Using MOSFET as load for voltage regulator?

Hi

I have a DC-DC voltage regulator (Input 12V, Output 3.3V/1A). I would like to use a mosfet as a dummy load to vary the output of the regulator, steadily increase the current. Would this be possible with the mosfet alone or would I have to use the mosfet as a switch for a seperate dummy load (resistor)?

Thank you.

Hi,

3.3V x 1.0A = 3.3W .. this is the power you need to dissipate as heat. --> You will need a heatsink.

I would like to use a mosfet as a dummy load to vary the output of the regulator

Click to expand...

A bit pedantic: You don´t want to "vary the output". Either you want to adjust the current or the load resistance or the load impedance.

Would this be possible with the mosfet alone

Click to expand...

A MOSFET alone will not work... because then there is nothing for adjusting...

****
There are several solutions:
* constant current sink circuit
Often used for precise measurements. One can select 0.5A for example and it will draw exactly 0.5A independet of applied voltage(as laong as possible) and independent of temperature.
Do a search for "constant current sink" or "low side constant current source"

* variable resistor circuit
Rarely used. You may set a resistance of 6Ohms and it will stay 6 Ohms independent of applied voltage (as long as possible) and independent of temperature.
(In opposite to the constant current sink here the current depends on voltage and follows I = U / R)
Do a search for "variable resistor circuit".

* or something simple inbetween
A simple raw load adjustment. You may optimize it in direction of "constant current" with the use of a zener, or you may use it more like a constant resistor. But both are not exact. They will drift with temperature and drift with applied voltage.
Often used because it´s cheap. For this you will additionally need a current meter to monitor the current .... and maybe re adjust for the current you want)
You need a MOSFET, maybe a zener, a pot, a power resistor and a couple of standard resistors)

Klaus

You can go a couple of ways but I have come to prefer
a bank of logic level power MOSFETs each with a binary
weighted "burden resistor" and logic control (DIP switch,
USB-DIO, whatever your data acq setup is). Trying to
hold a MOSFET where you want it as it heats up is not
easy or dependable, but a dumb ol' resistor will not cry
as long as you respect power limits - so work on making
it "not dumb".

Here's a hand wired pulsed load jig I built for my POL DC-DC
chips' design validation.

Hi

So I've built a dc electronic load as in the pic attached (courtesy of https://www.codrey.com/electronic-circuits/simple-electronic-dc-load/).

I'm having a problem adjusting the current draw though. As I understand it, turning the pot should adjust your current draw? That is having no effect in this case.

The value at the non-inverting input changes but the value at the inverting input and at the output of the op-amp remains the same regardless of the potentiometer setting.

The current draw is only increasing when I increase the voltage supply to the op-amp (because the op-amp output increases I suppose).

How is the op-amp in this circuit supposed to operate? Keeps the output constant regardless of change in inputs or keep the input the same regardless of output?

Thanks.

Hi,

the circuit has a couple of mistakes.
* violating "common mode input voltage range" of LM358
* OPAMP output voltage range vs gate_to_gnd voltage (= V_GS + V_RShunt)
* wrong load connection / or no load supply
* no noise suppression
* no stability improving circuit

Klaus

P.S.
That´s why I recommend to use reliable sources of information, like application notes from semiconductor manufacturers, universities, electonic designers with good reputation...

Hi,

Besides what Klaus has said, remember we're dealing with an op amp - afaik, it's goal is life is to do whatever it has to at the output to keep the voltage(s) at the input pins the same.

LM358 = inputs from ground up to V+ -1.5V. N-Channel MOSFET like that (above load), VGS needs to be - I think - 5V + whatever voltage it sees at the 1R point. I'm guessing that OA Vout or whatever it is won't get much, much beyond the dreaded 250uA syndrome but I may be wrong as no idea what MOSFET supply voltage is.

here is an ltspice sim of a current regulator that you could find useful