EEVblog #102 – DIY Constant Current Dummy Load for Power Supply and Battery Test - Page 1
Forum section: electronics / general

author: David
author_rank: Frequent Contributor
timestamp: August 01, 2010, 09:11:59 am
content: It's great to see another technical blog (no multimeter reviews!). Dave, any plans to post a blog on the latest power supply you designed? (I seem to remember you saying you were working on one...) There seems to be a lot of talk about SMPS/DIY bench supplies on the forum and it would be good to see how you went about the design.Keep up the good work,Dave

author: ddrl46
author_rank: Regular Contributor
timestamp: August 01, 2010, 09:27:06 am
content: That power supply might be his secret project.

author: EEVblog
author_rank: Unknown
timestamp: August 01, 2010, 09:38:14 am
content: Yep, all will be revealed in time...There will probably be a follow-up to that blog, actually going through designing a switchmode supply.Dave.

author: David
author_rank: Frequent Contributor
timestamp: August 01, 2010, 09:55:39 am
content: Quote from: EEVblog on August 01, 2010, 09:38:14 amBrilliant!

author: orbiter
author_rank: Frequent Contributor
timestamp: August 01, 2010, 10:41:00 am
content: Is that a scratch on your Fluke Dave?

author: ddrl46
author_rank: Regular Contributor
timestamp: August 01, 2010, 12:21:36 pm
content: Awesome, just in time!I'm working on a power supply project using linear regulators (aka LM317 / LM350) but i'll wait and try to make a switch mode now.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 01, 2010, 01:39:11 pm
content: For something bigger, wouldn't it be better to PWM the MOSFET in order to keep power dissipation in the resistors? High power resistors are pretty cheap in the form of heating elements and light bulbs.

author: RoelA
author_rank: Contributor
timestamp: August 01, 2010, 02:07:30 pm
content: Quote from: ddrl46 on August 01, 2010, 12:21:36 pmYou should also watch episode #90:Quoteit on youtube.

author: ddrl46
author_rank: Regular Contributor
timestamp: August 01, 2010, 02:27:36 pm
content: I've already seen that, i've seen all the EEVBlog episodes so far. I've already been experimenting with my own switch mode regulators but it hasn't been working out that well so far. I just need a little bit more explanation.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: August 01, 2010, 07:20:14 pm
content: Dave, can you post the complete schematic with values ? It does not matter it is done in DaveCAD.I hope you can to it.

author: ddrl46
author_rank: Regular Contributor
timestamp: August 01, 2010, 08:57:39 pm
content: He drew the schematics already in the video did he? It has all the values and stuff.

author: EEVblog
author_rank: Unknown
timestamp: August 01, 2010, 09:56:01 pm
content: Quote from: ddrl46 on August 01, 2010, 08:57:39 pmYes, all the values are in the video. It's just one opamp, one load resistor, one MOSFET, and one POT!Dave.

author: ddrl46
author_rank: Regular Contributor
timestamp: August 01, 2010, 10:06:47 pm
content: Just bought all the parts, just gotta build this.

author: smpowell
author_rank: Contributor
timestamp: August 01, 2010, 10:34:34 pm
content: Hi Dave,I've been watching EEVblog on my TV by downloading the Youtube file and transcoding onto my Tivo. But the latest ones can't be transferred.I'm assuming something changed to set a copy protection flag on the new videos.Could you change things back to the way they were? It's nicer to watch on the "big" screen.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: August 02, 2010, 01:25:41 am
content: Quote from: EEVblog on August 01, 2010, 09:56:01 pmYes, but I want to save it to my computer, so a picture or pdf will do the work.If not, I will copy from the video, just want to know if Dave has something already he can post.

author: EEVblog
author_rank: Unknown
timestamp: August 02, 2010, 02:19:27 am
content: Quote from: smpowell on August 01, 2010, 10:34:34 pmNo change at my end.I have no idea, sorry. Anyone else?Dave.

