Limit LED Current Only with Transistor and Base Resistor

Short answer, no. You need to either have a resistor in series with the LED (easy), or design the driving transistor to deliver a constant current (harder).Long answer, well yes, if you want to go to that trouble. If you use a large base resistor (I'd expect to see more like 10k/100k rather than 250 ohms) then a limited base current, multiplied by the hFE of the transistor, should give you a constant collector current. BUT hFE is not a well specified parameter. If you buy a cooking grade transistor, there might be a 6:1 spread of hFE. If you buy binned versions, so a -A or -B versions, then you might get down to 2:1 variation. Unfortunately, hFE varies with temperature, collector voltage, and base current as well. Given that LEDs have quite a wide current operating range, a LED that's happy with 10mA can still look fairly bright at 1mA. If you select a resistor for each transistor to get your target current, then you could get reasonable subjective performance. You're trading selection time and consistency for ease of build

I'm bit exciting about my new plan here, my goal is to make a small and simple 5W LED driver with Arduino. I know about current limiting resistor in series with led or in emitter pin of transistor, but just curious, can i driving LED with only transistor and base resistor to limit current going from collector to emitter?

simulate this circuit Schematic created using CircuitLab

thanks

·OTHER ANSWER:

I'm bit exciting about my new plan here, my goal is to make a small and simple 5W LED driver with Arduino. I know about current limiting resistor in series with led or in emitter pin of transistor, but just curious, can i driving LED with only transistor and base resistor to limit current going from collector to emitter?

