Load(s) are relatively small heating elements for heating chemical samples for analysis. The 555 on the right is a simple PWM limiting the power dissipated by the respective heating element, and the 555 on the left is a cheap comparator with hysteresis to keep the system off when the Li-Ion battery reaches "critical level". Typically some 20 to 50 Watts are dissipated in the various heaters, whose resistances are in the range of 0.1 to 0.6 Ohms.