Another Adjustable Current Load Anyone?

[prenato]

Ok, I know this is by now almost a "rite of passage" in this forum, so I decided to build yet another version of the Constant Current Load Check it out:

http://paulorenato.com/joomla/index.php?option=com_content&view=article&id=91&Itemid=2

The main difference some might appreciate (relative to other designs I've seen) is the added protection features (over-temperature protection and fan control).

Cheers!
Paulo

[Jimmy the Squid]

I won't comment on the electronics, since I am really not qualified, but I do have to say that that is a very nice looking package. I'm in the process of building my own - rite of passage and all - so I will enjoy reading your webpage in depth.

[toli]

Looks very nice, what case did you use for that?

I've actually built a more basic version in the past, and I've used a HS from an ATI video card as the HS and the thing that holds everything in place:
img803.imageshack.us/img803/3382/img0182ic.jpg
http://img10.imageshack.us/img10/5294/img0183yc.jpg

I thought about building a nicer unit many times, but found no need for that. I simply don't need it that much and for what I do need the one I have usually works just fine. I guess when I'll have the need I'll build something similar to what you did

[prenato]

@Jimmy: thanks for your comments. I hope you have fun building yours. The main issues with mine were stability but the rest was relatively straightforward. It's a fun project.

@toli: I used an old enclosure for a parallel port switch. This is a completely obsolete technology of course, so I just threw away the insides. The box was excellent quality though... They don't make them this good these days;not for something as basic as a switch anyways. It cost me only a couple dollars at a Thrift store:)

[saturation]

Great job, and nice documentation on your website.   

Just some thoughts.

"Overload protection" is really just overheat protection, as I can see.  I presume a user manually limits exceeding input ratings: 5W, or 7A @ 0.71VDC or 83mA@ 60VDC.  It may help to add some form of input protection, these are more likely to happen than overheating; there is at least a fan backup but nothing to prevent over current or voltage.   The body diode of the MOSFETs provide reverse polarity input protection, but it will short the inputs, which could be a shock for the item being loaded.

One could increase power dissipation with resistors in parallel, e.g. 5 watt resistors  more of the same, e.g. n=6, increases it: 5W x n= 60W. 

What is the let through voltage of the MOV?  If this 2 terminal MOV were to conduct it would short the inputs and absorb the excess energy, if it blows, it can cause quite a fire.  If not, it fails open circuit, leaving the inputs unprotected and no indication of its failure.  A better MOV is a 3 terminal variant with integrated thermal fuse that can be put in series with a power line, say with Vin, once triggered it disconnects the MOV and your circuit entirely from Vin. 

[prenato]

Hi Saturation,

Thanks for your feedback! You make some really good points. I agree with you on the power resistors. One often has to do with what's available in the shop so this is why I didn't go for higher power rating. I don't anticipate needing more than 4 or 5 A for most of my projects but someone needing that could certainly follow your good advice.

The voltage protection from the MoV is also admittedly somewhat of a compromise. It,s a 100 V device I had handy that is barely adequate. It should limit fast transients but a lower voltage would be preferable. I might also add a fuse in series as per your suggestion... I have quite a few of those handy:)

Thanks for your suggestions,

Paulo

[saturation]

Here's something I ran across just now, a good comparison of different types of MOVs and how they react to over voltage or current, the good stuff begins at 1 min :

[prenato]

Cool! this project was featured in the hackaday.com blog:

http://hackaday.com/2012/10/01/test-your-projects-mettle-with-a-protected-dummy-load/

[saturation]

Congratuations, pronato.  Here's an sample simple input protection circuit using TVS diodes.  Its simple to implement and similar to the zener version I deleted earlier except TVS can take enough current flow to blow a fast blow fuse.

[prenato]

Thanks saturation. That is a good protection scheme. I just need to find a suitable TVS in my parts bin...