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  • Author Author: jw0752
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DC Load Unit "La Carga" Episode I

jw0752

In January of this year, inspired by some of 's excellent tutorials I decided to begin to design and construct a DC load. I have been learning and improving my electronics ever since joining E-14 about two years ago. Those of you who are familiar with me know that I tend to be a little retro and basic in my approach to problems and projects. This DC Load unit which I have dubbed " La Carga " (named chosen by ) will be no exception. Up until now I have always been satisfied to grab what I called a "LOAD BULB" every time I wanted to load a circuit.

 

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While these have served my needs in simple experiments the progress of the last couple years has put me in a position where I want a little more precision and control of the load in my experiments.

 

As I mentioned I began in January to draw rough schematics and plans for the DC Load unit. Thanks to the inspiration offered by my friend I have put some effort into learning the basics of SPICE IV. For the first time in my life I am able to run simulations of many of my prototype circuits prior to bread boarding them. I also am planning so that the power supply and other circuitry is more appropriately sized to the job they are intended to do. When one is not very good at electronics there is a tendency to over build. This is why over the years many of my creations are "TANKS" when they should have been sports cars. As you will see I did not completely escape this tendency but I hopefully have made some improvements.

 

The plan is to use that same style enclosure that I used when I built my power supply in September of 2014.

 

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This Case and Chasis was from a 40 year old piece of Dental Equipment called a Cavitron. Above you can see the before and after pictures of the power supply. If you have an interest in that build here is a link to the blog post:  http://www.element14.com/community/people/jw0752/blog/2014/09/21/a-new-power-supply-for-john

 

Circuit Over View:

 

My plan is to build a DC Load that will cover the voltage range of 3 Volts to 35 Volts and be capable of 6 amps with a max wattage of 210 Watts. I want to have the unit sense the voltage and add or remove series current limiting resistors so that my control device, a salvaged IGBT, (Isolated Gate Bipolar Transistor) will never have to dissipate more than about 40 watts itself. I have decided to use digital LCD meters on this build so the unit will not be quite so retro, though no one will argue that I am completely out of the retro mode by any means. Something else that is new for me is that I have been inspired by and I am going to blog about this build as I am doing it. Up until now I have wanted to have a finished functional unit before putting my neck out. This will give everyone better insights into some of the mistakes and wrong paths that I take while building. I do reserve the right to not talk about the really dumb mistakes.

 

In Episode II , a couple days from now, I will publish my schematics for the build and show pictures of some of the major components.

 

John

Parents Comment
  • in reply to jw0752

    Hello John,

     

    Glad you are making progress and always happy to help if possible.

     

    On a good heatsink (blown < 0,45C/W) you could expect to put 80W into the IRFPC60LC MOSFET and see a junction temperature of about 120 - 130 where the fully turned on gate source resistance will be about 0.8R - but fo your 3V 6A you need 0.5R  - do you have 2 - then you could wire them in // and with a better heatsink and a max ambient of 35C you could just about reach your performance target with no resistors at all.

     

    (Thermal resistance 2 FETS to heatsink = 0.225C/W so with 210W the junction temp is heatsink + 47.25. If ambient = 35 and max safe junction temperature = 120 your heatsink needs to manage (120 - 35 - 47.25)   = 37.75C rise for 210W = 0.18C/W - this weedy thing from RS (158-556 60mm square) claims 0.5C/W so my guess (you would need to test) is that something like this StarTech.com 85x70x50mm Socket 478 CPU Cooler Fan with Heatsink and TX3 Connector: Amazon.co.uk: Computers & Accessories will do at least as well - use one per MOSFET.)

     

    MK

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