author: ThunderSqueak
author_rank: Unknown
timestamp: August 02, 2010, 06:51:05 am
content: I usually watch your blogs on my big screen too, via itunes podcast. I have it set to download it automatically (checks once a day) and then when I get home and sit down my ps3 or xbox360 tells me "you have a new video". I watch most of my podcasts this way. I had no issues with the last video at all.Only videos missing are the first 13 episodes as I do not think Dave uploaded them to itunes.I did try playing the youtube videos on my ps3, xbox360, and my bluray player (all 3 do youtube) and had no issues at all. Perhaps your tivo is just being stubborn? The format appears to be the same old standard format youtube uses.

author: Simon
author_rank: Unknown
timestamp: August 05, 2010, 12:08:35 pm
content: well as he did it in DaveCAD it looks like there is nothing better and it was probably only drawn (literally) because he was doing a video on it. So your homeowrk is to go back to the video, pause it and copy the schematic and values to your usualy schematic drafting program (of which there are many free) and then you can print that to a PDF making printprogram (and yes there are plenty of those for free too)

author: EEVblog
author_rank: Unknown
timestamp: August 05, 2010, 01:02:35 pm
content: Quote from: ThunderSqueak on August 02, 2010, 06:51:05 amThere is now an XBMC add-on available to browse and watch the blog!Dave.

author: EEVblog
author_rank: Unknown
timestamp: August 05, 2010, 01:04:46 pm
content: Quote from: Simon on August 05, 2010, 12:08:35 pmCorrectamundo!Dave.

author: Simon
author_rank: Unknown
timestamp: August 05, 2010, 01:44:35 pm
content: well it's that simple a circuit I'm sure you would have built it out of your head

author: toli
author_rank: Frequent Contributor
timestamp: August 07, 2010, 05:13:28 am
content: Sure is a great little thing to have. Especially now that I need something to check a couple of transformers, and than the PS I'm working on (once I find the right case for it).I only have other op-amp's and some PMOS (IRF9Z34N). I've drawn something that should work with these, and will appreciate it if someone more experienced would just take a look and let me know if its ok. It runs just fine on PSpice. R1 will be the pot. I might change the input and output pins on R1 since the current goes up with the value of R1, so that way I would still need to turn it clockwise to tune the current up.The op-amp might actually be the OPA604 as I have a few spare of these as well (they work about the same on the spice model).

author: smpowell
author_rank: Contributor
timestamp: August 08, 2010, 08:02:59 pm
content: Quote from: EEVblog on August 02, 2010, 02:19:27 amUpdating all the software (pyTivo/ffmpeg/python) fixed the problem. I'm guessing that the older version of the transcoder (ffmpeg) was choking on the video files.Stephen

author: jimmc
author_rank: Frequent Contributor
timestamp: August 09, 2010, 10:13:25 pm
content: Quote from: toli on August 07, 2010, 05:13:28 amSorry neither the NE5532 nor the OPA604 will work in your circuit if you want to set less than about 2A.For low currents (R1~0) both inputs will be close to the +ve supply of the OP-AMP.Neither of these types will work under these conditions.The parameter you need to look for is the 'Common Mode Input Voltage Range' which needs to include the positive supply for your circuit to work down to zero current.Simplest to look for an amplifier with 'Rail to Rail' input spec.(There are a few devices that typically include the positive supply but not the negative eg TL071 or LM301.)(Interesting to note that the LM324 in Dave's original circuit is only spec'd from 0 to (V+ -1.5v) at room temp and so 3 turns of the 10 turn pot are not usable.)

author: Unknown
author_rank: Unknown
timestamp: August 10, 2010, 04:40:36 am
content: Dave, it there a schematic or some kind of layout of this Dummy Load PSU as you show it off.Cheers.

author: EEVblog
author_rank: Unknown
timestamp: August 10, 2010, 05:55:21 am
content: Quote from: robo-x on August 10, 2010, 04:40:36 amThe schematic is in the video. Nothing on paper, sorry.Dave.

author: ThunderSqueak
author_rank: Unknown
timestamp: August 10, 2010, 07:36:05 am
content: because people keep asking, snapshot of the schematic from the video

author: Simon
author_rank: Unknown
timestamp: August 10, 2010, 11:50:31 am
content: hm did you check the terms and conditions of DaveCAD to see if reproduction of generated material is legal ?

