Low-Power Joule Thief Night Light

The classic Joule-Thief circuit can be modified to work in a low-power burst mode:

The base resistor R4 is increased to a value that provides just enough base current for startup. After startup, the base current is provided by C1 and transformer L1-L2. Each cycle, the B-E diode of Q1 charges C1 a bit, until the base voltage is negative enough to stop oscillation. C1 is then slowly discharged via R4 until oscillation can start again.

As a result, the circuit produces short (10-100 cycles) bursts of oscillation with a long pause (ms) inbetween.

LTSpice Simulation file:

JouleThief3 Low Power.ascDownload

The version I built in SMD uses a BCW30 transistor, a small 1:1 transformer from a 1000BaseT ethernet transformer module and a 250mW white LED:

The circuit draws about 200uA and runs for 6 months from a ~500mAh 1.2V NiCd battery .

2017-02-19 by rincewind