Current limiter resistance values - Page 1
Forum section: electronics / beginners

author: hermitengineer
author_rank: Regular Contributor
timestamp: May 05, 2023, 07:18:18 am
content: I was looking at the circuit that ElectroBoom used for his firefly lantern, and the two-transistor part in the upper left caught my attention. First job was trying to figure out what the circuit did, so I redrew that part in KiCAD and made some notes while attempting to analyze it. It quickly became obvious that its intent was current-related, but it wasn't actually a current source. Finally found a current limiter circuit that matched perfectly, so that's what it is.But then I looked at the resistor values. The 1 Ohm sense resistor makes "sense", providing an upper bound of 0.7A (though closer to 0.4A according to my experience with modern BJTs). But the 10K Ohm resistor seems to be too high unless I'm missing something. If we were to allow that 0.7V were developed across the sense resistor as well as the second BJT junction, then 13.5V - 1.4V = 12.1V should be across the second resistor. But that's 0.00121A, and even a decent beta of 100 on the second BJT would leave it with 0.121A, wouldn't it?Am I missing something, or does that second resistor make the sense resistor and BJT unnecessary components?

author: Zero999
author_rank: Super Contributor
timestamp: May 05, 2023, 07:47:23 am
content: It's a current source.Yes, the base resistor is too high. The hFEof a power transistor running at 700mA, with a couple of volts across it will probably be around 20. I'd go for 330R.It is theoretically possible to make a current source, with just a BJT and a resistor, but the current would vary depending on the power supply voltage and hFE. Even if the power supply voltage is regulated, the hFEcan vary widely and has a positive temperature coefficient, so it's generally a bad idea.

author: Benta
author_rank: Super Contributor
timestamp: May 05, 2023, 08:53:10 pm
content: Agree.The HFE20 is a bit pessimistic, though, it'll typically be higher (eg, BD137).A word of warning: this circuit is not as harmless as it looks. It it prone to oscillation in the MHz range.Two or three 1N4148 in series instead of the upper PNP might be a better option.

author: MrAl
author_rank: Super Contributor
timestamp: May 06, 2023, 04:25:21 am
content: Quote from: hermitengineer on May 05, 2023, 07:18:18 amHi,Yes, it looks like that 10k resistor has to be much lower like 1k or something lower like that.To discover the difference between this and the typical single transistor constant current regulator you could try to simulate both circuits and vary some parameters like the Betas and the nominal base emitter voltages. I think there will be a difference in the bias current where the two transistor circuit has less than the single transistor, but you could simulate both to see what happens.

author: EPAIII
author_rank: Super Contributor
timestamp: May 06, 2023, 09:36:34 am
content: 10000 vs. 1000 Ohms. Too easy to miss an extra zero. Good argument for using standard abbreviations. 10K vs. 1K: it's a lot easier to tell the difference.But then, computer geeks don't care.As for Electroboom, often breadboarded circuits are not tested very well. Who's to say he he ever tested it at full current? Perhaps it worked at the current level needed.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 06, 2023, 09:49:34 pm
content: This seems already pretty much answered, however, How am I supposed to know which one is "that other transistor". For the next time, run the annotation before you make a screenshot. Then everybody can just use "Q2 or "R1" in the circuit analysis description.

author: Zero999
author_rank: Super Contributor
timestamp: May 07, 2023, 09:10:02 am
content: Quote from: Benta on May 05, 2023, 08:53:10 pmExcept the BD137 is NPN, whilst the transistor used in the original poster's circuit is PNP. Yes, you could go for a much higher value. I just looked at the minimum hFEon a few datasheets. If it's not going to be used at low temperatures, 1k should be fine, especially given the transistor's beta will increase, as it warms up.