author: ThunderSqueak
author_rank: Unknown
timestamp: August 10, 2010, 08:39:10 pm
content: Quote from: Simon on August 10, 2010, 11:50:31 amno..hmmm, that might explain the men in black who showed up at my door this morning

author: Simon
author_rank: Unknown
timestamp: August 11, 2010, 06:44:27 am
content: no that must be because you have aliens in your loft but don't know it. it's ok the men in black are friend with the aliens.

author: btfdev
author_rank: Contributor
timestamp: June 22, 2012, 09:58:59 pm
content: Just re-watched the video and got a quick question.Noted at 2:10, Dave mentioned a LM317 could be used a constant current load when you ground the output.So *WHY* MOSFET solution you need a power resistor going to ground but LM317 does not need one?Any idea?Thanks, BTFDev.

author: SeanB
author_rank: Super Contributor
timestamp: June 23, 2012, 09:30:46 am
content: With the LM317 direct to ground you are just relying on the built in current limiter, at around 1.6A or so, to provide constant current sinking provided you just ensure the voltage across the device is above about 3V and it has a big heatsink. If you use a resistor across the output to ground then it will keep the voltage across the resistor constant at 1.25V over a wide input voltage range, from about 5V to 30V in most cases. The resistor allows programming a lower current, you can go from under 1mA ( but with a slight error from adjust pin current and quiescent current of the device) to around 1A and have a pretty stable current. It does need a heatsink that will keep the device under 60C for best stability though, especially at the higher range.

author: mianchen
author_rank: Regular Contributor
timestamp: July 08, 2012, 11:15:10 pm
content: Two questions:1. What are the benefits of using a mosfet compared to a bjt transistor? I have a few 2N3055, is it a bad idea to use them? I've tested it on a bread board, and it seemed ok and worked as 'expected'. I don't have a MTP3055, but have a few P36NF06L and I managed to kill one when the set current is >2 A with a 3c/W heatsink.2. How to calculate the heat dissipation for the transistor? The output voltage of the PSU to be tested (Vout) - Voltage drop across the resistor (Vr) x the set current going through the resistor?Sorry for the noob questionsThanks

