EEVBlog #102 yet another constant current load - help please - Page 1
Forum section: electronics / projects

author: marcusone
author_rank: Contributor
timestamp: December 09, 2015, 07:50:27 pm
content: I used this circuithttp://www.sleepyrobot.com/wp-content/uploads/2012/01/currentload-rev2-BOM.pdf(not issue with diagram: "test hook" is actually where PSU should be connected, and TP5-TP6 is where the voltage reads for amps 1A=1V apparently).(think the same as in this posthttps://www.eevblog.com/forum/projects/another-constant-current-102-implementation/msg77861/#msg77861)It works... Note:1. I used an FQP50N06L mosfet instead (high amp capability, as I hope to be able to go above or at least to 2A)2. I used an MP915-1.00F-ND 1ohm 1% resistor (15W) for the "IR"BUT:1V does not equal 1A...When my PSU (confirmed with meter) reads 1A, the reading at TP5/6 is well above that.How can I make the reading more accurate? I've tried a voltage divider, but it does't scale accurately as the load changes.i.e. If I setup a voltage divider so that the TP5/6 reading is 1A=1V, that is the only time it is accurate.Ideas?Thanks,

author: unit998x
author_rank: Contributor
timestamp: December 09, 2015, 11:38:59 pm
content: Hi there,The PDF you linked to doesn't seem to show a circuit layout, just a bill of materials. If you need help debugging this a circuit diagram will be essential.I found the diagram at:http://www.sleepyrobot.com/wp-content/uploads/2012/01/currentload-rev2.pdf, the diagram seems to make sense and should work, have you checked for oscillations in current?You say that you used an alternate FET for more amps, in the load circuit the heatsink is a bigger problem than the I_d of the FET. The current rating of the mosfet becomes irrelevant if you are using it in the linear region (as a small value resistor). Instead you need to calculate the maximum power dissipation that your load is designed to use and source a suitable heat sink. The heatsink listed on the BOM will probably be suitable for about 10/12W (with 60*C rise).Quote from: marcusone on December 09, 2015, 07:50:27 pmHow far above 1V is it? Is it very close or way out? If the error is significant you'll want to find out why rather than just add an offset to adjust it out.

author: rdl
author_rank: Super Contributor
timestamp: December 09, 2015, 11:57:00 pm
content: There's a very good chance it's oscillating. If you have some kind of oscilloscope, you can probably fix it. Replace R15 and R16 with trimmers and connect a substitution box in place of C4. Those are the components you have to experiment with.

author: marcusone
author_rank: Contributor
timestamp: December 10, 2015, 01:34:37 am
content: Quote from: rdl on December 09, 2015, 11:57:00 pmI don't have a capacitor substitution box... can I just take out C4? wasn't in Dave's original, but I assume it can be the cause of the oscillation?If not, can other ideas for size of capacitor, smaller, larger?Thanks!

author: yashrk
author_rank: Frequent Contributor
timestamp: December 10, 2015, 01:56:51 am
content: C4 here is used to reduced the oscillation just keep on replacing it different values till you find a stable signal and yes you can remove it and if it's still oscillating then try using other stabilizing circuit around it just check out other examples of dummy load online.

author: rdl
author_rank: Super Contributor
timestamp: December 10, 2015, 03:33:23 am
content: You could trying using short clip leads to add in different capacitors. I ended up with 220 nF for C4 and way smaller values for R15 and R16, but I was using a different op amp.

author: marcusone
author_rank: Contributor
timestamp: December 18, 2015, 01:44:03 am
content: Well, I got my hands on an Oscilloscope, and at the test point the mV reading is rather stable... I mean there is a bunch of noise on it, but not oscillating. If I remove the capacitor C4, it then turns to a sinus wave.changing R15 and R16 seems to have no effect at all!

author: EntropyWizard
author_rank: Contributor
timestamp: December 18, 2015, 01:32:21 pm
content: A silly thing to check, but it got me building one of these types of load. I thought the load was oscillating, but forgot about what power supply I was using to test. Make sure your DUT power source can handle the load otherwise it may go into current limiting which looks like oscillation.