Author Topic: Constant Current Dummy Load Kits (Read 17696 times)

chrisw957 · « **on:** June 03, 2011, 07:19:02 pm »

I created a version of Dave's constant current dummy load for myself, and I'm still trying to get rid of the few extra PCBs that I put into a kit with the required parts.

You can get one here: http://www.sleepyrobot.com/?p=95

thanks,
chris

Kiriakos-GR · « **Reply #1 on:** June 03, 2011, 07:32:48 pm »

What is the Min & Max current that your version can handle ?
And for how long time ?

sacherjj · « **Reply #2 on:** June 03, 2011, 08:09:14 pm »

Quote from: Kiriakos-GR on June 03, 2011, 07:32:48 pm

What is the Min & Max current that your version can handle ?
And for how long time ?

Parts list has 10x 1/4W 10 ohm resistors so it is 2.5W max for the shunt. MTP3055 which I believe is a 12 A MOSFET. With upgraded shunt (as long as the board can handle it) you could push more power.

With a 9V battery it looks like the op amp is getting fed from a 24k and 10k divider. So that comes out to 2.64V (which will be 2.64A) with a little drop depending on how close to the rail the op amp can get. So it is capable of about what the resistors are rated.

I think that is right, just guesses by looking at the schematic.

Kiriakos-GR · « **Reply #3 on:** June 03, 2011, 08:35:33 pm »

Hmmm ..

Well I need at minimum a 7A load that will work for an hour with out flames.
And the ideal is the 10A that it can keep up for an hour to five hours.
I have ultra large heat-sinks and fans , but I need the PCB and some details about the wiring.

JimmyM · « **Reply #4 on:** July 04, 2011, 11:15:13 pm »

Quote from: Kiriakos-GR on June 03, 2011, 07:32:48 pm

What is the Min & Max current that your version can handle ?
And for how long time ?

I just bought one. I was at the bench last night putting together an order at Digikey to build my own. I waqs experimenting with an op-amp I had on hand on a breadboard but couldn't get it to go don to 0V output. THEN I figured out why. It was the UA741 which can't get down to 0V with a single supply. I changed to +/- 9V and it worked perfectly. Plus it has an offset null adjustment. But then I'd need a dual supply... blah, blah, blah. But your kit has just about everything I was going to do with mine.
I'll be building it in a project box and will hack in a multi-turn pot I bought on eBay. I also bought a couple of CX101BG (or is it GB)s. One for volts and one for amps. I may fiddle with the 24k/10k divider to get more current out of it. But will be switching to an aluminum 1 ohm power resistor (10W) so I could potentially do 3+ amps. I also have several IRL8113 FETs to compare the 3055 to. nice going with the board though.

JimmyM · « **Reply #5 on:** July 08, 2011, 07:55:40 pm »

I just received the kit in the mail and put it together. Since I'll be mounting it in a project box with external heat sinks for the FET and 10W resistor I'll be able to reduce the board size appreciably.
I was able to get 1.69A out of it in the standard formation. When I disconnected the leg of R12 (10K) effectively removing the voltage divider between the Op-Amps I was able to get 2.5 amps out of it (5V at the gate) using a 5V input. But 5Vin through a 1 Ohm resistor (FET is <0.2Ohm) should be closer to 5A. I'm going to swap the included FET MTP3055 - 180mOhm, 2V Gth, 48W) with an IRL8113 (6mOhm, 2.25V Gth, 115W).

No luck. Still a max current od 2.5A. Although I did measure a 230mV drop across the Load+ connection and the FET drain. I'll keep fiddling.

JimmyM · « **Reply #6 on:** July 08, 2011, 08:47:47 pm »

I figured it out. As the current goes up, so does the voltage at the FET source pin. Thus, what used to be a 5V Vgs is now only 5V-Vdrop. So at 2.5A current, the FET gate is really only seeing 2.5V. Nothing can really be done about this unless you decrease the value of the 1 Ohm resistor. If you used a 0.1Ohm resistor the Gate would be able to receive 4.75V at 2.5A But the gate wouldn't need 4.75V. But you get the picture.
I'm going to use an LM317 to get 6V. So at 3A, the maximum the gate could get would be 6V, when disconnected and 3V at 3A.
Let's see how 6V works.

