Anyone ever design a DC Electronic Load? - Page 1
Forum section: electronics / projects

author: nixfu
author_rank: Unknown
timestamp: November 14, 2014, 03:02:18 pm
content: I was thinking about building myself a DC Electronic Load to use for testing my DC power supply circuits etc..For a hobbist, plunking down $300-500 US for a decent BK or HP one on Ebay, seems like a bit much when there is other gear higher up on the 'could use' list.Conceptually they are just fancy dummy loads right? I imagine you use nearly the same techniques in building one as you do building a constant voltage/constant current power supply, just sort of in reverse.I have not found much luck looking for designs/plans on google, and I guess that is not too surprising since its a very niche tool for the bench. But I figured maybe someone around here if anywhere would possibly have build one before.

author: lincoln
author_rank: Regular Contributor
timestamp: November 14, 2014, 03:27:52 pm
content: Well it depends on what you are looking for. For a basic constant current, this and the beginners section is lousy with them... Most of them will have a opamp based linear regulator dumping current onto a load / scene resistor.other interesting papers:http://pdfserv.maximintegrated.com/en/an/AN752.pdfhttp://cds.linear.com/docs/en/application-note/an104f.pdfrecent threads:https://www.eevblog.com/forum/projects/active-load/https://www.eevblog.com/forum/projects/dc-dummy-load/https://www.eevblog.com/forum/projects/dc-load-using-a-cpu-cooler/https://www.eevblog.com/forum/beginners/constant-current-load/

author: Kohanbash
author_rank: Regular Contributor
timestamp: November 14, 2014, 05:39:35 pm
content: I have not watched them yet (I just discovered these after Dave's latest Rigol video) but EcProjects seems to have some videos on thishttps://www.youtube.com/user/EcProjects/featured

author: wigman27
author_rank: Regular Contributor
timestamp: November 14, 2014, 09:44:08 pm
content: Hi,This is one I have designedhttp://www.instructables.com/id/Arduino-Programmable-Constant-Current-Power-Resist/https://www.eevblog.com/forum/oshw/arduino-programmable-constant-current-power-resistance-load/ThanksLee

author: nixfu
author_rank: Unknown
timestamp: November 14, 2014, 10:07:53 pm
content: Wow. I just found that one. It is EXACLY what I was picturing in my head. Very nice design.I was looking at the source code...did you ever considering adding any totals to the Arduino to automatically keep track of amp hours or anything else?I am going to order one of your "Dave approved"boards and try and build one of your design myself.

author: wigman27
author_rank: Regular Contributor
timestamp: November 14, 2014, 11:41:29 pm
content: That's great! I'm glad you have found it useful.The link to the store ishttps://www.tindie.com/products/Wigman27/pcb-for-arduino-programmable-constant-current-power-resistance-load/I only have 1 left until I can get some more (which shouldn't be too long hopefully!)ThanksLee

author: NZST205
author_rank: Contributor
timestamp: November 15, 2014, 01:19:12 am
content: I highly recommend these, I'm an absolute beginner and learnt a lot through the process, Lee was most knowledgeable and helpful when my ignorance (nee, sometimes laziness) got the better of me.

author: wigman27
author_rank: Regular Contributor
timestamp: November 15, 2014, 02:54:12 am
content: Quote from: 3roomlab on November 15, 2014, 01:23:16 amNo, the maximum current through the 1 ohm resistor is 8A therefore dissipating 8W.The MOSFET does dissipate most of the load.Thanks all! It's fantastic to see so many people getting help from this.Lee

author: janengelbrecht
author_rank: Regular Contributor
timestamp: November 15, 2014, 01:11:16 pm
content: Answer to headline in topic: Yes :http://elektronik-forum.dk/index.php?topic=850Is it any good ? Well to few MOSFETs people say for 4A driveAdd 2 more

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 15, 2014, 03:50:51 pm
content: Related question: why is a project like this so interesting to people?I have a box of power resistors, I test my project and that's that. I don't understand the motivation.Tim

author: janengelbrecht
author_rank: Regular Contributor
timestamp: November 15, 2014, 03:53:39 pm
content: Quote from: T3sl4co1l on November 15, 2014, 03:50:51 pmThink: Having an unlimitited amount of power resistors : unlimited amounts of valuesWell if youre into designing power supplies it could come in handy. But yes...if I design a 0-30V/0-5A power supply i could just use an above 150W 6R resistorOr a lot of theese in parallel .... to get 6 Ohm...less demand on the Power rating... nothing is impossible

