Monday, 11 April 2016

Accidental discovery!

Last night i put together a quick blocking oscillator circuit to test the feasiblity of it being supplied by Peltier junction devices, stacked and connected in series to develop a higher voltage from hand heat.  The stack certainly worked as i'd hoped - but - the very low impedance of the stack wasn't a good match for the oscillator (even with a 1F buffer capacitor) 

Although my intended test wasn't fruitful, i noticed that when the oscillator was allowed to discharge the large buffer capacitor, on disconnection of the battery (2 very depleted 750mAh NiMHs in series, just under 2.4V total) the circuit sometimes seemed to enter a sustained regenerative feedback mode, after the circuit had reached its low-voltage supply cut-off point

The feedback appeared to be caused by the piezo buzzer component i was using as the capacitive coupling from the flyback output winding to the base of the switching transistor

I replaced a few components with different values and then found that the circuit pulsed at around 20 Hz and the battery terminal voltage was slowly increasing. Red trace below is the AC voltage pulse on the battery terminal, blue trace is the flyback output voltage to the LED



The circuit has been running now for nearly 3 days and the battery terminal voltage has continued to increase slowly, see datalog of on-load battery terminal voltage below (this is NOT the typical 'battery relaxation' effect after previous heavy loading of the battery followed by a low-current loading - the cells had been left depleted and unused for a week or so before this test, and the loading at all times has been at a similar level)





i've tried different output configurations and the best arrangement, so far, seems to be a full-bridge rectifier with red LEDs in the positive output positions



The transformer core is a split ferrite toroid used for reduction of EM interference pickup via cables (toroid is approx 25mm OD, 10mm high); windings use 0.45mm magnet wire

Q1 is a high-gain, low-power device (eg. BC327); C1 is currently 1000uF; D2 & D3 are BAT42 Schottky diodes (not suitable for use as D1);  L1 is around 2mH, using approx 60 turns of 0.45mm magnet wire on a 12mm OD ferrite tube, approx 35mm long

The pnp device is biased 'On' using the reverse-leakage current from D1, which can be either a Germanium or suitable Schottky diode, and the device is switched 'Off' by the inverted output signal fed back via the piezo element;  pulse width is approx 20us

i've used a pnp part only because that is the first high-gain device i happened to pick up in my spares tin - the circuit should achieve the same *interesting* behaviour if it is re-arranged, polarity-wise, to suit an npn device (eg. BC547)

...meanwhile...

Tests with the Flyback PSU setup continue - and they're giving very interesting results - Watch this space!