JimmyM · « **Reply #7 on:** July 08, 2011, 09:25:02 pm »

OK. Using a 6V supply (Max for the MCP6004) I get 3.7A using the IRL3118. Thins are getting a little warm at that point using a 10V input. The resistors (I switched to 3 x 3.3 Ohm 10W ceramic power resistors for testing = 1.1 Ohm) are dropping ~3.4V and dissipating ~12.7W. The FET is dissipating the remaining 21.3W. So I can't really run it all that long at that output. However, the minimum current is 1.6mA. Due to the offset in the 1st and 2nd Op-Amps. If I don't use the first op-amp (voltage buffer) and connect the pot directly to the "output" op-amp that drives the FET gate. I can get down to 0.6mA (measured on my Fluke 189). So I think that's the design I'm going to use for my actual Dummy Load.
6V instead of 5V, 1 op-amp instead of 2, IRL3118 instead of MTP3055, 25W 1 Ohm Aluminum body resistor.

It occurs to me that I could use a 0.1 Ohm high precision resistor and use one of the op-amps as a x10 non-inverting amplifier to Still get 1V/A output. But that would take a little fiddling and maybe a dual supply with Offset Null adjustable Op-Amps for correct readings when the current is very low. Then again. I could just build 1 for low current stuff and another for higher current.

Projects projects projects.

ivan747 · « **Reply #8 on:** July 09, 2011, 02:24:30 pm »

Very nice!
You may want to add a combination layout that includes a 5mm screw terminal. I have found multimeter leads to fit nicely in there.

JimmyM · « **Reply #9 on:** July 09, 2011, 08:05:03 pm »

I've decided not to use the board I purchased. It's too large to fit in my project box.
So I've purchased an MCP602 (Not a MCP6004). The MCP602 has 700uV input offset. The MCP6004 has 4500uV. The MCP6004 I have actually shows 600uV offset in operation. This leads to a 0.6mA load when it should be zero. I know it's not much. If I wasn't already getting things from Digikey I'd stick with the MCP6004, but since I was ordering anyway. I figured I'd get the 8-DIP dual op-amp that has a lower Input Offset voltage. I'll be using an externally mounted heat sink for both the FET and the 1 Ohm resistor. I also have a pair of CX101s on the way for current and possibly voltage display. Since I'm using the CX101, there's not much sense in going over 2A any way. It will be configured for a maximum of 1.999 counts. I'll probably add a 10K trimmer between the 5.11V rail and the 10K 10-turn adjustment pot I bought. Thus limiting the maximum amperage.
Which begs the question... If I'm not going over 2A, why use an LM317 and 5.11V? Answer: I had it configured for 5.9V (capable of 3.4A) when I remembered the CX101 can only accept 5.25V MAX and it can't display over 1.999 anyway. So I just swapped in a different Vset resistor and called 5.11V good enough. In operation it uses ~1mA from the 9V battery. 5.5mA with the LED.
If I need a dummy load > 2A, I can build another unit with a much bigger FET and a 0.1 Ohm resistor. In that case mA level accuracy will not really be an issue.

chrisw957 · « **Reply #10 on:** January 21, 2012, 04:32:42 am »

I revised the kit for a little better performance. I was seeing a slight oscillation at higher currents which I addressed by shortening up some of the traces and adding C4, C5, and R15. The schematic, BOM, and gerbers are available for download at the link below, and I'm again trying to unload the extra pcbs in a kit if anyone wants to build their own. Gotta love those 10 boards for 10 dollar PCB services.

http://www.sleepyrobot.com/?p=136

Yes, the max current is set to around 1.5 amps by the resistor divider R11 and R12, which you can tweak if you would like. I thought that was reasonable for the load resistors.

