1W LED driver circuit

This circuit is designed to drive the 1W LEDs that are now commonly available. Their non-linear voltage to current relationship and variation in forward voltage with temperature necessitates the use of a 350mA, constant-current power source as provided by this supply.

In many respects, the circuit operates like a conventional step-down (buck) switching regulator. Transistor Q1 is the switching element, while inductor L1, diode D1 and the 100mF capacitor at the output form the energy transfer and storage elements.

The pass transistor (Q1) is switch-ed by Q2, which together with the components in its base circuit, forms a simple oscillator. A 1nF capacitor provides the positive feedback necessary for oscillation.

The output current is sensed by transistor Q3 and the two parallelled resistors in its base-emitter circuit. When the current reaches about 350mA, the voltage drop across the resistors exceeds the base-emitter forward voltage of transistor Q3 (about 0.6V), switching it on.

Q3’s collector then pulls Q2’s base towards ground, switching it off, which in turn switches off the main pass transistor (Q1). The time constant of the 15kW resistor and 4.7nF capacitor connected to Q2’s base adds hysteresis to the loop, thus ensuring regulation of the set output current.

The inductor was made from a small toroid salvaged from an old computer power supply and rewound with 75 turns of 0.25mm enamelled copper wire, giving an inductance of about 620mH.

The output current level should be trimmed before connecting your 1W LED. To do this, wire a 10W 5W resistor across the output as a load and adjust the value of one or both of the resistors in the base-emitter circuit of Q3 to get 3.5V (maximum) across the load resistor.

Willetton, WA.

Simple cable tester

Here is a simple RJ-45 cable tester that can be assembled in quick time. It is intended for use with patch cables or similar, where both ends of a cable can be brought together and plugged into RJ-45 sockets on the tester.

A PICAXE micro drives four of the eight possible connections on one end of the cable, feeding the anodes of four LEDs at the other end. The cathodes of the LEDs are returned via the remaining four conductors, themselves is series with four more LEDs.

By flashing the LEDs in sequence and at varying intervals, it’s therefore possible to visually determine not only continuity but also shorts between conductors.

The accompanying program is self-explanatory and easily modified to individual taste.

South Carolina, USA. ($40)

'PICAXE-08M Cable Tester

main:
   if pin3 = 1 then step1  	'start test when switch pressed
   goto main

step1:
   high 0			'output 0 high
   pause 1000		'for 1 sec.
   low 0			'output 0 low
   pause 1000		'for 1 sec.

   for b0 = 1 to 2
      high 1		'output 1 high
      pause 500		'for 1/2 sec
      low 1 		'output 1 low
      pause 500		'for 1/2 sec
   next b0		'loop twice

   for b1 = 1 to 3
      high 2		'output 2 high
      pause 333		'for 1/3 sec
      low 2 		'output 2 low
      pause 333		'for 1/3 sec
   next b1		'loop 3 times

   for b2 = 1 to 4
      high 4		'output 4 high
      pause 250		'for 1/4 sec
      low 4		'output 4 low
      pause 250		'for 1/4 sec
      next b2		'loop 4 times

   pause 250		'leave off for 1/4 sec
   if pin3 = 0 then step1  	'run tests again unless switch pressed 

step2:
   if pin3 = 1 then step2  	'wait until switch released
   pause 100		'short delay
   goto main

Improved LED torch

The Novel LED Torch circuit presented in the February 2005 instalment of Circuit Notebook is an interesting approach for a variable intensity LED torch and can be improved upon with a few small modifications.

The shortcomings of the initial design were the necessity of a separate power switch and the increase in intensity by only one LED at a time at higher illumination levels. The accompanying circuit addresses both of these issues.

A double-pole, 6-way rotary switch is used as both a power switch and a 5-step intensity selector. The values of the resistors in the divider string were chosen so that 1, 2, 4, 6 or 10 LEDs are illuminated simultaneously when positions 1 to 5 of the switch are selected, with position 6 being "off".

Of course, the values of resistors in the chain may be altered to change the number of illuminated LEDs at switch positions 2 and up. Keep the total resistance as close to 100kW as possible.

Other changes to the circuit include the use of the reference voltage output (pin 7) to power the voltage divider and the 100W resistor, so ensuring consistent behaviour with varying input voltage.

An old torch housing or even a simple plastic tube could be used to house the circuit assembly and its three AA (or larger) cells, with the rotary switch positioned at one end.

Load sharing multiple supplies

A recent correspondent ("Ask Silicon Chip", March 2005) asked about the possibility of running multiple laptop power supplies in parallel to increase load handling. This simple circuit shows how it can be done.

As shown, the circuit will allow four laptop power supplies to be connected in parallel, each supply sharing a portion of the total load current. The nominal output is 13.8V, suitable for powering audio or radio gear. More supplies could be added just by adding more parallel branches to the circuit.

The circuit is a variation on an arrangement often used for paralleling power transistors. The 0.1W emitter resistors help the transistors share the load more evenly. By splitting the transistor collectors (these are usually connected together), each transistor can be fed from a different supply.

Supply 1 will carry a slightly higher load (up to about 5%), depending on the gain of the pass transistors. Emitter resistor values can be adjusted to compensate for variations in the rated maximum loads of the supplies. For example, if one supply can provide twice the current of its counterparts, its associated emitter resistor would need to be halved (or all others doubled).

Load sharing will be improved if the transistor specifications are closely matched but in practice, this would be difficult to achieve unless you have access to quantities of transistors for comparison.

The regulation of this simple design is not great. Expect around 14.5V at no load, dropping to a bit over 12V at full load. Supply 1 will need a minimum output of 17V, whereas the others will need a minimum of 16V. The output fuse needs to be rated for the total load.

Each pass transistor should carry no more than 7A and all devices must be mounted on a large, common heatsink. Attach the 7815 regulator to the middle of the same heatsink to afford a measure of thermal overload protection.

Diodes D1 & D2 are included for polarity protection and are required only if the supply is used for battery charging.

Kelvin Lawrence,
via email. ($40)

Paraphase tone control

Here’s a tone control circuit for audio applications. It incorporates passive low and high-pass filters and uses only three transistors.

The first transistor (Q1) amplifies the audio signal before it is AC-coupled to the second stage based on Q2. This second stage uses a high-linearity 2SC945 transistor and acts as a driver and phase splitter. Its collector feeds a high-pass RC filter network, while its emitter feeds a low-pass RC network.

After traversing the filter networks, the two signals are summed at the gate of Q3. The result is that some frequencies are attenuated while others are amplified, depending on their amplitude (adjustable by VR1 & VR2) and phase difference. Using the values shown, the crossover point of the two filters is approximately 1kHz.

An N-channel JFET was chosen for Q3 due to its high input impedance. The JFET is available as part number 2SK30ATM (stock no. 317-5983) from RS Components at www.rsaustralia.com or phone 1300 656 636.

The 2SC945 is available from Wiltronics Research, on the web at www.wiltronics.com.au or phone 1800 067 674.