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emitter follower?

I followed the links in the article and eventually ended up reading San Bergmans' discussion of an emitter follower circuit which keeps the IR current constant even with changing supply voltages.  Since the USB IR Toy uses the unregulated USB voltage, his design seems like a better choice at first glance.  Basically, San moved the resistor to the emitter and added a couple of diodes.  Not sure what this would change (and the USB IR Toy is already on order), but I wanted to raise the question to see what folks have to say.  Meanwhile, I'm breaking out the old college texts to see what sort of difference there is between the common emitter design in the USB IR Toy and San's common collector design.

Re: emitter follower?

Reply #1 –
I'm not sure that would be all that useful  : the USB 5V should be pretty constant, and if it does fall, you're going to have problems. If it doesn't, then the led power will be fairly constant anyway.

It may not be very repeatable between examples, though - some LEDs and some transistors will introduce extra loss. Perhaps a constant current drive will help make things consistent, and help LEDs with a high forward voltage attain full brightness without risking low voltage examples.

A more useful modification might be a method to get more brightness out of the LED for larger range. This will usually require some sort of energy storage, as for example in this Microchip  app note TB062

Re: emitter follower?

Reply #2 –
San Bergmans used the term emitter follower for a circuit that's also known as common collector, and can be configured as a constant current source. Particularly, San used two diodes - one to mirror the base-emitter drop (and cancel any temperature drift), and another to set a constant voltage drop for the emitter resistor. In this way, the emitter and collector current can both be made independent of USB supply voltage. I'm not really sure if it would still be considered common collector with the IR LED on the collector, but it should work.

You're somewhat correct that the USB 5V should be pretty constant, but it all depends upon how you define constant. VBUS can be anywhere from 4.01V to 5.25V, representing a 25% change, and still be legal according to the USB spec. I don't expect that the voltage would drift much in any one situation, but each configuration where you plug in a USB device could see a drastically different voltage, especially with a bus-powered USB hub.  Unfortunately, if the voltage varies by 25%, then the IR LED current will vary by 25% as well in the USB IR Toy circuit.

San's circuit should work as long as the IR LED has an active forward voltage drop of 2.7V or less, and you could dial in any desired constant current simply by changing one resistor.

I've worked with inductor-based voltage boost circuits like the one you suggest, e.g. the MAX1561. Even with a constant current circuit, this may be necessary if the IR LED needs more forward voltage.

Re: emitter follower?

Reply #3 –
hi everyone, im not really that familiar with the circuit you guys are referring to..i have a query and a suggestion though.

how about using a resistor-zener diode pair at the base side of the transistor, where the zener is connected to the ground and to the base, while the resistor is connected to the base and to the supply so there will be a constant voltage at the  emitter to ground side..that is Vz-0.7

im thinking that if i have only the IR led as the load (it has constant forward bias resistance) and if ill have the voltage regulation fixed then the IR LED would have a constant brightness..im not really sure of this though. the configuration im referring to above is actually a transistor based voltage regulator i found at my boylestad book..

i hope you guys can comment on this and help me understand more how problems like this should be approached.

i have a simple diagram attached..

Re: emitter follower?

Reply #5 –
Philip: Sorry for the delay in responding. I went researching an answer to your question and got lost in my college texts.

The voltage regulator circuit that you described would be quite similar to San's. The differences are that a voltage regulator is always on, and thus an IR driver would need to be switched off and on. Disconnect the resistor from the Supply and you probably have an input for the switch voltage. Also, the "Constant voltage here" is used to place a resistor that will set the current. Since V=IR or I=V/R, then you can set "I" by selecting "R" based on the "V"

vimark: The "Simple transistor current source" is also always on, just like the voltage regulator. Disconnecting R1 from Vs(+) and connecting it to a PIC port pin which swings from GND to Vs(+) should do the trick. If you continue on down that Wikipedia page, the "Simple transistor current source with diode compensation" is getting closer to San's circuit.

San Bergman's circuit is here:
http://www.sbprojects.com/knowledge/ir/ir.htm
Scroll down to "The Transmitter" and read the working description of the final circuit. I like the possibility of ordering two of the same part (San uses two identical diodes) instead of two different parts.

The smallest Zener I could find is 1.8V, but a pair of simple diodes in series is 1.2V, giving more room for the IR LED forward Voltage drop.

