EEVblog #102 - DIY Constant Current Dummy Load for Power Supply and Battery Testing

Dave grabs a few junkbox parts and builds a useful constant current load for switch mode power supply, battery testing, and other applications.

[elye]

I am blown away. A really great exposition. Simple, practical and I like the verification bits. I learnt more about thermal resistance of heatsinks here than all the books and articles and datasheets I have been reading.

[Networkengie]

That dave CAD sure is some piece of sophisticated software you got. :)

[ForViewingOnly]

Excellent video. So good to see electronics theory applied in a practical way. I studied electronics 20 years ago at college as part of my apprenticeship at a telecoms company, and although I came away with useful qualifications I didn't really learn much that I could apply: the course was 95% theory and 5% practical!

[testep]

You know, I really wish I had found Dave's videos last. After watching these vids, it makes it really really hard to watch the videos from other people, even though they contain good information as well. His teaching style is just so in tune with how I learn and his explanations are in depth, but don't treat the listener like an idiot. This series is just brilliant.

[clifffiftytwo]

Had a vision of factories full of engineers waiting impatiently for Dave's latest video to come out, designing away as he talks, then quick into production, the first on eBay wins!!!

[enriquecourtade]

5:19 absolutely important thing no explained at all. This is a video for truly advanced people. I suggest to search Rail to Rail Op amp to understand what wasn't said here.

[RexxSchneider]

The other part of the heat dissipation calculation is that the MTP3055 has a thermal resistance between junction and case of 3.13 degrees C per Watt. That means with your 11 W example, that the mosfet junction will be another 3 x 11 = 33 degrees above the case/heatsink - a little over 100 degC with a 20 degC ambient. That's within the maximum operating temperature of 175 degC for the device, but a little toastier than I like.
The corollary is that your heatsink/mosfet combination has an absolute maximum power dissipation of around (175-25)/(4.5+3) = 150/7.5 = 20 W with a 25 degC ambient. You might want a better cooling solution if you start looking at higher voltages and currents - 12 V and 2 A would well exceed the capabilities of that heatsink. And to get past 50 W, you'll need a mosfet in a different package.

[Laogeodritt]

Notice that the op-amp feedback controls the DC current depending on the PWM input (because the voltage across the 1R is proportional to the current), NOT resistance. For power, P=V*I, which is the same as CC iff the supply voltage is constant (not necessarily true! Think batteries as they discharge). Constant-resistance would be to vary current according to supply voltage according to Ohm's law, i.e. you have to actively maintain the constant resistance since the OPA/NMOS system is CC.

[ernestuz]

Dale, doing it in that way creates a constant resistance load, so the current will change when the power supply voltage under test changes. Wicth Dave's circuit the current sucked from the power supply is constant.

[jeremiahgeo]

Excellent tutorial Dave, Hats off to you .Like your Dave CAD .tells almost everything on this CKT

[EEVblog]

@cborrero2000 The opamp output gives whatever Gate voltage is required to keep the constant voltage across the load resistor. It's a standard building block circuit.

[exallievididonbosconcrgrou]

Dave, you're GOOD! Keep the video's coming. This video is already 7 years old when I saw it this Dec 2917. So you are now 7 years a little bit older since you made this video. But keep up the good work.I LEARNED A LOT FROM IT AND SO DO OTHERS. ATTABOY! Thanks.

[johanvandebelt]

I have a remark about the diagram at 16:40. In constant resistance mode, to justify Ohms law, you'll need to know the input voltage and adjust the current accordingly. Since the microprocessor sets the FET current with PWM, it also needs to monitor this input voltage. The inputvoltage (=Voltage across FET and current shunt resistor) can be monitored using a resistive divider over the input connectors. The divided voltage is then connected to a second channel of the microprocessor ADC (or use an analog multiplexer). For constant power, same story.

[Debraj]

Opamp is required to maintain the constant current. With a pot + manual intervention, you could vary the current, but the current will vary the moment supply voltage varies. Using opamp in closed loop will make sure that the current is same irrespective of any input voltage.

[ChrisRid]

Awesome little circuit! I had thought about trying to make a constant-current load before but wasn't sure which route to take. New project I think!

[EEVblog]

@thenaimis Correct, that's what the original board does. Micro with constant current/resistance/power and logging capability.

[exgenica]

Very well explained and executed.

...If you'd only gone one step more and shown how to relate the allowable maximum temperature of the MOSFET to the power dissipation of the heatsink to verify that the heatsink would adequately protect the MOSFET for the maximum current you were allowing the device to draw.

[prakashveereswar]

Thanks for the video Dave..very useful...Please give some tips on building higher currents sources like 7 Amps constant current source.
Thanks Regards.

[joshstube]

Great Idea Dave!
I want to build one!

A request:
Please add 17:17 to the next "how to entertain a geek Highlights!
Especially the part about doing it with "intelligent control"
;)

[mjlorton]

Excellent...might have to put one of these together.