The "JOULE-THIEF" voltage boost circuit

LED ribbon strip light is a very useful thing.
I have fitted it inside car dashboard instrument clusters, with great effect.
It comes supplied on a reel, you simply cut it to length,
affix with its self adhesive backing,
connect to 12 volt DC and it works.

I did try to run some from a 5 volt (USB) DC supply.
It simply wouldn't work because of the fact there are groups of 3 led's in series,
'bussed' together along the length of the strip
meaning in excess of 9 volt (3 x 3v) is needed to 'strike' these LEDs.
I overcame this drawback by use of the "Joule-Thief" circuit
which boosts up the 5 volts to 12V or so
and will light the LED ribbon strip.

Graphic shows a bench lash-up of Joule-Thief in use.
I used an un-branded toroid (as with All toroids!) which was of the high resistivity Ni-Zn (HF/VHF) type.
I did try some low resistivity Mn-Zn types (meant for lower frequencies) but they don't seem to work?
despite the fact the circuit oscillation is in the lowish 50-200KHz range!
The joule-thief circuit is tradionally called a "blocking oscillator"
See also:- Ferrites - the 2 types

Some background:-


In the November 1999 issue of EPE (Everyday Practical Electronics),
a small and intriguing circuit was published in the Ingenuity Unlimited section by Z. Kaparnik.
It was a very small implementation of a typical transformer feedback single transitor invertor.
The transformer was a standard ferrite bead with two windings wound on it
and the circuit was using the high voltage pulse generated when
the transistor turns off to light a LED from a single 1.5V battery.
These are the components used in the circuit, and as you can see there are very few.
One metre of enamelled wire will be used to turn the ferrite bead into a transformer,
the resistor will be used to limit the feedback current to the transistor,
the transistor will switch on and off at about 50 KHz and the white LED will light.
Take a metre of the 38swg (0.15mm) enamelled wire and fold it in half
making sure that the point of the fold is nice and sharp since
t will be used to thread the wire round the ferrite bead.
It is important to lay the wire on evenly and pull it firmly
round the form since you will need to make 20 turns
of the doubled wire and its a tight fit.

A simple circuit - what makes it work?

Refer circuit re-draft, above.
My understanding...........
The transistor is biassed 'on' by current flow thru the resistor (and L2)
Collector current steadily rises due to transistor amplifying action.
When the transistor (and T1 core) saturates (current value levels out)
The rate of change of current flow is such, that by transformer action a pulse is induced in L2
Because the pulse polarity; transistor is biassed-off (this is 'blocking')
Collector current drops to zero.
Collapsing field in L1 produces a series aiding potential to the DC supply
Boosted voltage appears across the LED (which illuminates)
Cycle then repeats (ad nauseum) i.e. oscillation
Frequency of oscillation determined by 1K resistor and T1 inductance
About 50~200KHz
Great little idea.......whatever way it works!

The joule thief is AKA 'boost-convertor' type circuit
FYI I have also used, and recommend the XL6009 boost-convertor circuit.....