author: ModernRonin
author_rank: Contributor
timestamp: July 09, 2012, 01:36:40 am
content: Quote from: mianchen on July 08, 2012, 11:15:10 pmIn this particular case, the major advantage is that a voltage on the gate of a MOSFET controls the current through the drain-source. With an NPN transistor, it's acurrentthrough thebasethat controls the current from collector to emitter. Voltage control is usually easier than current control.In the general case, the advantage of MOSFETs is that when used as a switch, they have a lower resistance than an NPN. They are more power efficient, when used as a switch. But in this case we're not using the MOSFET as a switch. We're using it as a variable resistor. So that advantage is not useful to us here.Also, NPNs can go into thermal runaway. NPNs conduct better the hotter they get. With MOSFETs it's the opposite, the hotter they get the worse they work. So MOSFETs can't go into thermal runaway nearly as easily.QuoteFirst, you need to know that there is a minimum voltage drop across an NPN transistor operating in linear mode of about 0.7 volts. This will put a limit on the maximum current that the dummy load can sink. However, the limit is pretty high. Something in the neighborhood of: (1.5 * input voltage) amps.Second, NPN transistors can go into thermal runaway as mentioned before. This probably won't be a problem with the circuit Dave used, because the opamp will automatically throttle back the NPN if it goes over-current. But just be aware that thermal runaway can happen.Third, it depends on how you're driving the base of the 2n3055. If there's X amount of current flowing through the collector of an NPN, then you need (X / beta) flowing through the base. The 2n3055 has a beta anywhere between 20 and 70. So in the very worst case, you will need to supply X / 20 amps of current into the base. If the opamp you use to drive the base of the 2n3055 can supply that much current, then no problem. But if it can't supply that much current...Assuming a "typical" opamp with 20mA of output current capability, that would mean your dummy load would be able to handle a maximum of 20mA * 20 = 4000 mA = 4 amps. This is a lot less than the 15A maximum that the 2n3055 can handle.To ensure the full 15A limit can be reached, you would need an opamp that can output 15A / 20 = 750 mA. There are opamps that can supply that much current, but they're not very common and may not be cheap.BTW, you can go back and do all of the above math with 70 instead of 20 to see what kind of base current you would need in the "perfect world" case. But I don't recommend you count on the world being perfect.(There's a relatively simple base driving circuit that you can make with a resistor and small MOSFET that will greatly amplify the output current of your opamp. But you're such a n00b I don't want to confuse you with it just now. It looks basically like this, except with the transistors swapped:http://www.neufeld.newton.ks.us/files/electronics/mosfet-motor/MOSFET-motor-driver.png)QuoteThe usual answer is "P = IV", but in this case we should probably use the equivalent but alternate form:P = I^2*RorWatts = (Current through component)^2 * Resistance of componentNow, the current you know. You just decide you want 1.3 A constant current, and that's that. The constant-current load will adjust its own resistance until that constant current is reached.The resistance you will have to calculate, based on the input voltage and constant current. Example: You have 12V and set a constant current of 400 mA (= 0.4 A). Use V = IR to calculate the equivalent resistance:V = IRV/I = R12/0.4 = RR = 30 ohmsNow use P = I^2*R:P = 0.4^2 * 12P = 1.92WThis is much less than the ~100 watts than the 2n3055 is rated for.But don't get a false sense of security. Remember that the equation depends on currentsquared.As the current goes up, wattage also goes upvery quickly.Another example:Constant current = 4 A; Vin = 24 volts; R = V / I so R = 6; P = I^2*R so P = 16 * 6 = 96 watts! You're getting close to the power dissipation limit of your 2n3055.

author: btfdev
author_rank: Contributor
timestamp: February 08, 2013, 06:37:28 pm
content: Hey folksIn the winter holidays we did our own dummy load based on Dave's circuit.http://www.binarytaskforce.com/Weblog/2013/02/08/dummy-load/]http://www.binarytaskforce.com/Weblog/2013/02/08/dummy-load/Sure it is a great tool!EnjoyTeam BTF

author: rodcastler
author_rank: Regular Contributor
timestamp: June 21, 2014, 01:17:05 pm
content: Dave says he adds the second OpAmp because he had the dual package lying around but what is exactly the benefit of the second opamp? Apologies if this is a newbie question but I wish he had explained it a little deeper in the video.

author: Lee697
author_rank: Regular Contributor
timestamp: July 05, 2014, 11:34:19 pm
content: Quote from: rodcastler on June 21, 2014, 01:17:05 pmCurious about that question myself - I breadboarded the dummy load yesterday, and just used the pot as Dave indicated in the video, and it works well....

author: JayJones
author_rank: Newbie
timestamp: May 29, 2015, 03:54:39 am
content: Reviving an old thread...Can anyone tell what I might need to modify to run this from a 9VDC adapter, and any way to use a standard N-Channel Power mosfet?I blew up 4 FQP50N06 mosfets already trying to figure out which parameter I'm exceeding in the datasheetThanks!

author: rqsall
author_rank: Regular Contributor
timestamp: June 03, 2015, 08:04:08 am
content: At least you will need to reduce the source resistors (1 ohm in Dave's version) to something less than 0.5 ohm or so. You need more Vgs with a non-logiclevel mosfet to turn it on, and if the 1 ohm source resistor is already eating 2 volts at 2 amps, and you have 5V supply on your opamps, then there isn't much left for Vgs.If you blew them up, however, I suspect you ran too much current through it, perhaps the gate voltage was too high due to faulty wiring?

author: brutester
author_rank: Contributor
timestamp: June 03, 2015, 08:27:57 am
content: Attached is even simplier way of doing it. Just use a power NPN. The one I've used was removed from old computer PSU. It is not as good as Dave,s approach, but hey - it is powered from the device and does the job