Strange Regulator/MOSFET behaviour

Thread Starter

sirch2

Joined Jan 21, 2013
1,037
Quick summary... I am running a MOSFET as a switch driven by an Arduino. The MOSFET switches a voltage regulator which powers a bridge circuit and instrumentation amp. It all seems to run okay, the MOSFET switches, the regulator outputs 5v the bridge circuit turns on but the bridge/instrumentation amp is always at about 4.7V. If I power the bridge circuit from a power supply at 5V it works fine and I can tweak a pot to get the bridge to a set point but not if I power it from the MOSFET/regulator.

The MOSFET REG -
1676146642612.png



The bridge circuit
1676146676468.png

Any thoughts? One thing I did notice is that if I connect the bridge ground to the Arduino ground rather than pin 2 of the LM78L05 the bridge is at about 2.7V but from pin 2 of the LM78L05 it is at 5V
 

ronsimpson

Joined Oct 7, 2019
2,904
What is the voltage on 78L05-1?
What do you want on "5VJOut?
Q1-1 is that ground?
What does R1 connect to?
There are so many questions.
1676147755813.png
 

BobTPH

Joined Jun 5, 2013
8,687
You MOSFET is wired entirely wrong.

The gate should be controlled by the Arduino, you have it grounded.

But then, an N-channel is the wrong type, anyway. You need p-type controlling the input to the regulator.
 

Thread Starter

sirch2

Joined Jan 21, 2013
1,037
Hmm, good spot about the MOSFET, the diagram is wrong, not sure how I managed that the Gate goes to the Arduino the Source to ground. I am running the Arduino at 3v3 but the bridge circuit needs 5V so my understanding is that I can't use a P-Channel because the gate would not go high enough to turn on? I'll double check the board to make sure I have the MOSFET correct though ...

For info JOut is the power to the bridge circuit, basically the Output of the LM78L05 is connected to X1 of the bridge circuit. Thes are built as two separate units and I don't have a combined diagram. The rest of the first circuit is really just a bunch of sockets for various modules so wouldn't add much
 

Thread Starter

sirch2

Joined Jan 21, 2013
1,037
So this is the actual MOSFET arrangement
1676150353153.png

VBattery is about 8V from two 3.7V Lithium batteries, JOut is 5V when the MOSFET is on.

As I said it seems to work as expected, the MOSFET turns the LM78L05 on and off, its just leading to odd behaviour from the bridge circuit. The bridge circuit works fine when powered from a power supply
 

BobTPH

Joined Jun 5, 2013
8,687
my understanding is that I can't use a P-Channel because the gate would not go high enough to turn on?
You need another transistor, n-MOSFET or NPN BJT to switch the gate of a p-channel. I try to avoid switching ground on anything involving electronics and signals. It is fine for a light or a motor or relay.
 

dl324

Joined Mar 30, 2015
16,732
I am running the Arduino at 3v3 but the bridge circuit needs 5V so my understanding is that I can't use a P-Channel because the gate would not go high enough to turn on?
You just need to use an N type device to drive the P channel MOSFET.

Why do you have so much whitespace, extraneous connection dots, and the awkward flow?
1676152473391.png
R1 isn't needed.
 

ElectricSpidey

Joined Dec 2, 2017
2,753
I can't possibly see how that configuration can possibly turn a 3 terminal regulator on and off.

If you let the common connection(ground) float the output will go to the supply minus the dropout.
 

ronsimpson

Joined Oct 7, 2019
2,904
Some one tell me how this works.
I see that it works when the MOSFET is on.
When ground is cut open, 78L05 pin 2 can float up or be pulled down to -1V before it hangs up on the diode in the MOSFET.
1676153399821.png
 

ElectricSpidey

Joined Dec 2, 2017
2,753
I think the correct way would be to set up a MOSFET as a switch on the output of the regulator and before the output cap if it's a big one.

And leave a small cap right at the regulator to prevent oscillations.

Or the switch can be before the regulator to save a little power.
 
Last edited:

ElectricSpidey

Joined Dec 2, 2017
2,753
I just ran an LTSpice sim using the supplied LT1083-5 regulator with a 10 ohm resistor and 8 volt supply.

The output was just as expected 5 volts and 500mA.

Then I opened the ground pin and the regulator did just what I expected.

The voltage went up to 7 and the current went to 700mA. (aprox)
 

ElectricSpidey

Joined Dec 2, 2017
2,753
I know that's not the same design but I couldn't find a 78XX model and I assumed that it would expose any inherent reason that a 3 terminal regulator wouldn't behave the same.

The only reason I know how the 78XX regulators behave when you pull the ground pin is I have seen it on the bench, long before I used any simulators.

I will have a look see if I can find a 78XX series model and do the simulation again.
 

Ian0

Joined Aug 7, 2020
9,533
I know that's not the same design but I couldn't find a 78XX model and I assumed that it would expose any inherent reason that a 3 terminal regulator wouldn't behave the same.

The only reason I know how the 78XX regulators behave when you pull the ground pin is I have seen it on the bench, long before I used any simulators.

I will have a look see if I can find a 78XX series model and do the simulation again.
Any sort of 5V regulator will be trying to maintain 5V between gnd and output terminals. If the ground terminal is not connected to ground, the only way it can go is up, due to the quiescent current.
 
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