Re: emitter follower?

Reply #6 –
I tried to calculate the values for San's circuit and ran into so many temperature-based variables that I decided to back up a few steps and try again with the zener circuit.

I'll start with an outline of the steps and then jump into the details.

1) Select the target current for the IR LED, then determine the forward voltage.
2) Select a bipolar transistor which can handle the current, then determine its base-emitter voltage.
3) Select (zener) diode(s) to determine the controlled voltage drop.
4) Select R1 to dial in the target current (R1 fits in Philip's circuit where it says "Constant voltage here")
5) Select R2 based upon the PIC port output voltage and operating levels of the transistor and diode(s).

1) The whole goal here is to get maximum IR range without blowing anything or suffering from variable power supply voltages. We'll start with Ian's IR LED, the SFH480. It has a maximum current of 200 mA. It can handle a surge of 2.5 A, but we probably don't need to push it that far for great remote control performance. The data sheet specifies 1.5 V at 100 mA and 2.4 V at 1 A. Looking at the graph of Forward Voltage versus Current, it appears that our target 200 mA current falls around 1.6 V, and certainly less than 1.75 V. At the surge current of 2.5 A, the Forward Voltage is around 3.7 V! By the way, this LED has a capacitance of 25 pF, which affects the switching frequency.

2) Scanning Mouser for cheap SMD bipolar transistors, it's doubtful that we need a power transistor for 200 mA, just as it's doubtful that RF performance is needed for 36 kHz. Near the top of the price-sorted list is the ON Semiconductor MMBT4401 with 600 mA capacity. Looking at the data sheet around the 200 mA target range for the collector current, it appears that the transistor gain (hFE) is just above 200, probably 210. The base-emitter turn on voltage is about 0.8V at 25°C for a collector current of 200 mA.

3) The Zener voltage must leave room for the 1.6 V to 1.75 V LED voltage, or else the transistor might go into saturation, so I searched for the smallest Zener voltage. ON Semiconductor has the MMSZ4678 at 1.8 V, or 1.89 V maximum. (Note: I gave up on the series diodes, but was looking at the MMDL6050)

4) R1 determines the current, and depends solely upon the Zener voltage less the base-emitter voltage. R=Vr/Ir and Vr=Vz-Vbe, where Vz is 1.89 V max and Vbe is at least 0.8 V, and Ir should be 200 mA. That suggests a resistance of at least 5.45 Ω. Mouser shows a 1% 5.49 Ω that just barely makes it. A better choice might be 5.6 Ω to keep from blowing the 200 mA limit.

5) R2 is perhaps more forgiving, but it should provide at least enough current to keep the transistor and zener going. With 200 mA of collector current, and a gain of 210, the base current should be less than 1 mA, probably around 0.95 mA. The Zener has a 50 µA current, which puts the total current back to 1 mA minimum. Worst case, the PIC may put out as little as 4.3 V, and the Zener may be 1.89 V, leaving 2.41 V across R2. This suggests that R2 should be a standard value of 2.37 kΩ or less.

Note: the collector current will be about 1 mA less than the emitter current set by R1, but that should serve as a safety margin if anything.

Keep in mind that all of the above values are calculated from data sheets with no breadboard testing. If you change any of the parts, especially the IR LED, then just look up the new values and go through the steps again. Designing from specifications is important because temperature and other factors can make a design that works once on a breadboard fail when you get to Burning Man.

If the USB IR Toy PCB ever needs to be modified for a new version, it would be great to see a constant-current LED driver on the board!

Re: emitter follower?

Reply #7 –
Shall we test this? I'd like to get it in the next version of the IR Toy.
Got a question? Please ask in the forum for the fastest answers.

Re: emitter follower?

Reply #8 –
Here's a wild idea that's doubly untested:

In another thread where someone was looking to use the IRToy as a 3.3 V UART, I suggested that it "might" work to supply 3.3 V over the USB power.  A quick check of the PIC data sheet should tell whether this would actually work or not, but it's an idea at least.

To some degree, an emitter follower would allow the IR LED to operate at the same current, even with the USB voltage reduced from 5 V to 3.3 V, because it operates at a constant current.

Of course, the problem here is that a 3.6 V LED will not turn on with only 3.3 V power, so the emitter follower is not a magic solution for everything.  But it will at least provide consistent results over a fairly wide range of input power.