Current Source Dummy Load Design - For Negative Power Supplies - Page 1
Forum section: electronics / general

author: DaveFan
author_rank: Contributor
timestamp: February 03, 2014, 08:51:35 pm
content: I found Dave's Current Sink Dummy Load to be PERFECT for a project that I'm currently working on and just wanted to say THANKS, and for all the posts on the internet from people just like me, willing to give back in return for receiving assistance. I solicited help on creating a NEGATIVE version of the Dummy Load, and just wanted to say thanks for those who gave input, it helped. I decided to simulate the circuit and that helped to get my creativity going. I'm a Multisim user and find it to be a good place to play around with ideas.I've attached a snap shot of my circuit which contains Dave's original design idea, along with my INVERTED VERSION (Current Source) of the same.The FETs can be any which meet the needs of your circuit, I simply tried different ones and it didn't make a different to the operation of the circuits.If you see any issues with the circuit please feel free to give input, I welcome it!And thanks for sharing!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 03, 2014, 09:48:50 pm
content: Hi,You can make a few improvements that will reduce the risk of the circuit oscillating.I provide some details in this thread:https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/msg288313/#msg288313To convert the design for a negative supply I came up with this:These are the results of the simulation:I have attached a zip file with the LTspice model.Regards,Jay_Diddy_B

author: DaveFan
author_rank: Contributor
timestamp: February 04, 2014, 02:55:37 pm
content: Hello Jay_Diddy_B,Nice work! The filter and snubber will be great additions to my circuit.How did you calculate the values?Thanks!

author: DaveFan
author_rank: Contributor
timestamp: February 10, 2014, 03:05:10 pm
content: It would be nice to get some input from you guys...

author: mrflibble
author_rank: Super Contributor
timestamp: February 10, 2014, 04:40:17 pm
content: Why not breadboard it and see what the sucker does?

author: DaveFan
author_rank: Contributor
timestamp: February 10, 2014, 04:56:34 pm
content: I have bread-boarded that little sucker, and it works, but I still want feedback if someone has other ideas.Thanks!

author: mrflibble
author_rank: Super Contributor
timestamp: February 10, 2014, 05:19:44 pm
content: Well, one thing you could try is use an opamp for which the input range comes close to the positive rail. The LT1013 mostly makes sense in the other flavor (load for positive power supplies), because the LT1013's input range goes right to the negative rail. But for this arrangement you want an opamp that can get close to the positive rail. Why? because that way you would only need the N9V supply in that schematic.Also you could try to change V3 (the current set voltage) to a negative voltage, and tie it to the other opamp input node. That way you can create that V3 voltage from the N9V supply.Just an idea if you want to get rid of the dual supplies, so you can run it off a single 9V battery for example.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 10, 2014, 08:50:12 pm
content: Hi,Just a little additional information:1) used the LT1013 because it is included with LTspice. The LM324 will work just as well in this application.2) mrflibble describes how to remove the positive supply. The original poster was looking to design a load for testing dual (positive and negative supplies.) If the positive supply was eliminated from the negative supply it would be needed for the positive supply.3) There is some advantage to having a positive reference, for the negative load, especially if you want to replace the pots with a DAC.Regards,Jay_Diddy_B