Transistor as a current limiter

"Jon Kirwan" schreef in bericht news: snipped-for-privacy@4ax.com...

Suppose Q2 slipped through non reversed?

Don't worry about your brains. Without them you couldn't make a mistake :)

petrus bitbyter

Of course there are ICs for every purpose nowadays, but "buy this chip" is kind of a boring solution to anything.

Besides, I'm not convinced that the examples you show are ideal for my purposes.

This is a boost converter intended for Li-ion batteries. I don't think step-up conversion buys me very much if I work with a 12 V power supply, already enough to drive several leds in a series.

This is a switching current limiter. It sounds great, but since it's expensive relative to leds, one would ideally run it from mains to ensure it runs a maximal number of leds. Unfortunately dabbling with mains power directly is illegal for hobbyists where I live. (Also, I wouldn't dare touch it even if it weren't.)

This looks more like what I need:

It controls 32 led chains of 13 V each. However, it has lots of extra features and costs 10 bucks, far more than 32 cheapo transistors. Also, it's surface mount. I'll stay with through-hole for the time being.

Thanks for the suggestions, anyway.

Lauri

I actually caught that mistake, right away. Note my self-reply shortly after?

Hmm. An argument for being a machine, perhaps. ;)

Jon

On Sun, 7 Jul 2013 08:13:30 +0000 (UTC), Lauri Alanko wrote: . . .

--- If you're going to be using tens of chains in order to turn on some total number of LEDs, then you'd be well advised to use a higher LED supply voltage with more LEDs in each chain than for 12V.

For example, with a 24V supply and 2.2V LEDs, you could string ten LEDs in series, drive them from the 24V supply, and still have about

I understand the advantages of higher voltage, but I have decided to stick with 12 V, since 12 V power supplies are cheap and ubiquitous, and I'm not comfortable dabbling with higher voltages yet.

I don't even know which leds I'm going to use yet. The ones I have currently are too directional. I also don't know yet how many I'm going to need. I intend to experiment and see how many I need to add until it looks good to my eyes.

The idea is to make gadget that glows in different colors. The leds will be surrounded by a diffusion shell. It doesn't need to illuminate an entire room, but I certainly want the light to be noticeable.

I'm now considering using 0.5 W leds. They are probably a bit less efficient, and won't spread the RGB colors quite as evenly, but at least there's less wiring. Heat might be a problem, though. We shall see.

Lauri

"Not knowing" which LEDs may be used emphasizes the need to be able to support more than one series chain, since you don't know how many you can get into one series chain and the lighting effect may drive the total number of LEDs needed. A flexible design approach may then be an option worth considering.

So I don't think time was entirely wasted. Though a better focus would certainly help select between design tradeoffs.

Jon

Here's a mosfet version of the generic current limiter:

R1 can be most anything, 5K maybe. The current into the LEDs is about

The mosfet needs to be a logic-threshold part. What PWM voltage do you have available?

I'm sorry if I gave that impression. I merely gave an example calculation, with realistic sample values, to demonstrate my concern with the circuit in question. I hoped to emphasize that this was merely hypothetical by using words such as "suppose", "load would be", "if we designed" etc.

Besides, in the same example the supply voltage was 12 V, and that didn't prevent you from suggesting otherwise. Surely there was nothing to prevent you from suggesting different leds, as well?

In any case, I don't see how anyone's time has been wasted. All the replies I've received have been most pertinent and educational, and the circuits I've seen seem useful regardless of whether one plugs in four 2.2 V leds or three 3.6 V leds per series.

Lauri

I didn't feel misaligned. (Well, I'm a hobbyist anyway.)

In any case, this is the 'basics' group. Does a question have be asked as though it were framed by an expert here? No. And you framed the question better than many I've seen (and some that I've asked.)

...

So... are you going to try something out? Or will this remain hypothetical for a while?

Jon

"Jon Kirwan" schreef in bericht news: snipped-for-privacy@4ax.com...

Missed it completely. That's to say my newsserver did. Now you mentioned it, I found it in Google groups.

My IP, though "the best IP of the country," considers its newsserver as experimental so it comes without any guaranty at all. Sometimes my own postings do not show up even if wrote on that actual server.

Thanks for the note about it. Yeah, I get it. Nice you caught it, too. I might have missed it myself and that's a good catch from you!

After my internet service (Verizon) decided to remove from their contract, unilaterally and without my permission of course, NNTP services... I pretty much was forced to "go find something." I'm paying 10 euros (despite being in the US) per year for a service offered in Europe and it appears to be working well these last years (fast.) news.individual.net.

Seems to get everything I care about, perhaps except for alt.binaries.schematics.electronic. I have to just live with that for now.

Well, at least you HAVE a newsserver with your internet service.

I grew used to the idea that my internet service is supposed to provide the usual suite, including domain name services, mail, and NNTP. Traditional, I had thought. You take the good with the bad to get the business. But it's really a monopoly, so... they can take the good with the good and sell off the bad.

Jon

Having read some of the suggestions, I notice very few of the minimalist solutions I favour.

The minimalist 1 transistor current source: give the transistor a fixed base bias (like the Vf of a LED) and ground the emitter via a resistance (the value of which determines collector current). If current tries to increase; the voltage on the emitter resistor creeps upward and cancels some of the fixed base bias - which acts to regulate the current.

Its that simple.

I think that was already in the OP's original, very first post... along with some comments from the OP in that same post that showed thinking going on about it.

Jon

perhaps next year send $50 to astraweb and receive a terrabyte of usenet with no use-before date.

Hmm. On first blush, that sounds like a lot. But when I think about what a terrabyte will likely mean to me... hmm. Do they carry binaries like alt.binaries.schematics.electronic? (I guess I can go look and find out... but if you already know, I wouldn't mind your opinion about it, too.)

Thanks, Jon

I meant "a lot of money." In case that wasn't clear.

Jon

Just looked at the site and it's not clear what they carry and what they don't carry. I'll have to email them about details, I suppose.

Also, I used to get unlimited NNTP, a few email accounts, access to their dial-up "56k" modems, and a shell account for $10/month in a bundle. (Did NOT include my ground line at the home, of course.) The ground line cost me $45 per month, then. (And I had to buy my own modem, of course.) That was more than 15 years ago.

Now I still pay for home fiber on three optical wavelengths if I can believe what they told me. Phone service, broadband, and internet access. Then I pay for NNTP, separately. And pay for shell accounts, a name on name servers, and email as a bundle, separately. And I pay more, of course.

My NNTP access has to be around a megabyte or so per day. Unless the protocol is totally stupid and intentionally inflates packets with random garbage, it can't exceed 10 meg per day. I don't do news server binaries, except perhaps the electronics one. 1000 gig would last me a while, I imagine. I might die before I get my money's worth. ;)

Jon

ABSE is pretty much a dead group with occasional political rants from JT - even JF is having trouble finding people to rag on!