Another Adjustable Current Load Anyone? - Page 1
Forum section: electronics / projects

author: prenato
author_rank: Regular Contributor
timestamp: September 30, 2012, 01:24:24 am
content: 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 LoadCheck it out:http://paulorenato.com/joomla/index.php?option=com_content&view=article&id=91&Itemid=2The 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

author: Jimmy the Squid
author_rank: Regular Contributor
timestamp: September 30, 2012, 02:29:01 am
content: 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.

author: toli
author_rank: Frequent Contributor
timestamp: September 30, 2012, 12:50:15 pm
content: 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.jpghttp://img10.imageshack.us/img10/5294/img0183yc.jpgI 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

author: prenato
author_rank: Regular Contributor
timestamp: September 30, 2012, 03:12:05 pm
content: @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:)

author: saturation
author_rank: Super Contributor
timestamp: September 30, 2012, 10:15:46 pm
content: 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.

author: prenato
author_rank: Regular Contributor
timestamp: September 30, 2012, 11:08:28 pm
content: 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

author: saturation
author_rank: Super Contributor
timestamp: October 01, 2012, 02:19:09 pm
content: 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 :

author: prenato
author_rank: Regular Contributor
timestamp: October 01, 2012, 09:23:51 pm
content: 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/

author: saturation
author_rank: Super Contributor
timestamp: October 01, 2012, 10:58:26 pm
content: 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.

author: prenato
author_rank: Regular Contributor
timestamp: October 01, 2012, 11:55:45 pm
content: Thanks saturation. That is a good protection scheme. I just need to find a suitable TVS in my parts bin...