Electronic Load Without Current Sense Resistor

Greetings,

I want to make a 300W capable electronic load but in every schematic I saw a current sense resitor connected to an op amp. That means I need a 300W resistor .The 300W resitors are not very cheap. I searched and then found this schematic. What are the advantages and disadvantages of using an op amp and not using a current sensor.


(Sorry for the bad writing)

Hi,
There is very little energy dissipated in the current sense resistor, it is not the load, so as long as the current sense module is rated for the max current you will be sinking, it should be okay.

Tom.... :grinning: :+1: :coffee: :australia:

Thanks for the answer.

For example: My load is drawing 10 amps and my resitor is 1 ohm.
Power = current^2 * Resistance so 10101 = 100 W

Is my calculation wrong?

Hi,

Yes for your LOAD resistor. 100W correct.
Do you have a circuit?

Tom... :grinning: :+1: :coffee: :australia:

1 Like

Thanks for the answer,

This is not my circuit but I will make somethig similar.(R1 is the problem. I will use 1 ohm reistor for simplicity)

electronic_load

Hi,
Your 1 Ohm resistor is the load and current sense resistor.

That is an old circuit LM741 is one of the first opamps ever marketed.
You will need a +12V and -12V supply.

google

electronic load circuit LM358

Here is a good reference.
https://www.gammon.com.au/forum/?id=11525

Tom.. :grinning: :+1: :coffee: :australia:

Thanks for the answer,
So in conclusion I need a resitor equal to my power rating right?

Hi,
Yes, you should be able to get ones that have two bolt holes for you to bold it to a heatsink.

Tom.. :grinning: :+1: :coffee: :australia:

Thanks for the answer,
Or I can use a current sensor with an mcu right? I want to display the current value on an LCD. One way or another I will use a current sensor.

Hi,

If you include a current sense module like the ACS712 module, you still need the 100W resistor.
The resistor is part of the feedback for the MOSFET driver op-amp and dissipates the energy.

Tom.. :grinning: :+1: :coffee: :australia:

But neither are FETs rated at 300W - which will need to be combined with a very large heatsink and cooling fans.

Actually, 300W resistors are not really that expensive.

But what voltage are you dealing with?

Thanks for the answer,

One more question. Can I make a circuit without an op amp?

Like this

Thanks for the answer,

"But what voltage are you dealing with?"
Maximum 30 volts with a current of 10 amp.

I have to use MOSFET and heatsink either way.

With the ACS712 you don't need a "current sense" resistor. (You might want a "current limiting" resistor, depending on your application.)

I assume "electronic load" means the MOSFET serves a variable load resistance... The MOSFET will have to dissipate ALL of the power and 300W seems like a lot. You might need more than 1 MOSFET.

If the MOSFET only needs to switch the load on/off, then you CAN use a load resistor and the load resistor can double as the current sensing resistor. When the MOSFET is off, almost no current flows. When its fully-on the resistance is low so it doesn't have to dissipate the full 300W, but you probably still need a heatsink.

...If you do go with a current sense resistor, 10A through 1-Ohm is too much voltage for the Arduino. If you are using an Arduino you can use the optional 1.1V reference you can get resolution of about 1mV which means you can get-by with a much lower resistor value.

Thanks for the answer,

I looked at the datasheet of acs712 and the datasheet says it is a hall efect current sensor. Why do I have to use a current sense resistor with an acs712?

Datasheet of acs712

Also I found this schematic and there is no current sense resistors.

current-sensor-circuit

Hi,
With hall effect devices like this you don't have a sense resistor.

Tom... :smiley: :+1: :coffee: :australia:

Does you current need to be steady through the load or can it be pulsed?
If pulsed you can use PWM to control the FET and have minimal FET dissipation.
If the load is a heater, pulsing should be OK.
To 'sense' the average power you will need a voltage divider across the load into a RC filter and then the Arduino A/D.

By the way, do you access to both sides of the load. i.e. both terminals.

Thanks for the answer,

"Does you current need to be steady through the load or can it be pulsed?"
It has to be steady.

"If the load is a heater, pulsing should be OK."
My load will be a power suplly or a battery.

"By the way, do you access to both sides of the load. i.e. both terminals."
Yes I do.

That will also depend on the accuracy required on the ohms value of the load and its stability over temperature (temp coefficient) . The heater proposed for example is likely to exhibit lower ohmic resistance when connected to power which will then decrease at it heats up.

Some years ago I built an electronic load which needed a stable sense resistor, 0.1Ω if my memory is correct.
It was cheaper to use 10 resistors of 1Ω /2W 1% each in parallel than buy a single 1Ω 20W resistor. This way you also get better total tolerance. (Probably 1% /10 ) , better heat dissipation (distributed) and easier to handle package. I was able to use SMD reistors