Let's make an example: Suppose you have a lead-acid battery that you want to discharge at 8 A. The battery goes from 14 V to 9.5 V. That would mean 112 to 76 W, well beyond the maximum for standard MightyWatt. However, if you put a 1 Ohm power resistor (properly rated) in series with the battery, it will dissipate 64 W while producing an 8 V drop in voltage. The load will only have to dissipate the rest: 48 to 12 W. Thanks to the differential voltmeter, you will always have the accurate battery voltage when using 4-wire mode as well as the total power of the device under test.
|Schematic of connection with power resistor|
A suitable power resistor would be for example this one from Vishay: HL10006Z1R000JJ or this one from Multicomp: MC14668. The first one is 7 USD on Mouser, the second one 9 USD on Farnell/Newark. That's a pretty cheap extension of the power dissipation!
|Typical shape of a suitable power resistor|
Now, there are some important points about this approach:
- Because the power dissipation is calculated simply as the product of voltage and current, you will have to tell MightyWatt that you are using a series resistance so it won't show overload. In the Windows control program, select Advanced => Series resistance and type the value of the external power resistor in the pop-up window. The load will automatically keep track of the true dissipated power in 4-wire mode. This value only affects the thermal management, nothing else. But be careful, if you put wrong value or you remove the power resistor later, it may lead to a damage of the load.
- This only works as long as the power difference between charged and discharged state (start - end) is not higher than the MightyWatt rating. It is great for batteries but if you have to go all the way down to zero with the voltage, you will have to split the experiment (voltage ranges) into two or more parts.
- The temperature coefficient and precision of the power resistor do not matter much. The voltage is measured directly on the device under test and current measurement is not affected either. Don't waste money on high-end power resistors.
- There are power resistors in aluminium heatsinks that look like the one below. These have to be mounted on a large heatsink to dissipate the rated power. See their datasheet. These resistors cannot dissipate the rated power only with the small heatsink that is around them.
|Power resistor that has to be mounted on additional heatsink|