Does anyone have a constant Watt dummy load circuit to share ?

I've been googled but all I could find are constant current types.

Humble request from a electronic noob to EE guru, is it possible to make one using common discrete components such as op-amp (precise one), volt-reference and power transistor bipolar/mosfet ?

Working range like voltage from 0.9 to 12V and current range from 0 to 2 Amps so the max. wattage will be -> 2 Amp X 12 V = 24 Watt, imo this should be more than enough for most flashaholic.

Thanks in advance.

Hi there,

What do you need this for?

Hi MrAl,

Currently I'm planning to use it for profilling battery power, but I believe it will be usefull for other test as well in the future.

Do you have any in your drawer or circuits collection right now ? Mind share it !

Thanks for responding !

Cheer

By constant wattage you mean V x I is constant. While this might be a good theoritcial approach, in practice battery loads are never constant wattage. Usually constant resistance or some non-linear load like leds.

Having said that I have a circuit for a constant current controller which also measures the voltage so with some re-programming you could measure the voltage and adjust the current to suit.

The particular circuit I have needs 2.4V to 16V for the controller but can measure voltage from 0.3V to 16V min and control current upto 2 to 6Amp depending on the sense resistor independent of the controller voltage You need to be a programmer to modify it to suit your needs.

Send me a PM if you are interested

Try this circuit from Analog Devices, its not fully complete, but it should get you going.

I was thinking of doing something like this too, let me know if I can help.

Crux

Hi there,

Before you go through the trouble of trying to build a constant power load
that works up to 12v and 2 amps perhaps you should look at the
reason why you need a constant power load in the first place. Is this
really necessary? Most batteries are rated in Ampere-hours anyway.

Also, another way to get power data from a battery is to load it with
a resistor and then measure the voltage and current over the discharge
cycle. You can compute the total power from those measurements.
A nice way to do this would be to use a uC like a PIC to measure
voltage and current and send this data to a computer over an RS232
interface. The computer can then calculate the total power after
each discharge run.

To add to MrAl's reply above - If you already have a contsant current buck or boost (switching) LED driver, you can use it for battery testing. Since the driver is pretty constant current and the LED is pretty constant voltage the resulting load on the battery is pretty constant power. And any deviations from ideal are pretty well balanced out if the end goal is to use the batteries with this driver/LED combo. Pretty neat!

Measure the battery voltage (and even the LED) periodically during the discharge to get a graph. Use a big heat sink on the LED to minimize voltage droop from heating, or substitute 4 or 5 rectifier diodes (ones with a heatsink tab or stud). Or use a power resistor (10 ohms for a 350mA driver, 5 ohms for a 700mA) in place of the LED.

Keep in mind that this setup is limited to the operational parameters of the selected driver. However, you may already be possesion of everything you need to get started!

Good luck, Crux

Crux said:

To add to MrAl's reply above - If you already have a contsant current buck or boost (switching) LED driver, you can use it for battery testing. Since the driver is pretty constant current and the LED is pretty constant voltage the resulting load on the battery is pretty constant power.

Click to expand...

Hmmmm... doesn't a constant current driver deliver constant output but compensates for the diminishing input voltage by drawing more current(meaning increasing batt load with decreasing voltage)?

...oh, we're talking about power. Just ignore my question.