Hi all,

I have interested in IoT projects and exploring this subject as a basis to learn electronics.

I am currently trying to create a learning remote control circuit (see attached schematic) but my electronics experience is novice and although I have recently acquired a basic understanding of ohms law and am able to apply this to simple circuits I am a little confused with base resistor values when including the 2N2222 npn transistor and how one would choose the ballast resistor as (if I understand correctly?) I think the 2n2222 can act as an amplifier that amplifies the current through the collector and emitter. which further confuses me about how to size the ballast resistor.

I would like the TSAL6400 IR LED to run as bright as possible within the confines of the current definitions in the spec sheet.

Looking at the spec sheet it specifies that the TSAL6400 has a forward current of 100mA. But I have read in various places that this may be specifying a continuous on state and that IR LEDs may be able to operate happily at higher current levels if they are to be pulsed, like in a remote control application. The other current definitions in the spec sheet are peak forward current 200mA and surge forward current 1.5A. The spec sheet also displays Forward Voltage @ 100mA to be 1.35V (typical) and 1.6V(max). Also it specifies Forward Voltage @ 1A to be 2.2V(typical) and 3V (max) but I don't understand whether that implies that the TSAL6400 can operate at such high amperage if pulsed.

I think the above should be enough information to work out the resistors in the circuit but I don't know enough to be able to interpret these values and apply them tor my application and was hoping that someone might shed some light on how I would approach this?

Many thanks in advance.

Robert

 

Welcome to AAC!

You should read the datasheet more closely to understand under what conditions you can drive the LED at 100mA.

You're using the transistor as a switch, not an amplifier. The rule of thumb is that the base current should be a tenth of the collector current. For Ic=100mA, you would require Ib=10mA.

This can be seen in the 2N2222 datasheet:

Note that Ib is a tenth of Ic for the saturation specs.

The typical and maximum forward voltage spec is just that. A "typical" LED will have a forward voltage of 1.35V @ 100mA, but the maximum (for some LEDs) will be 1.6V. The 2.2V and 3.0V numbers you're using are for a current of 1A and a pulse width of 100us. I didn't notice any mention of duty cycle, but 10% is typical...

 

Hello,

@dl324 , in the datasheet there is a graph given for current / pulse length and duty cycle:



Bertus

 

What are the frequency and maximum pulse width of the signal at the base?

ak

 

Thanks to all the replies, things are really beginning to click now

@dl324
I've sized the ballast resistor at 39R to allow for just under 100mA on the Ic/LED branch. From the information you've highlighted for Ib, I calculated 260 ohms to achieve 10mA @ 3.3V (minus 0.7 voltage drop from base to emitter) so a 270R resistor? Please let me know if my workings are incorrect as this is all very new to me.

@bertus
Thanks for pointing out the graph. I'm still trying to get my head around it
I am not sure exactly what duty cycle is. is it another way of saying 'pulse' or is it something else?

@AnalogKid
I am yet to analyse the remote signal that I will be reproducing, so I don't have this data yet.

 

Hello,

The duty cycle is on-time/total-time.
For example a duty cycle of 10% (or 0.1 as in the graph) can be 10mS on in a period time of 100 mS.

Bertus

 

Please let me know if my workings are incorrect as this is all very new to me.

It's close enough. The datasheet gives typical BE saturation voltage of 0.6V and CE of 0.3V max.

From my interpretation of the datasheet, you can operate at 100mA with a 100% duty cycle at an ambient temperature of 25C.

 

Thanks for the clarification!
Ok, so to recalculate ballast resistor; (5-1.35-0.3) / 0.1 = 33.5 ohms, rounding up (even though only .5 from 33R) back to 39R resistor
base resistor; (3.3-0.6) / 0.01 = 270R resistor.

Re duty cycle at 100%, I see that in the spec sheet Forward Current is stated as 100mA. Why does the Forward Voltage specify it's value under test conditions with a 'tp' value of 20ms? What does tp stand for?

 

Ok, so to recalculate ballast resistor; (5-1.35-0.3) / 0.1 = 33.5 ohms, rounding up (even though only .5 from 33R) back to 39R resistor

The closest E24 (5%) value is actually 36 ohms, but I might have opted for 33 ohms because there's a range of forward voltages. The deciding factor would be the lowest forward voltage, which the datasheet doesn't give.

Re duty cycle at 100%, I see that in the spec sheet Forward Current is stated as 100mA. Why does the Forward Voltage specify it's value under test conditions with a 'tp' value of 20ms? What does tp stand for?

There's so much wrong with that datasheet...

In some cases, they give too much information; in others, they give too little information...

They give no limitations on the 100mA current, but they list it under absolute maximum ratings; which we usually try to avoid.

Giving a pulse width here for 100mA seems irrelevant, while it is relevant for the 1A spec:


The bottom line is that you want to limit the junction temperature to something that the device can survive. They specify a maximum junction temperature of 100C and a thermal resistance from junction to ambient of 230K/W. So you can calculate the power dissipation in the LED and see if you'll be close to 100C; taking ambient temperature into consideration.

 

@dl324
Thanks for the info Dennis, I've picked up a range of resistors from 27 ohm to 39 ohm and will test which one gives the closest result. Spec sheets becoming clearer to decipher but as you say they can confuse matters if they don't follow a standard.

@AnalogKid
I've just plotted the signal I'm trying to reproduce. (See attached plot image).
The signal starts with 1 preamble pulse of about 9000us (highlighted in green on the plot) and then follows with a series of short pulses at about 600us each followed by either an off gap of about 500us or 1700us until the next pulse.

With these timings in mind, could someone please help interpret the data from the TSAL6400 spec sheet as to what current value would be recommended for the brightest result?