Heavy duty DC load

Here is a heavy duty DC load that can be used up to 800W, something you can not do with a single MOSFET since they usually give up the ghost at about 100W or at a junction temperature of 125C (which you cannot directly measure). If you want to go beyond this limit then either use more MOSFETs (please say no here), or do something simpler. I believe in the KISS approach (google for it) and came up with this design:

IMG_0398

The schematic of the high power DC load is embarrassingly simple and thus part of the KISS philosophy:

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To build the heavy duty DC load is a bit of a challenge because the resistors will become hot (temperatures like 200 C), so that you need a blower. The essence of the design is resistance wire (I used 2.2 Ohm per meter) and a ceramic coil element which can withstand the heat of the wire.

Settings:

  • For a 10 Ohm load use terminal 0 and C. This is tested up to 60 Volt where you can initially get a current of 6 Amp, which quickly reduces because resistance wire that heats up will lose its conductivity, so the resistance will increase. Atoms in the wire become more chaotic, and the efficiency by which a current can flow will reduce.  You can model this effect with the Stefan-Boltzmann law which says that anything that is hot will radiate and the hotter it becomes the more it will radiate effectively by temperature to the power 4 times a constant.
  • For a 5 Ohm load use terminal 0 and B
  • For a 2.5 Ohm load use terminal 0 and B and short terminals A and C
  • In any configuration you measure the voltage between 0 and A, and multiply times 0.159 to get the current. You can also use this feature separately for current measurements up to 15 Amp or so, the 0.15 Ohm resistor is rated up to 25W, so 10*10*0.159 = 15.9W. You can not go beyond approximately 15 Amp with this device, but this is more than enough for its intended use which is to test power supplies and to discharge batteries.

Some experiments:

  • Take any power supply or battery and measure the internal resistance, that is step-wise increase the load and measure the voltage over the source, plot the current and voltage in excel and compute the slope of the regression line, this gives the internal resistance of the source which is a quality of the power source.
  • Measure the ripple of a power supply for various loads, for this use the AC voltage measurement of your multimeter or a scope.
  • Measure the efficiency of a power supply under various loads, so, what is the ratio between the power put into the supply, and the power that you measure over the load. Also this is a quality factor of a system.
  • Connect several identical batteries in series and discharge them, then measure the voltage across each battery to find out whether they are all ok. In a pack of four dryfit 12Volt batteries I was able to locate one bad cell which was otherwise hard to find.
  • Discharge any battery and charge it again, try it several times, this should be a habit in your collection of lead acid batteries, otherwise you can toss them after several months. All NiCd and Pb batteries need this type of maintenance.
  • If you want to do more then demonstrate the Stefan-Boltzmann law, for a description see http://fisica.uc.cl/images/stefan-boltzman_lamp.pdf

Last update: 25-Dec-2019 12:30

Adjustable DC load

The Adjustable DC load is on the left, I use it to test power supply circuits such as the one on the right. The question with any power supply is always: 1) what is the internal resistance, 2) what is the ripple voltage, 3) what happens when we gradually increase the current, i.e. what part gets warm in the power supply 4) does the power supply limit the current at some point, or, will it trip a fuse, and finally, 5) how does a power supply react to a short circuit?

The adjustable load design is inspired by https://forum.arduino.cc/index.php?topic=90343.0 The idea is that you set the pot-meter, the first part of the opamp is a voltage follower, next you take half of that voltage and it goes in the second opamp which has a negative feedback of the voltage over a 1 ohm resistor at the source of a N channel power MOSFET. Effectively the both opamps will control the N channel MOSFET such that the current running from Drain to Source is proportional to the pot. meter setting.

Any IRF N channel power MOSFET will do, the used IRF840 can take up to 8 Amps  as long as you keep the junction temperature under 150C. Temperature is more you problem than Amps. With the IRF840 and the used heatsink you can continuously dissipate 50W, in that case the heatsink becomes something like 90C. If you want to you can dissipate 100W shortly, but at some point the MOSFET will give up. You should not dissipate more than 125W into a IRF840 at 25C. The alternative is to also put a separate resistor above the drain so that you can use the adjustable DC load for higher voltages. A drain resistor of 15 Ohm could be handy for voltages up to 60V, 5 Ohm would be helpful up to 30V, etc.

A separate PC fan  will increase the efficiency of the heatsink and the range for the DC load, you can put several MOSFETs and resistors and PC fans on different dummy load to even further increase the range of the adjustable load. Imagine what it will look like when you try to dissipate 1KW at 60 Volt, it should be a lot of plumbing.