simulate this circuit Schematic created using CircuitLab

thanks

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Can a Few Inexpensive Components Light a White Led From 2 AAA Alkaline Batteries? Closed
(1) Processor pin or similar driveIf a processor or other device was available with enough current drive and a rail to rail square wave (which some 6th sense tm suggests to me may be the case in this case) then a simple diode pump from a single pin may suffice. This could be driven from any point with a rail to rail rectangular wave present with a suitable mark space ratio range plus drive capability as above. simulate this circuit Schematic created using CircuitLabVLED max 2 x V - 2 x Vdiode_drop.At V 2V, VLED max 2 x 2 - 2 x 0.3 3.4V.Many modern White LEDs could be operated from this voltage.As an indication of what could be achieved, an LED rated at say 150 lumen/Watt and with 1 mA LED current would make about 3V x 0.001A x 150 lumen/Watt 0.5 lumen.If this light was distributed evenly over an A4 sheet (hard to do evenly) then light level 0.5 Lumen (0.21 m x 0.3m) 8 lux.At 2.5 mA LED current illumination level rises to 20 lux over an A4 sheet and text and colour pictures would be easily read by most (despite what many text books say.)LED current can come in pulses as long as the frequency was high enough to avoid LED flicker and as long as peaks of capacitor discharge current does not over-stress the LED. (2) Asian (or other) IC.This circuit meets your spec rather well.The main qualification is availability of the IC in Western markets. The IC is available as aQX5252F (TO94 pkg (similar to TO92 but with 4 leads)) andQX5252E in a DIP pkg.The 5252x operates from a single NimH or NiCd cell and the 5253x is the same but operates from 2 cells. The latter would be best suited to your spec. The DIP version adds variable light level sensitivity when used as a solar lamp controller. The IC costs around 10 cents in modest volume in China and probably a few tens of cents in 1's. Features includeLED driver.Brightness level set by inductor valueInternal PV isolation diode and switch MOSFET so ...Total circuit comprises IC, LED, inductor, battery.The IC goes into shutdown mode when Vin is above a preset voltage and drives the LED when Vin is below a preset level. This feature is used to provide auto turn-on turn-off lawn light control. SBAT can instead be used as an enable line or tied low for permanent operation. From memory efficiency is in the 85% region - better than many informal circuits. (3) From the archives ... from this stack exchange question is a good start.This circuit will flash an LED of any colour and forward voltage (or potentially even several LEDs in series) or will pulse a load using one cell - probably about 1 volt will be enough to operate it. I "designed" this circuit but it is based on a design that has not only long been used in transistor form but existed in pre-transistor thermionic valve days and, while I have never seen it used elsewhere, I would be surprised if it has not been independently "developed" by many other people.As shown Q1 collector is driven negative below ground when Q1 turns off until energy in L1 is dissipated. Swap ground and supply and transistor types for ve supply. Add diode from output to use as a DC supply. L1 - small potted "resistor like" inductor or many others - experiment. Q1 Q2 - almost any "jellybean" small pnp & npn transistors. C1 polarised only to get high capacitance per size.Can be eg ceramic if capacitance high enough for needs. Use only either LED2 (best) or LED1 at one time.Use either LED2 (most efficient) or LED1Time constant R2 x C1.Long time constant leads to discrete flashes.Short time constant produce apparently permanently on LED.Use resistor between Q1b-Q2c for higher supply voltages.Resistor in series with C1 will extend pulse length.More soon ...Closed. This question is off-topic. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Electrical Engineering Stack Exchange. Closed 5 years ago. Improve this questionPlease help with finding an energy-efficient LED driver circuit for 2 AAAs(all the way down to 2V), for example for this LED: It has to be:1. Cheap2. As few parts as possible3. Energy-efficient·OTHER ANSWER:Closed. This question is off-topic. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Electrical Engineering Stack Exchange. Closed 5 years ago. Improve this questionPlease help with finding an energy-efficient LED driver circuit for 2 AAAs(all the way down to 2V), for example for this LED: It has to be:1. Cheap2. As few parts as possible3. Energy-efficient
2021 07 11
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Looking for the Right LED Driver, Roughly 4V, 1.5W Or 3W, 10-20Hz
I would use a LM555; it's a very flexible, cheap, and easy-to-find timer chip. And you can find lots of examples on how to use it. Here's one to start with. The 555 will source around 125mA, which isn't quite enough for your application. If want to drive all of the strings at once, you can do it with an NPN transitor switch hooked to the output of the 555. I would probably start with a TIP120; that's hefty enough to handle the current you need to switch. You will need a way to power all of the strings at once. If you are using batteries, I would use 3 D cells to drive all of the strings. If you are okay with a plug-in solution, you can find lots of small wall wart power supplies that put out 5V at 1Amp; they are used to charge cell phones and other kinds of electronigsI have bought 6 of the 20-string LEDs for a hobby project of mine. I wish to make them blink at a fixed frequency anywhere between 10Hz and 20Hz. I wish to use only one or two battery packs (3 to 6 AA batteries total) to power all 6 strings (the final circuit needs to be compact).A single string of LEDs draws 120 mA of current. Fresh AA batteries supply 1.5 Volts each. So, each string should draw 3 * 1.5V * 0.12A approx 0.5W per LED string.I am looking for an LEd driver that will make the LEDs blink. My estimates is that I need 4 /- 0.5 V, 1.5W or 3W /- 20% for both, 15 /-5 HZ LED driver.I have tried a few searches and did not find anything that seems appropriate, probably because I am a beginner.Please suggest a component. It should draw the same power as it modulates I assume it should not be bigger than a US quarter coin. Hopefully I have done my arithmetics right.UPDATE:I ended up using the 555 / astable circuit like this one: Here is a calculator I used: So far so good.·OTHER ANSWER:I have bought 6 of the 20-string LEDs for a hobby project of mine. I wish to make them blink at a fixed frequency anywhere between 10Hz and 20Hz. I wish to use only one or two battery packs (3 to 6 AA batteries total) to power all 6 strings (the final circuit needs to be compact).A single string of LEDs draws 120 mA of current. Fresh AA batteries supply 1.5 Volts each. So, each string should draw 3 * 1.5V * 0.12A approx 0.5W per LED string.I am looking for an LEd driver that will make the LEDs blink. My estimates is that I need 4 /- 0.5 V, 1.5W or 3W /- 20% for both, 15 /-5 HZ LED driver.I have tried a few searches and did not find anything that seems appropriate, probably because I am a beginner.Please suggest a component. It should draw the same power as it modulates I assume it should not be bigger than a US quarter coin. Hopefully I have done my arithmetics right.UPDATE:I ended up using the 555 / astable circuit like this one: Here is a calculator I used: So far so good.
2021 07 09
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