Another counstant current load for critics - Page 1
Forum section: electronics / projects

author: 001
author_rank: Super Contributor
timestamp: November 03, 2019, 05:49:25 pm
content: HiIs it good or bad project?https://diyodemag.com/projects/dummy_load

author: floobydust
author_rank: Super Contributor
timestamp: November 03, 2019, 08:42:57 pm
content: Bad.

author: Whales
author_rank: Super Contributor
timestamp: November 03, 2019, 09:55:16 pm
content: From the article:(Viewing this image required javascript + a lightbox.)

author: Whales
author_rank: Super Contributor
timestamp: November 03, 2019, 10:01:54 pm
content: Wait, they consider this a switching device? But this looks like a linear regulator.Hmm, they have a scope shot of something that looks switching. There are no grid squares, so I can't tell what the 100mV scale means, but I suspect they have just zoomed into the ripple.Also I am not sure you really need the 9V regulation stage. Ditch it and use your 12V directly unless your 12V source is unstable. I suspect it was born of them changing it from the "5V" design they mention.Overall: it's a simple design built with jellybean parts. The IRF540 most people will use will be a greymarket fake (it works fine but won't meet datasheet specs, be wary of their remarks regarding the 175degC die limit). My only thought is to whether or not this device is stable and under what conditions.

author: Whales
author_rank: Super Contributor
timestamp: November 03, 2019, 10:08:29 pm
content: Some specific quotes about this design:Quote"inefficiencies in the MOSFET" isn't the right wording or line of thought here. That's too easy to construe in many incorrect ways, this class of device always has the same "efficiency" regardless of the MOSFET used.QuoteI would have thought this device operated the FET mostly in the linear region, not the saturation region.I once bought or obtained an issue of Diyode after being recommended it. I didn't end up liking it, but I can't remember the full details.It looks like they're focusing on the whole round trip of making a device from breadboard to box. That's nice. It's made from jellybean parts, it's performance won't be anything fancy, and there are a lot of tweaks I would do to the PCB for better perf; but I think that's outside their aims and misses the point. It's not intended to be an amazing spec dummy load, but instead shows you how you can start making your own (simpler) alternative to the off-the-shelf products the mention in the opening.

author: bson
author_rank: Unknown
timestamp: November 04, 2019, 08:42:27 pm
content: Quote from: Whales on November 03, 2019, 10:08:29 pmFor a FET the saturation region is the linear/ohmic region.

author: Dave
author_rank: Super Contributor
timestamp: November 05, 2019, 09:09:41 am
content: Quote from: bson on November 04, 2019, 08:42:27 pmSource

author: mikerj
author_rank: Super Contributor
timestamp: November 05, 2019, 10:50:23 am
content: Quote from: Whales on November 03, 2019, 10:08:29 pmThe FET in a linear regulator has to operate in the linear/ohmic region. If the FET becomes saturated then the current/voltage is no longer under control.

author: magic
author_rank: Super Contributor
timestamp: November 05, 2019, 05:58:30 pm
content: No, when the channel saturates then current stays nearly constant regardless of minor variations in drain voltage, controlled solely by gate voltage. Quite a desirable condition in a constant current sink, actually

author: tooki
author_rank: Super Contributor
timestamp: November 05, 2019, 06:56:44 pm
content: Quote from: Whales on November 03, 2019, 09:55:16 pmNo, as with many lightboxes, the JavaScript merely traps the click and redirects to a lightbox, but the actual link itself is a plain link to the destination image, so that without JavaScript, it’s a functional link. (That’s how the inline thumbnails on eevblog work, too.) You can try this by opening the link in a new tab, which sends you to this URL:https://diyodemag.com/_images/5cbed096c672e05c2cb5a1fc

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 05, 2019, 07:04:07 pm
content: This project is a pretty basic voltage-to-current converter. There's nothing fundamentally "wrong" with it. As is, it has some limitations, but it can work fine for a range of conditions.(I hate schematics with basic elements like opamps not drawn as actual opamps...)As it is, one of the opamps is used as a voltage buffer for the "reference" voltage coming from the pots; then the buffered voltage is divided by 2 (to limit the current range). Let's call it Vref = Vpot/2. With R6, the resulting current range would be 0 (there will be an offset though!) to 4.5A: Iload = Vref / R6.With the LM358 and an IRF540, the probability of it oscillating is small IMO, but not zero. You can add a capacitor in the feedback loop to avoid oscillations.Due to the topology, the working voltage range is also limited. R6 basically drops the voltage reference (= R6*Iload). Obviously the input voltage must be greater than Vref + Vds.This basic topology has been discussed many times on here, so you should be able to find a number of threads dealing with it and the possible traps (including oscillation).As to whether the MOSFET mostly works in the linear or saturation region, I'll leave it as an exercise.

author: 001
author_rank: Super Contributor
timestamp: November 05, 2019, 07:41:14 pm
content: Thanx a lotCan U link me with better project at these level?

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 05, 2019, 07:49:00 pm
content: You can take a look at this thread:https://www.eevblog.com/forum/projects/mosfet-and-opamp-load-tester-constant-current-load