author: nctnico
author_rank: Super Contributor
timestamp: November 15, 2014, 04:38:43 pm
content: Quote from: T3sl4co1l on November 15, 2014, 03:50:51 pmAdjustability for starters. And a DC load can mimic a true current source better than a resistor. The advantage of a current source is that it's resistance is infinite so if your PSU tends to oscillate you will find out. I recently build one myself which uses resistors switched with MOSFETs. Because it uses a feed-forward control scheme it can load a PSU with extremey sharp pulses (I measured 20A/us) so it is ideal to test the load step response of a power supply.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 15, 2014, 07:31:44 pm
content: But what kind of loads are even constant current? Anymore, that is. A resistive load is probably more representative for some purposes (e.g., CMOS logic?), but just as well, anegative resistancemay be more illustrative (subsequent switcher loads)!Transient testing is handy, in which case I put the signal generator into a MOSFET on the breadboard, same story with load resistors. As for selection, it's hardly critical (I'm testing above and below rating anyway, which isn't usually a hard limit anyway e.g. peak current mode converters under nominal conditions), so I just grab whatever.Tim

author: janengelbrecht
author_rank: Regular Contributor
timestamp: November 15, 2014, 07:39:15 pm
content: So it could be an advantage to have an AWG to control an opamp and this the MOSFETS in an active load ? To simulate a Non constant load ? hmm

author: nctnico
author_rank: Super Contributor
timestamp: November 15, 2014, 09:14:26 pm
content: Quote from: T3sl4co1l on November 15, 2014, 07:31:44 pmThe collector of a transistor is a current source and ideal for testing power supplies. I used to have a self build transistor based DC load which didn't have a feedback loop. Thermal runaway is an issue (could be mitigated with a control circuit) but the load behaves almost like an ideal current sink.

author: wigman27
author_rank: Regular Contributor
timestamp: November 15, 2014, 10:05:48 pm
content: Well I can think of quite a few.1. Testing battery discharge, if you have a project that runs on a constant current which most regulated devices will. You can test how long your batteries will last when drawing a certain current over their dropping voltage. Can't do that with resistors.2. Testing power dissipation over a wide voltage range. If you want to test your supply to its limits and change the voltage but draw the same power, this will do it. Resistors won't.3. You can set this to be a certain resistance also.4. Testing voltage ripple in a rectified power supply designed for a particular project. You can draw various currents and test it over a specified range of currents, in 2ma increments.The box of high precision power resistors you would need to test 0-8A 0-24V at 50W I would hate to think how much space that would take up let alone the cost!That's why I built one anyway. If you prefer resistors then use resistors :-) simple.

author: wigman27
author_rank: Regular Contributor
timestamp: November 15, 2014, 10:18:18 pm
content: Quote from: janengelbrecht on November 15, 2014, 01:11:16 pmThis MOSFET is capable 100A and I must say, I haven't heard of 4A per MOSFET. I wouldn't think they would make them capable of 100A if you were only supposed to put 4A through it. I can understand for heat dissipation but that's about it.

author: robrenz
author_rank: Super Contributor
timestamp: November 15, 2014, 10:19:32 pm
content: Some Ideas inthis threadExample

author: janengelbrecht
author_rank: Regular Contributor
timestamp: November 15, 2014, 10:54:45 pm
content: Quote from: wigman27 on November 15, 2014, 10:18:18 pmThe total power is divided...that makes some senseAnyway I will use 3

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2014, 08:15:46 am
content: Quote from: wigman27 on November 15, 2014, 10:05:48 pmJust discharging a battery goes fine, but if you require dropping it to zero, yes, a resistor will get quite slow through the tail. Though not proportionally so, because the battery itself has an exponential decay rate, so the last 50% of voltage takes less than 50% of the time, even with much less than 50% of the current draw.QuoteCough... that requires negative resistance, not constant current.One that's programmable for positive or negative (or infinite in the middle) resistance would cover that nicely, of course.QuoteAre you expecting that the ripple will vary in an unnatural manner from zero to nominal?Digitizing as many points as possible can be handy, and makes nice spreadsheets, but isn't always useful. This relates to the analytical relevance of information and sampling: unless the power supply design is shitty (weird operating modes at certain oddball currents), it should be sufficient to define its operation with just a few points and an interpolating curve. More points are redundant.QuotePrecision?1. If I need to know something that accurately, my power supply must be a pretty crappy (razor thin margins) design!2. If I need to know something that accurately, I'll measure it directly (volts and amps, or measure the resistance at temperature), which is better than watching it through the offset, gain and nonlinearity of a test box's digital readouts.To check the operating area of a 0-8A, 0-24V supply, I would use probably twice as many steps as suggested by the design (presumably this example is a stepped linear regulating supply?), and also let it sit for an extended period of time on the highest current tests to verify that it can handle that power continuously (hint: those cheap Mastek and myriad clones (e.g. 30V 5A) you find on eBay won't). If it's single range (200W square operating area instead), a grid of two or three voltages and currents will suffice to verify its operating range.Load step is usually done with a moderate pre-load (10 or 50%) and the remainder switched; if the transient is well damped on both sides (i.e., for a load of R or 10*R), it's reasonable to expect it will tolerate a load of infinite R, and perhaps even negative R before going unstable. (The worst negative resistance a power supply should reasonably expect is dV/dI = incremental R = -V^2 / P, the constant power curve typical of switching supplies.) If the controller is well-behaved, the output impedance should be a well-defined RLC equivalent, which can be solved for based on transient response to find stability range.Tim