Archive for December 14th, 2018

Astable Multivibrator: SMT LED Ballast Resistor

The original astable multivibrator ran from a dead CR123 primary lithium cell:

CR123A Astable - front

CR123A Astable – front

With a terminal voltage falling from barely 3 V, the LED drew about 3 mA (1 mA/div), tops, without a ballast resistor:

Astable - CR123A 2.8 V - 1 mA -green

Astable – CR123A 2.8 V – 1 mA -green

Hacking in a charged NP-BX1 secondary lithium cell boosted the supply to 4 V:

NP-BX1 Holder - SMT pogo pins

NP-BX1 Holder – SMT pogo pins

Which, diodes being the way they are, raised the LED current to nearly 400 mA (100 mA/div):

Astable - NP-BX1 4V - base V - 100mA-div

Astable – NP-BX1 4V – base V – 100mA-div

Somewhat to my surprise, a few weeks of abuse didn’t do any obvious damage to the LED, but I added a resistor while I was soldering up another holder:

Astable - 51 ohm SMD ballast

Astable – 51 ohm SMD ballast

There’s not quite enough room for a 1/8 W axial resistor, so why not blob in a surface-mount resistor?

Which cuts the current down to a mere 15 mA (10 mA/div) from a lithium battery at 4 V:

Astable - NP-BX1 - 51 ohm ballast - 10ma-div

Astable – NP-BX1 – 51 ohm ballast – 10ma-div

It’s still blindingly bright, but now I don’t feel bad about it.

 

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