0-180W adjustable dummy load for testing cooling systems
Page 1: 0-180W adjustable dummy load for testing cooling systems

Many people designing their own waterblocks and phase-change systems often wonder: 'How good IS my thingie actually? How does it compare to XXXX?'

To measure this, one needs a standard load that puts out the same amount of heat over and over again. The device described here does just that, and is easy and cheap to build. Heat output can be adjusted from 0 to about 180W heat output, and remains fairly constant over temperature.


Schematic of the dummy load


The device consists of an adjustable constant current source connected to the power supply. And since P=U*I, we can adjust dissipated power by adjusting the current.

Now, how does it work? The output Darlington consisting of Q1 and Q2 try to maintain a voltage of about 1.4V between base of Q1 and emitter of Q2. Thus, voltage on the emitter of Q2 is 1.4V lower than the voltage at it's base. Now, if we set the voltage on the base of Q1 to 2V for example, the voltage over R3 and R4 must be 0.6V. Since R3 and R4 form a resistance of 0.075 Ohm, this means that the current through the emitter of Q1 must be (I=U/R) 0.6 / 0.075 = 8 amps. Approximate power dissipation is now (P=U*I) (12-0.6) * 8 = 91 Watts.

If you intend to build this device, and you are not an European, swap out the TIP142 and BC547B for local alternatives.

Also, mount the TIP142 or similar power transistor using screw terminals or something similar. If you turn on the dummy load without heat sink attached and a high load dialed in, the transistor will melt within a second.

To know how much power is actually dissipated in the device, you can calibrate the variable resistance by measuring voltage across Q1 and the collector current of Q1, and multiplying these.

A last tip: if you are interested in the transistor temperature, you can use the physical law that the voltage drop over a silicon PN junction varies with -2.2mV / °C (1.22mV / °F). Measure this voltage at room temperature, and again when interested. If the voltage is 22mV lower for example, then temperature has increased 10C over room temperature.