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Building your own lab equipment

09 Dec 2013  | Steve Hageman

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The Mark II version used a really huge 180-A MOSFET in a bolt-on package, simply because it will survive nearly any abuse thrown at it. Of course, on this big MOSFET the input capacitance is > 10,000 pF, so driving it took some trial and error in gain shaping to keep the control loop stable even when using an unlimited capacitive load op amp (Figure 3).

Building your own lab equipment

Figure 2: A side view shows the simple construction of the Mark II Constant Current Load. The Power MOSFET and sense resistor are mounted on a small PCB on the heatsink itself, then the drive electronics are on another PCB and finally the controls and display are mounted on a PCB that serves as the front panel. All simply screwed together with standoffs. Truly a quick and fun way to build a useful piece of lab equipment.

This project actually used three PCBs to make the load:

  1. The MOSFET and Power Sense Resistor were mounted on one small PCB down in the heat sink channel
  2. The rest of the electronics and 9V battery were mounted on a PCB that fit the top of the heatsink
  3. The front panel was another PCB that had the holes for the pots and held the meter, etc.

Building your own lab equipment

Figure 3: The essential innards of how to make a MOSFET behave like a constant current source. R1 is a power sense resistor that is amplified by IOP2 and fed to the main loop amplifier IOP1. Components R3, R4 and C2 shape the loop gain, reducing the high frequency AC gain to insure stability of the overall loop. This compensation is needed because of the > 10,000 pF input capacitance of the Power MOSFET.

Stepped load testing power supplies

Using a loop gain tester is the best way to make sure that our power supply loops are stable, but this is not always possible due to equipment constraints or if the power supply is a closed box. What is always possible however, is to attach a constant-current load to any circuit and sometimes this is the most useful in debugging a running system. In a running system we can see the stability of the power supply when it is driving all the normal inductances and load capacitances that affect its stability in operation.

What you are looking for on every edge of the load is a nice low impulse and a quick recovery without excessive ringing.

In just seconds one can get a good feel for a system's overall stability without the need to grab the loop gain analyser and start breaking into the control loops.


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