Trying to build a current limiter.

Thread Starter

woozycactus

Joined Jan 4, 2021
80
Right, and I am aware that in some areas electronic parts are not avaiklable locally, and in some places the prices are really high.
yeah here we have one local small store where its 5 bucks for 4 1/4 watt resisters and then there is micro center which is an hour away and kinda lacking in the components sections
 

MisterBill2

Joined Jan 23, 2018
9,034
Since the TS has internet access I suggest using on-line suppliers of electronic parts, BUT NOT e-bay or any general sales sites. There are many distributors of electronic parts that are actually exclusively electronics. Digikey, Newark, Pioneer, Avnet, and Mouser are just a few that pop into my mind. Prices for resistors vary widely, so a bit of research will be rewarded. Be aware that there is a minimum shipping charge that is the same if you buy one resistor or a hundred of them, so it makes sense fo order more parts than the moments requirements.
 

neonstrobe

Joined May 15, 2009
165
Audioguru - an LED is, these days, about 50% efficient. A 1W LED takes 1W power in but gives out about 500mW of light. So it only dissipates 500mW.
However, a heatsink is still needed. And as the LED ages, its efficiency will reduce, so its dissipation could well creep towards 750mW by the time it is worn out.

as regards a resistor, that is OK for low power LEDS but higher power LEDS where the temperature may rise will cause a voltage reduction, about the proverbial -2mV/C. A constant current is better for the higher power LEDs.

Bertus gave a circuit earlier. Two transistors were shown. The TIP120 is a Darlington and probably OTT, but you need a medium power transistor in that position. Power dissipation is Vcc-VLED -VE multiplied by the current, so you also need to decide the power supply voltage. Which will be, for example, 6-3-.7 or 2.3V x 350mA. One of the small 20x20x20mm "finger" heat sinks would suffice, as long as it had air to circulate.

The lower power transistor (2N2222 or similar) only has to shunt current not needed by the power transistor.
Suppose the gain of the power transistor was only 30. for 350mA it needs a base current of 12mA. A resistor for 6V operation would be 6-1.3(the voltage drop across the base of the power transistor and emitter resistor) divided by 12mA or ~390 ohm. (Using a Darlinton reduces the current needed but probably is little advantage, and note that the base voltage drop is two Vbe's not one). Allow for a little more current, say 330 ohms, for lower supply voltage, and so the lower power transistor could be almost any: BC547, BC548, BC337, etc.

It's a pity to use a linear current regulator (but simple and reliable, as long as thermal considerations given) because you need a low PSU voltage really, or you throw more power away in the regulator transistor (or resistor) which obviates the efficiency of the LED.

Using an ordinary medium power transistor like a TIP31 or TIP41 (or BD139) should work given a lower base resistor as mentioned.
 
Top