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

Monday, 3 February 2014

Related developments in 2013/14...

What's been happening?

i decided to try and apply the DIY cell 'technology' to something slightly more demanding than a flashing LED load circuit

over the past year, i've powered several LCD-based clock & weather-station units, using the same basic approach as the cells described here

the higher current-drive requirements of these devices has been achieved by increasing the water content - the electrode area has been reduced

normally, a galvanic cell arrangement such as these, using a few ml of water as a single electrolyte, would have polarised within a few months - needing the water to be changed and probably requiring the electrodes to be cleaned

these devices have been operating for nearly a year now, without attention

at the start of 2014, i wanted to move up to powering something more than just electronics, so i've started an experiment using the basic LED flasher circuit to energise a coil with just enough power to maintain a spring pendulum

it uses a dual transistor pulse circuit, based on the drive part of my original 3 transistor LED flasher; the coil is air-cored to prevent drag on the magnetic 'bob' of the spring pendulum;
the device is powered by 3 DIY cells in series providing a total of approximately 1V on continuous load (current draw is slightly less than 10uA)

Spring Pendulum drive circuit

it retains the energy capture and feedback from the coil field-collapse, via an LED, back to the supply (as featured in the earlier circuits) - and since the coil/magnet of the spring pendulum is similar to a 'Shake Flashlight' type arrangement, some kinetic energy is being converted back to electrical energy and returned to the supply, too

the system is contained within a clear plastic 'bell' cover to reduce effects from any ambient air movement

the LED flash period is approximately 20 seconds, the pendulum period is approximately 1 second (full-cycle):


(apologies for the creaky sound-effects - must be my knees!)

in the video, the DVM is displaying the supply voltage of the relaxation oscillator, which triggers at approx 1.2V and drops to approx 0.75V on pulsing the coil; the oscillator supply then re-charges from the battery supply
the components for the circuit aren't critical

C2, C3 and R1 form the timing of the relaxation oscillator - i selected values for C2 (10uF) and C3 (300uF) which produced sufficient pulse width to give an acceptable 'kick' to the pendulum, and then i selected R1 (150K) to give a repetition period of approx 20 seconds

a certain amount of feedback in the circuit triggers each pulse at a regular point in the pendulum movement (which can be either a vertical linear path, or an arc)

Q1 and Q2 are general-purpose, low-power, high-gain transistors with hFE >= 400

diode D1 is Schottky (for low reverse-leakage/forward-voltage), D2 is Germanium (to provide some leakage current)

C1 needs to be a suitable value to buffer the high impedance of the DIY cells - i have had the system operate with a value as low as 300uF, but 1000uF seems more reliable in the long term

the battery output voltage, on-load, is less than 2V, so low-voltage capacitors can be used, to reduce physical size 

LED1 is a hi-brite white type

the drive coil was hand-wound onto a card spool;

DC resistance is approx. 20 ohm
(a few hundred turns of multi-strand insulated copper wire, 7/0.09mm, air-core);
approx 30mm diameter, 10mm high, 10mm diameter air-gap
the battery consists of 3 DIY cells, 1 strip each of 15mm-wide copper and 5mm-wide zinc, approx 35mm submerged in about 2mL of 1:1 honey:tap-water solution, well-mixed; reasonably-well sealed with a plastic cap

the copper foil is bent into a 'U' shape to partly overlap the zinc strip on either side; a piece of sponge/foam is used to keep the two electrodes separate at the bottom of each cell

Friday, 13 July 2012

Cell #2: now 1+ years operation, voltage increased

Cell stack #2 has now been operating continuously for nearly 17 months, on load to the same LED flasher circuit

the graph below shows that the average terminal voltage for the cell stack, as indicated by the trend of the data, has been increasing since March 2011 (approx. 1.3 years so far, as at this post)

the voltage data still shows a strong inverse correlation with the day-to-day ambient temperature readings

Tuesday, 1 November 2011

4 Comparative results (Nov. 13)

the Graphs below show the behaviour of DIY cells #5 & #2 compared to two control experiments: one with a NiMH AAA cell (with medium charge), one with a 'sister' cell to Cell #5 (ie. both cells constructed at the same time and with the same materials & method)

  in each experiment the cells are continuously loaded: Cell #5, cellstack #2 and the NiMH cell both have similar pulsed LED loads, Cell #5b has a constant passive load (6M6R resistor in parallel with 200uF capacitor) with an equivalent power-draw to the Cell #5 experiment

 all graphs use the same
voltage scale for easier comparison; they clearly show that the DIY cells with pulsed loads are gradually increasing in charge whilst in the two control tests the cell voltage is gradually discharging

click on graphs to view full size

Monday, 1 August 2011

New Cell - New Circuit (Sept. 21 update)

Cell #5 is another Zinc/Copper pair, again with a cotton separator impregnated with honey

the on-load terminal voltage (Blue data) decreased initially during the first 400 hours of continuous operation, and then in the following 300 hours it steadily increased again

it regained just over 100% of its early voltage discharge and is now sustaining at an average of this level, having supplied the LED flasher so far for 1800 hours
(2.5 months) continuous operation

                                      (click images to enlarge, use browser Back btn to return)

the circuit is contained in a mild steel enclosure to ensure that it is not influenced by local utility or radio signal energies:

Cell #5 was originally tested in a 2 Cell battery supplying my low-power LED flasher circuit, which requires a supply voltage greater than about 0.9V

one of the weak points of the '2 cells in series' arrangement has been that the battery has relied on a good contact between the Zinc of one cell and the Copper of the other - any oxidation of the copper surface has tended to increase the internal impedance of the 'battery' and reduce the total voltage supplied by the pair of cells

for this test i'm using one of my variants of Professor Jones' 'SJ1' circuit - a 'common collector' oscillator similar to the 'Joule Thief' circuit (which is usually 'common emitter')

i've inverted the circuit to use a PNP transistor and, as with with my original low-powered LED flash circuit, i've also connected the LED to feedback some of the energy stored in the coil each cycle into the supply (a 200uF capacitor in parallel with the voltage cell) and connected a piezo sounder in parallel with C1 to give an audible click when the LED flashes


  the benefit of this new circuit is that it can operate down to approx 0.4V and still flash the LED - so i only need to use 1 cell for this arrangement, therefore the internal impedance of the supply is lower and there is less likelihood of connectivity issues