Emitter follower conofiguration

emitter-followerinfrared

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I need help with the following configuration , using LIRC for as the source signal for infrared led transmitter circuit with raspberry pi .

when putting the infrared led emitter led above the collector , I couldn't get enough range for led ( wasn't bright enough using a digital camera ) even with higher voltage and current ( 5 volt )

so I've chosen the following configuration to get current gain without voltage gain , the led was bright and got range about 6 meters and even can handle 2 IR leds in series .

my question :
1- I know the emitter follower configuration has a current gain and no voltage gain ( the IR led can handle up to 1.5 A for a short time of pulses ) , so how can i calculate current flowing from the emitter to the IR led .

2- if the emitter follower has no voltage gain , it should have voltage drop about .7 , so vout should 3.3-.7 = 2.6 volt . I have used a digital mulitmeter between the emitter and ground and shout that voltage is about .2 volt .
so , is it a wrong reading ? how could the led be bright if the IR led typical forward voltage is 1.3 volt ?

how can I calculate current of Ie ?

3- how can I simulate IR pulses using ltspice (generting squrewave from independent signal source tool)

attachments :
example for IR pulses http://bit.ly/1N4GHRv

Best Answer

I think you are seriously abusing the TSAL6100; it's not meant to take those 1.5A surges on a repeated basis, but only 200mA pulses. If you keep putting 1.5A through it with your remote application, you will probably damage it soon enough. And then no wonder it has no range.

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I think you need to read Visay's guide their diode (=LED) datasheets.

Peak surge forward current, IFSM: The maximum permissible surge current in a forward direction having a specified waveform with a short specified time interval (i.e., 10 ms) unless otherwise specified. It is not an operating value. During frequent repetitions, there is a possibility of change in the device’s characteristic.

As badly written by [non-native speakers of English] interns as that doc might be, the message is clear I think.

I don't know exactly how Vishay determines IFSM for their stuff, but a different manufacturer does this (for rectifier diodes):

IFSM, Non-Repetitive Forward Surge Current

IFSM is the maximum allowable nonrepetitive half-sine wave surge current under the following conditions: TJ = 45°C and the base-width of the half-sine wave surge pulse is 8.3 ms. A sample of diodes is selected and one-by-one the diodes are tested to destruction. This is done by hitting the DUT with a single surge pulse and checking to see whether the diode was destroyed. If so, the peak value of the surge is recorded as that diode’s pulse-height capability, and the next diode is tested. If not, the junction temperature is allowed to return to 45°C, the peak value of the surge is increased, and the DUT is hit again. This process is repeated until all of the diodes in the sample have been destroyed. Then the pulse-height capabilities are averaged and IFSM is set equal to half of the average.

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