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Project #017 - Electronic Constant Current Dummy Load V2.0

DISCLAIMER: This design is experimental, so if you decide to build one yourself then you are on your own, I can't be held responsible for any problems/issues/damage/injury that may occur if you decide to follow this build and make one yourself.


Building or repairing power supplies, testing DC motor drivers etc often calls for using dummy loads, i.e. raking through your parts bin for a suitably rated high wattage resistor of the right value. However, rarely is the one you want to be found! What's needed is an adjustable dummy load, or an Elecronic Dummy Load!

So, inspired by V1.0 of my Electronic Dummy Load which has 2 amp maximum I thought I'd build a bigger unit capable of up to 10 amps @ 25vdc (250watts).



1 * LM8272 op-amp
2 * IXTK46N50L N-type Mosfet
1 * 0.27ohm 35w resistor
1 * 78L05 +5v regulator (100mA)
1 * 1uF 16v tantalum capacitor
1 * 4.7uF 16v tantalum capacitor
1 * 10uF 16v tantalum capacitor
2 * Ferrite beads (fitted to Gate of Mosfets)
1 * 2-way pcb terminal
1 * 5k 10-turn potentiometer (panel type)
1 * 82k resistor
1 * 100ohm resistor
1 * 5k pcb trimpot
1 * 40mm 12v Fan (110mA)
2 * 450maH 3S 11.1v Lipo Battery (used to power 12v fan as well as circuit). Note: 1 battery only will work also.
1 * Heatsink 0.5degC / Watt (RS 271-870 or similar)
1 * DVM (Beckmann & Egle EX2068/EX3068)



The design is based around an N-type mosfet and an op-amp. A 10-turn potentiometer controls the load and displays this in amps via the on-board LCD. Basically, the circuit provides a constant current sink to the supply being tested and irrespective of the voltage of the source.

I have optimized this design around a 10A max. load. Working backwards from R1 (35W shunt resistor), at 10A it'll see 2.7vdc across it, i.e. 27W. This means pin 2 of the op-amp will see 2.7vdc and so pin 1 needs to be setup to see 2.7vdc max. A 5k pot in series with a 4k7 resistor off +5vdc will achieve this.
The 2.7vdc max voltage also drives the LCD with an adustable voltage divider R2, R3 allowing for calibration.

These are N-type Linear Power Mosfets, the IXTK46N50L (500v, 46A). Conventional mosfets can be used but you would be limiting the range of voltages/loads the Dummy Load would be able to cover due to running the mosfet in it's linear mode. The IXTK mosfets overcome these limitations by extending the transistors’ FBSOA. I.E. ETI (Electro-Thermal-Instability) as a result of positive feedback within the mosfet when used in linear mode. The IXTK is ideally used in programmable loads, battery chargers & current regulators etc.
There are 2off Mosfets paralled together in order to spread the load both electrically and thermally, however I should note that this can cause oscillations (due to un-matched ON resistance, gain and threshold voltage) so if it does then try a ferrite bead on the gate of each Mosfet, or try 1off Mosfet only.
PS. You may get away with standard power mosfets (much cheaper!), and you'd more than likely be limitting the voltage range of the load.

This is single supply capable and rail to rail in order the output can swing down to 0vdc. The LM8272 chosen here is also ideal as it is able to drive large capacitive loads such as power mosfets. This is crucial for the operation of this circuit across the full voltage range of the load.
PS. Standard op-amps may work, but again more than likely you'd be limitting the voltage range of the load.

This is a 200mV FSD DVM and capable of a common SIG- / V-, i.e. the DVM SIG- is tied to V- (supply 0vdc). Some DVMs do not work this way and will not work here.

The 5k trimpot on the PCB allows the FSD of the DVM to be adjusted.
The 4.7uF capacitor across the op-amp is used to stabilize the mosfet especially at higher voltages where oscillations can occur.
To limit the max. current the load can be set to then adjust the value of the resistor in series with the main 10k potentiometer.
To house the electronics and rather than use an enclosure a large heatsink encloses the electronics, including housing the 11.1v Lipo batteries which powers the electronics & fan.

The device being load tested is connected to the large screw terminals. Polarity is important.

The maximum load current that can be used is down to heat dissipation. The IXTK46NL50 FET is capable of 46amps drain current (at 25degC), and with 2off them should be able to run up to 92amps, however this would require a heatsink capable of dissipating over 2000watts (@ 24vdc). Clearly this is not going to be the case here, and with the heatsink I've chosen, the mounting of the FETs, the small fan and with a heatsink rating of 0.5degC / Watt gives me a more realistic figure of 10amps @ 25vdc........and even then it does get quite hot!
I have tested my Dummy Load to 10amps @ 24vdc.

V = 10vdc
A = 5.0amp
Temperature measured after 15mins = 60degC at the heatsink close to the Mosfets

Total Electrical Power = 50watts
0.5degC / Watt heatsink dissipation = 25degC rise at heatsink calculated
Workshop ambient temperature = 23degC
Therefore 23 + 25 = 48degC calculated heatsink temperature.

The err in the above (12degC) will be due to the heatsink not being used at it's optimum, i.e. mounted vertically and with a good clear air-flow. Mine is horizontal and with the central section air-flow being blocked by all the electronics/battery etc.

CURRENT CONSUMPTION (11.1v Lipo Battery):
115mA (including 12vdc Fan).




18/05/12 - Photos.