this is an old video but i just came across it. i have only used FETs in SMPS and never tried them in the linear mode. Very interesting and informative. just yesterday i watched ur other one dissipating 400 Watts but didnot pay much attention to it. now i am going back to it so i can learn more about linear loads. thanks for sharing.

I had an old plasma tv that broke down so I decided to pull all useful parts from the PCB's and I found a couple big IGBT's, some smaller ones and a few MOSFETS. So I made a DC load using just one little MOSFET which is a TO-247 package, marking on the MOSFET is: ST W13NK60Z, max. 600V, max RDS(on), 0.55 ohm, max 13A and max power dissipation is 150W. I've put it in a box and used a tiny fan from an old broken Xbox that someone gave me to recycle, it's one that blows the air sideways. And for the heatsink I took the biggest one I had from an old computer power supply, so it is not very big but it's the one with the thickest aluminum plate and the most fins, taking this into account I know I have to be very conservative with the maximum power I can push through the MOSFET. So nothing is optimized for what I'm using it for and I've pushed this little MOSFET pretty hard and tested it a substantial amount of time at the highest I dared to go. What do you think? I hooked up my 70V DC transformer, rectifier and capacitor bank that I made also from old stuff, big audio transformer that can handle quite a lot on the 70V output, it also has a 50V and 25V output after rectification which is pretty nice to have for testing stuff that can handle higher voltages as the usual bench top power supplies. Well, I pushed 70V and 0.8A at first and it didn't get too hot with that so I pushed it a little bit further to 0.9A and it was still fine, keeping my eyes on the voltage level and amperes drawn watching it stabilize to my surprise, so I pushed it to the maximum I thought was still safe which is 1A... That's a 70W constant current load that I left running for about 15 or 20 minutes to see how hot it gets. It's pretty toasty in that little box with that small heatsink but I didn't smell anything funny so I consider the test a success. Before I used the MOSFET that I mentioned above, I tested a couple other MOSFETs that I took from the plasma tv PCB's, two different types that looked pretty good on the datasheets but I've blown those to smithereens pretty quick which was a lot of fun to see the magic smoke come out, sadly I didn't see any flames or sparks. However, I never thought that the MOSFET that I'm using now would be as strong as it is, able to push it far beyond what I thought was possible. I've watched quite a few videos from you and used your ideas for my own DC load so thanks for sharing your knowledge online, I appreciate it very much. Best regards, Ricardo Penders.

Excellent video Kerry, thank you for your excellent work.

Nice project and I just wanted to asked about the mosfet. It looks like you have all three leg of the mosfet shorted and soldered togther.

Nice video. Definitely worth watching. I am a little confused as far as how the electronic load and the DUT (the power supply in this case) are grounded. Where is the common ground in your circuit. Is it at the shunt resistor? How is this not shorting?

so it works like a potentiometer? until fully on?

Am not very well versed and already of age but have been fiddling with a simple controller for a house fan for a long time. If you want to control it with a triac then faza cut-off takes place. It works but makes the motor hum. The only solution I've found is with a variac, but that's a big device to control a (a)synchronous motor. More than once I have tried to do this with MOSFETs, but these need control, which is not so easy.

im newbie but you explain very well. thankyou for sharing.

Emitter degeneration resistor (source degeneration) will help solve the tempco problem. Resistor will increase in voltage to counter the VGS voltage vs current. It's related to Gm during linear part of plot. Solves thermal runaway IIRC.

Hello Kerry, I am wanting to build an adjustable electronic load capable of draining up to 250 Amperes, I initially build one using 32 IRFP260N mosfets but I believe they will not support for a long time, I am using 4 LM324 comparator based control module doing gate control of the mosfets, As a Shunt resistor each mosfet has a 0.10r 5W resistor, My question is, Is it possible to use 8 mosfets IXTK82N25P to obtain a good current drain? Each control module controls 4 mosfets and would it be necessary to change each Shunt resistor of 0.10r for a smaller value, for example, 0.05r? Would it be necessary to do this or would I have much more efficiency just changing the IRFP260N mosfets for the IXTK82N25P linear mosfets?

Excellent vid, learnt ton's...cheers !

does the larger packages dissipate heat better? are there and advantages of the larger packages?

Sir.. You know how to make dc electronic load with pic microcontroller?

Thanks !

Are there linear MOSFETs that are capable of few Amps at least 1A by 400VDC ? I would like to build an electronic load for mains applications. You know PFCs can go up to 400VDC and deliver 1-5A so there must be someway to load them down without resistors in kW range.

I was wondering what the deal with the crazy priced FETs was!

This MOSFET is a BEAST ;)

Thank you!

This guy might have saved my life.

Nice video! I didn't know these linear region mosfet existed. This also explains why I might of killed allot of mosfets with my dummy load experiments.

I have had problems using electronic loads based on Mosfets for measuring low voltage sources. I have found that around 500mV Vds and below, the mosfet doesn't work.

thanks, I didn't know such devices exist.

Well information. I suscribed. But one thing: 1st term is Y-axis and 2nd term is x-axis, not reverse. So, Vgs vs Ids is wrong. The correct way is Y-axis vs X-axis or Ids vs Vgs.

Just like bjt with more simple current unput

The connections on the metal board are not clear. Everything seems to be in short circuit which obviously is not the case. I would like to see how drain, gate and source are isolated as well as the two copper plates. Apart from that, it is a good video about the subject.

Great video Kerry. I have an electronic load schematic I was wondering if you would like to review? I emailed your youtube address with no response..

Your massive heat sink is wasted and so are the fans. The cooling air should go through the rib area. You could gain some natural cooling, if the heat sink would be oriented so it allows a chimney effect. Better yet, the fans should add to the chimney effect!

$18 EACH! F**k that. I ain't paying that much.

Wrong, wrong, wrong! You should read a text book before you try to teach. For example, when a MOSFET is used in a "linear" application, like a voltage regulator, that does NOT mean the MOSFET is operated in the "linear" region of the I/V curve. Second, "saturation" is NOT when the MOSFET is "fully turned on". A bipolar transistor is "saturated" when it is fully turned on. A MOSFET is "enhanced" when it is fully turned on. A MOSFET in saturation means that it is "pinched off", so that the drain current is a function of gate-to-source voltage, and depends relatively little on drain-to-source voltage; this is what you showed when you sketched the ID vs VGS curve (but forgot to show the threshold voltage). It seems that you don't understand that the curve you drew applies only when the MOSFET is saturated. in other words, a saturated MOSFET acts like a voltage-controlled current source, not a voltage-controlled resistance. Also, a power MOSFET is not comprised of a bunch of separate little MOSFET chips inside the package; it is one silicon die that contains many MOSFET cells. I could write about 10 more pages about what you did wrong. Don't mislead people!

It would be nice if Kerry had not mixed up some semantic differences for clarity. I also think it is a bit confusing to try to promote a particular product and present it as tutorial on using mosfets at the same time getting people to buy one. It muddies the water.