N-Channel Mosfet as a Low Side Switch

Thread Starter

AnthonyB

Joined Nov 12, 2015
5
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.
 

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MrAl

Joined Jun 17, 2014
11,256
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.
 

Ian0

Joined Aug 7, 2020
9,455
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.
 

Thread Starter

AnthonyB

Joined Nov 12, 2015
5
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.
 

sghioto

Joined Dec 31, 2017
5,368
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.
 

LowQCab

Joined Nov 6, 2012
3,935
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.
.
.
.
 

sghioto

Joined Dec 31, 2017
5,368
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?
1699985279763.png
 

LowQCab

Joined Nov 6, 2012
3,935
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.
.
.
.
 

Ian0

Joined Aug 7, 2020
9,455
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.
 

Thread Starter

AnthonyB

Joined Nov 12, 2015
5
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.
 
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