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)


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

Saturday, 9 January 2016

Charging cells using Flyback PSU type circuits

experiments during the last year have focussed on pulse motor operation adapting brushless fan motors, and also battery charging using flyback switchmode power supply type circuits (Boost Converters)

an interesting development has occurred with one particular variant of these test circits:

the circuit was being used to charge a battery of 2 NiMH AAA cells using a similar battery as input; the initial offload voltages of the i/p & o/p pairs were 2.62V & 2.63V (ie. both batteries approximately 50% charged)

after the first test, the end voltages were 2.3V (i/p) & 2.9V (o/p), as might be expected, input nearly fully discharged, output fully charged; the batteries were then swapped and the test repeated

observing the in-circuit terminal voltages as the test progressed the run was interrupted at an appropriate point to determine the 'crossover' voltage where the two batteries become equally charged (after a rest offline)

this voltage was found to be approximately 2.65V (within a few millivolts) - higher than both the original offline battery voltages

it appears that this circuit/battery combination is able to charge its output batteries slightly faster than it discharges its input batteries

more tests are underway to investigate this behaviour - the next test certainly appears to confirm that first observation:

this test circuit is similar to a small flashlight - it has a single white LED which is bright enough to cast a clear shadow on a white surface approx 7' away (>2 metres) in a darkened room; the circuit is powered by a single AAA NiMH (750mAh) and it is also charging a second similar cell on its output

the in-circuit voltage of the input cell (B1) dropped by 3mV (holding around approx 1.3V), while the output cell (B2) has increased by 30mV (from approx 1.29V)

i'll upload some of the graphical output from the datalogging PC, showing the voltage traces, as soon as i can prepare images & co-ordinate file transfers between my Windows & Linux systems

Update: the graph here shows the in-circuit terminal voltages logged for the input & output batteries, B1 & B2.  It can be seen that B2 charges whilst B1 discharges, as the LED is illuminated. After the flashlight has been operated for a while, switch S1 can be used to swap the 2 batteries between i/p & o/p for the next time that the flashlight is used.  In this way, the operation of the flashlight can be extended from the original charge of the NiMHs.  My application here has been for a small flashlight, using 1 run battery, 1 charging battery and a single LED lamp;  the same principle could be extended to use more cells per battery and a larger number of LEDs. VR1 can be used to alter the intensity of the light output (which will alter the discharge/charge rates accordingly) Switch S1 has a central 'Off' position, in addition to the the 2 'On' positions which select the current input battery

Summary, 2015/16...

A quick summary of the state of the 'experiments' listed below:-

Charge Anomaly - i've now learnt that what appeared to be an increase in  total 'charge' stored in a circuit's capacitors, in one of my earliest experiments, is in fact an increase in 'gorge' (as defined by John Denker at - unlike charge & energy, gorge is not conserved and my experiment clearly shows an increase in this quantity within test runs

my DIY cells (those which haven't been cannibalised) are still able to power their circuits; some circuits (eg. digital clocks) have been powered by a more liquid version of the electrolyte in sealed capsules, rather than the original 'gel' version sandwiched between the electrode plates

my spring pendulum worked well for about a year using three DIY cells with liquid electrolyte, but at present the circuit has latched-up pulling down the supply to 0.6V - i think i need to review the circuit and possibly return to the earlier single transistor with transformer version, rather than the 2 transistor driver with inductor