Constant Current Source with BJT Transistor

Рет қаралды 6,922

4 жыл бұрын

These BJT transistor constant current source circuits can provide a reasonably constant current if precision isn't critical, such as driving LEDs.
The advantage of a constant current source over a single resistor to drive LEDs is that the circuit can supply the same current while the supply voltage fluctuates, or as different LEDS (or a different number of LEDS) are swapped in the circuit. With just a resistor, the current would fluctuate as these other parameters change, which may not be desirable.
BJT Transistor Biasing:
en.wikipedia.org/wiki/Bipolar...)
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Пікірлер: 25

@henrikjensen3278 4 жыл бұрын

It is often a good idea to do power calculations for led drivers, with 10mA you will seldom have problems, but at 50mA and a small transistor you might have problems. Other ways to make simple current generator is JFET and constant current diodes

@DrexProjects 4 жыл бұрын

That's cool. Who knew? You did. Thanks.

@pileofstuff 4 жыл бұрын

I've been meaning to play with constant current supplies one of these days. Thanks for the head start.

@GadgetReboot 4 жыл бұрын

remember to use a good quality breadboard instead of those cheap eBay ones, I lost at least four hours debugging the wrong part of my set up.

@BobWa43 3 жыл бұрын

Thanks, good video with clear illustrations and explanations.

@Lionel_Walder 4 жыл бұрын

Toujours au top tes explications

@MarcRidders 4 жыл бұрын

I learned a lot, thanks for the video.

@newburypi 4 жыл бұрын

Good one👍 Well explained and demonstrated.

@TheEmbeddedHobbyist 4 жыл бұрын

Nice video, now we need to move on to the current mirror and the long tailed pair. ;)

@GadgetReboot 4 жыл бұрын

I was going to do current mirrors but the whole thing was starting to get too long, but that saves more for another day!

@electronic7979 4 жыл бұрын

Helpful video I liked it

@hugobracamontesbaltazar 4 жыл бұрын

Good video! Regards.

@andymouse 4 жыл бұрын

Great stuff, I'm quite into this fundamental stuff at the moment, it's interesting how much I have forgotten...cheers!

@GadgetReboot 4 жыл бұрын

Yeah I need to fill in the blanks that have crept up on me over the decades. One thing I learned in this experiment - ebay breadboards can't be trusted for signal integrity. Until I switched to a proper breadboard, current was jumping all over the place!

@andymouse 4 жыл бұрын

@@GadgetReboot Massive problem, I'm doing small current stuff to and it's practically impossible on cheapy boards. Maybe a breadboard review ?

@GadgetReboot 4 жыл бұрын

yeah it’s similar to the problems I have with test leads that have alligator clips on them and they are poorly made so they build up resistance and you try to use them to provide 5 V to a circuit from a power supply and you’re getting 4.1 V and then it jumps to 3.7 V, I wonder if it’s possible to easily measure or demonstrate breadboard connectivity issues. Something to put on the list. lots of videos out there take apart the cheap breadboards and show the contacts are poorly made and it’s easy to see why it’s hard to shove components into those but I don’t know if anyone’s demonstrated connectivity issues and reliability.

@WistrelChianti 2 жыл бұрын

By jove, I think she's got it!

@aurthorthing7403 4 жыл бұрын

Sweet. I've been thinking about building a similar circuit to improve cheap LED flashlights. Something to tame it down while squeezing as much juice as possible out of the batteries.

@johncoops6897 4 жыл бұрын

Then DO NOT use this kind of circuit (with series resistor). You lose light but the excess power just gets wasted as heat (in the series resistor). If you want maximum juice from the batteries to be converted to light, then use a proper LED driver. If you want control, use PWM. Don't use "linear" type drivers - you want a high quality switching controller, Buck to step down, or Boost to step up..

@GnuReligion 4 жыл бұрын

Some blue LEDs will light up with very small currents ... sometimes far less than 1mA. The BJT mirror configuration can be a convenient to globally trim constant current to a series of loads. www.allaboutcircuits.com/uploads/articles/current-sinking-and-current-sourcing.jpg

@GadgetReboot 4 жыл бұрын

I remember regularly using around 2 mA for surface mount LEDs in designs but I don’t think I’ve had one less than 1mA, now I want to investigate that, maybe see if I have any already in my collection of surface mount and through hole. One thing I did notice recently with my through hole red LEDs, out of the same batch I got on eBay, some work fine and some look like they are only operating at about 1/3 brightness in the same circuit so that’s weird, I think I have to go through and do a quick audit and just throw away the weird ones. A few months ago I was researching current mirror circuit configurations but now I can’t remember a few of the things I learned, need to start over. i’ve never used them before and I can’t remember if they need matched transistor pairs or something else might have been a critical parameter, maybe I’ll get it the breadboard and see.