EEVblog #102 – DIY Constant Current Dummy Load for Power Supply and Battery Test

[David]

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

[ddrl46]

That power supply might be his secret project .

[EEVblog]

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.

[David]

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.

Brilliant! 

[orbiter]

Is that a scratch on your Fluke Dave?

[ddrl46]

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  .

[NiHaoMike]

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.

[RoelA]

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  .

You should also watch episode #90:

Just how different are linear, LDO and switching buck voltage regulators? I'm glad you asked!
You might be surprised...
It's tutorial time.

it on youtube.

[ddrl46]

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.

[DavidDLC]

Dave, can you post the complete schematic with values ? It does not matter it is done in DaveCAD.

 

I hope you can to it.

[ddrl46]

He drew the schematics already in the video did he? It has all the values and stuff.

[EEVblog]

He drew the schematics already in the video did he? It has all the values and stuff.

Yes, all the values are in the video. It's just one opamp, one load resistor, one MOSFET, and one POT!

Dave.

[ddrl46]

Just bought all the parts, just gotta build this .

[smpowell]

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.

[DavidDLC]

He drew the schematics already in the video did he? It has all the values and stuff.

Yes, all the values are in the video. It's just one opamp, one load resistor, one MOSFET, and one POT!

Dave.

Yes, 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.

[EEVblog]

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.

No change at my end.
I have no idea, sorry. Anyone else?

Dave.

[ThunderSqueak]

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. 

[Simon]

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)

[EEVblog]

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. 

There is now an XBMC add-on available to browse and watch the blog!



Dave.

[EEVblog]

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.

Correctamundo!

Dave.

[Simon]

well it's that simple a circuit I'm sure you would have built it out of your head

[toli]

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).

[smpowell]

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.

No change at my end.
I have no idea, sorry. Anyone else?

Dave.

Updating 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

[jimmc]

...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).

Sorry 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.)

[avrfreaks]

Dave, it there a schematic or some kind of layout of this Dummy Load PSU as you show it off.

Cheers.