What is the purpose of a stabistor in this IR remote circuit?

Thread Starter

szabikka

Joined Sep 3, 2014
116
Dear Forum Users,
I'm reading the datasheet of the IR remote control transmitter IC SAA3004. In the attached picture, you can see that to its REMO output (which drives the transistor in turn driving the IR LED) there is a stabistor (BZX75-C1V4) connected. What is the purpose of the stabistor there? I'm guessing that in conjunction with the resistor connected to the transistor's emitter it forms a current limiting circuit. Am I right? Is it necessary for the circuit to function properly or can it be omitted?

1716404285431.png
 

Pyrex

Joined Feb 16, 2022
309
hi,
i guess, it's to maintain stable current thru the LED. BC368' base voltage is kept constant, Ub-e is constant too and equal to 0.6V, so emmiter current is (1.4V-0.6V)/Re. Therefore, the remote control emits the same light flow when the battery is new and when it is weak
 

Thread Starter

szabikka

Joined Sep 3, 2014
116
hi,
i guess, it's to maintain stable current thru the LED. BC368' base voltage is kept constant, Ub-e is constant too and equal to 0.6V, so emmiter current is (1.4V-0.6V)/Re. Therefore, the remote control emits the same light flow when the battery is new and when it is weak
Thanks for the tip, Pyrex. Your idea about the battery is brilliant. I thought it has to do something with current regulation but I would've never guessed that it has something to do with the battery's life cycle.
 

ErnieM

Joined Apr 24, 2011
8,382
I have not a clue what the REMO component is, but I once used a similar scheme to drive a LED.

My needs were to have a PWM controlled LED with a controlled current. As my "driver" was a microcontroller running off 5V I had a stable source, but my supply voltage would vary from 8 to 28 VDC. Placing an emitter resistor gave me a decent pulsed current sink for the LED.
 

Thread Starter

szabikka

Joined Sep 3, 2014
116
I have not a clue what the REMO component is, but I once used a similar scheme to drive a LED.

My needs were to have a PWM controlled LED with a controlled current. As my "driver" was a microcontroller running off 5V I had a stable source, but my supply voltage would vary from 8 to 28 VDC. Placing an emitter resistor gave me a decent pulsed current sink for the LED.
Hi ErnieM,
The component on the REMO output of the IC is a stabistor. It is an obsolete component nowdays. Basically, it was a stabilized voltage source used for low voltages mostly in the 1980s. It is essentially two or more silicone diode pieces with "tightly controlled parameters" in one package connected in series. They got a niche market, since they were more reliable voltage sources than low voltage zener diodes. Speaking of which, low voltage zener diodes are not too reliable today either. As Pyrex pointed out, the circuit in the picture is a current source which supplies the same current for the IR LED regardless of the battery voltage, much like the emitter resistor circuit with the stable MCU output you have described in your answer.
 
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