Another Constant Current #102 Implementation - Page 1
Forum section: electronics / projects

author: IanJ
author_rank: Unknown
timestamp: February 14, 2012, 10:30:43 pm
content: Hi all,Decided to build one of these today in order to load test a Psu................I got fed up raking for large wattage resistors! Anyway, thought I'd share.All parts robbed from my stock (ok, my bits'n'bobs box) and as such had a few hurdles along the way including a DVM which won't work with S- tied to V-, so I had to offset the signal voltage. I thought this might create some drift/innaccuracies but it isn't too bad.I had the novel idea to house the electronics within the heatsink including the PP3 battery, and again I thought this might create some drift/innaccuracies due to heat but again it isn't too bad!Tested up to 4A.......photo shows it running at 3.02amps. It's a 1degC/watt heatsink so runs pretty cool considering.PS. Trimpots in photo only there for testing......and I've still got to set limit of control pot so as not to run above 4.5A.Ian.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 15, 2012, 03:13:34 am
content: Quotenovel you say? how do you beat my darwin award invention?... and i had the dvm replaced with led indicator! thats darwinian novelty!

author: sacherjj
author_rank: Frequent Contributor
timestamp: February 15, 2012, 03:26:45 am
content: I'm thinking of doing the same thing with a massive PC heatsink. I think it is from an old Pentium 2. With a fan that I can kick on if I need additional cooling. I'm hoping to push 4-5 A as well.I'm designing mine with a microcontroller and rotary encoder. I'm going for a ATMega328, as I have a dozen or so of them. This would allow me to display Source Voltage, Current and Power. Also serially control it if I needed to have a profile, rather than a static load. Or log a discharge curve to the PC and throw the MOSFET into high impedance when the battery is discharged to a set limit.

author: IanJ
author_rank: Unknown
timestamp: February 15, 2012, 07:28:59 am
content: Quote from: sacherjj on February 15, 2012, 03:26:45 amI had the same thoughts as I'm big into Arduino's.......but thought heck it's only a dummy load.....However, knowing me that won't stop me...........Ian.

author: EEVblog
author_rank: Unknown
timestamp: February 15, 2012, 08:04:39 am
content: Quote from: IanJ on February 14, 2012, 10:30:43 pmNice build!Dave.

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2012, 08:07:46 am
content: I always want to see the underside of those perfboards or protoboards to see how people do the connections and soldering. I'm never quite sure what the best technique is.

author: IanJ
author_rank: Unknown
timestamp: February 15, 2012, 08:17:18 am
content: Quote from: IanB on February 15, 2012, 08:07:46 amHere's an example from another of my projects. I prefer to use vero board with 1*3 copper grid rather than full strip board.........that way not much cutting necessary.Some folks go way over the top in terms of neatness and layout such that a minimal amount of wire is used.......but that ain't me........I just want the job done.Hmmm, vero board prototyping might make a useful blog for Dave........Ian.

author: Rerouter
author_rank: Super Contributor
timestamp: February 15, 2012, 08:30:40 am
content: so many mod wires, great to see the level of complexity possible with perfboards .

author: grumpydoc
author_rank: Super Contributor
timestamp: February 15, 2012, 08:47:57 am
content: QuoteOh, I don't know - the back of those three boards is pretty neat.I agree that tri-pad or just single pad board is the way to get the best component layout on prototyping boards. I quite like the "self fluxing" wires that you can get which reduces the amount of wire stripping that you have to do.

author: Unknown
author_rank: Unknown
timestamp: February 15, 2012, 07:51:42 pm
content: Quote from: IanJ on February 14, 2012, 10:30:43 pmThe thermal resistance will be somewhat higher in this configuration, since that figure is probably for the heatsink in vertical orientation, when convection will provide more efficient cooling.

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2012, 09:49:57 pm
content: Quote from: IanJ on February 15, 2012, 08:17:18 amThanks, that's interesting to see.Do you wrap wires around component leads and then solder in one operation, or do you first solder components to pads and then solder wires to pads in a second operation?

author: IanJ
author_rank: Unknown
timestamp: February 15, 2012, 10:12:52 pm
content: Quote from: IanB on February 15, 2012, 09:49:57 pmI never wrap wires since part of the point of vero board is so that you can prototype and that means moving wires around, so you want it easy to reheat and move a wire.Step 1 - Solder component to pads.Step 2 - From the reel, strip end of wire (about 1mm) with side-cutters.Step 3 - Solder to pad/component leg.Step 4 - Lay down the the wire to gauge the length you need and snip it where you want the other end to be.Step 5 - Strip 1mm off the end.Step 6 - Solder to pad.For short runs, i.e. say you wanted a 8mm length then you would find Step 4 above extremely hard, so what I do is from the reel of wire strip back about 10mm, then, 8mm further down strip again, but don't let the outer insulation come off the end, let it come up the wire until it's about your 1mm from the end of the wire and so on........and with this method there is little waste.Other people may do it a different way, but the above works for me and you get quite fast at it.Ian.

author: Mint.
author_rank: Frequent Contributor
timestamp: February 15, 2012, 10:37:17 pm
content: Awesome design. Good job!

author: Freighternut
author_rank: Regular Contributor
timestamp: February 15, 2012, 11:54:37 pm
content: Hi Ian,Great design!What did you use for the shunt resistor?

author: IanJ
author_rank: Unknown
timestamp: February 16, 2012, 08:06:51 am
content: Quote from: Freighternut on February 15, 2012, 11:54:37 pmSMD D2PAK power resistor 35W 0R27RS 733-2694£2.58eaThere's enough of a tag on it I managed to get a washer on it and bolt it down onto the heatsink.Farnell sell similar in a TO220 package but £12.93.......Ian.

author: Freighternut
author_rank: Regular Contributor
timestamp: February 20, 2012, 08:10:19 am
content: Thanks Ian,I have ordered same from RS here in New Zealand.