-chris

blackman · « **Reply #11 on:** January 21, 2012, 02:25:20 pm »

Are you shipping to Aus?

benemorius · « **Reply #12 on:** January 22, 2012, 11:02:47 pm »

You got 10 10x10 boards for $10? Where?

metalphreak · « **Reply #13 on:** January 23, 2012, 12:40:18 pm »

That would be 10 5x5cm boards for $10. Seeed Studio or Itead Studio

J4e8a16n · « **Reply #14 on:** August 09, 2012, 10:24:24 pm »

I builted it but the dual pots does not seems to work properly. Is there another way to set theese pots?
JP

J4e8a16n · « **Reply #15 on:** August 10, 2012, 06:44:44 pm »

Quote from: chrisw957 on January 21, 2012, 04:32:42 am

I revised the kit for a little better performance. I was seeing a slight oscillation at higher currents which I addressed by shortening up some of the traces and adding C4, C5, and R15. The schematic, BOM, and gerbers are available for download at the link below, and I'm again trying to unload the extra pcbs in a kit if anyone wants to build their own. Gotta love those 10 boards for 10 dollar PCB services.

http://www.sleepyrobot.com/?p=136

Do you ship to Canada?

JP

Yes, the max current is set to around 1.5 amps by the resistor divider R11 and R12, which you can tweak if you would like. I thought that was reasonable for the load resistors.

-chris

J4e8a16n · « **Reply #16 on:** August 12, 2012, 02:42:04 pm »

Hi,

I am trying to find why the regulator heat so much. I checked everything.
How can I check the IC without desoldering it ?

JP

JimmyM · « **Reply #17 on:** August 12, 2012, 03:12:28 pm »

Oh, jeez. I'll have to look back through my notes.

J4e8a16n · « **Reply #18 on:** August 17, 2012, 01:08:19 pm »

Hi,

Ichecked the regulator for 24 mA , 8Volts battery, I get .024*3 = 0.072 mW.
The spec does not give the Jc degrees C. It gives a general 200 degrees C.

0.072 * 200 = 14.4 D/ W + 23 Ambiant = 37
I read 60 degrees with my multimeter wich is accurate.
Maybe I should change the regulator for a 7805ct..

JP

J4e8a16n · « **Reply #19 on:** August 17, 2012, 01:16:19 pm »

Quote from: JimmyM on July 09, 2011, 08:05:03 pm

Since I'm using the CX101, there's not much sense in going over 2A any way. It will be configured for a maximum of 1.999 counts.

Could you post your shema?
I have tried 2 ways to make the CX101b work without success.

Jp

J4e8a16n · « **Reply #20 on:** August 21, 2012, 09:45:21 pm »

Quote from: J4e8a16n on August 17, 2012, 01:16:19 pm

Quote from: JimmyM on July 09, 2011, 08:05:03 pm
Since I'm using the CX101, there's not much sense in going over 2A any way. It will be configured for a maximum of 1.999 counts.

Could you post your shema?
I have tried 2 ways to make the CX101b work without success.

Jp

Corrected.

J4e8a16n · « **Reply #21 on:** August 23, 2012, 09:55:46 am »

Here is a link: http://pianointune.blogspot.ca/2011/04/voltage-division-with-coarse-and-fine.html

Why dont you use the .33uF capacitor recommended in the datasheet at the input of the regulator?

JP

J4e8a16n · « **Reply #22 on:** August 24, 2012, 01:43:41 pm »

Hi,

This is a dead thread?

Dont forget to let us know if you find out how to make it work with a shematic.

J4e8a16n · « **Reply #23 on:** August 24, 2012, 01:50:28 pm »

Hi,

Here is the cx101b datasheet . I still cant make it work.

Rerouter · « **Reply #24 on:** August 24, 2012, 01:52:35 pm »

there is the A version (9V), and the B Version (5V) but there is also the BG varient that is the b version thats comapatible with a common ground, make sure you have the right one, else you need an isolated supply to the meter,


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Author Topic: Constant Current Dummy Load Kits (Read 17696 times)

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