Measuring Inductors

Often you find yourself in the position of needing to wind your own coil for a project, or maybe you come across an unmarked coil in the junkbox. How can you best find out its inductance? An oscilloscope is all you need. Construct a resonant circuit using the coil and a capacitor and connect it to a square wave generator (often part of the oscilloscope itself) Adjust the generator until you find the resonant frequency f. When C is known (1000 pF) the inductance L may be calculated from:

L = 1 / (4π2⋅f2⋅C)

If you are also interested how good the coil is i.e. what is its quality factor or Q, you can use the oscilloscope again. If the level of the damped oscillation drops to 0.37 (= 1/e) of the maximum after about 30 periods, then the Q factor of the coil is about 30

The Q factor should be measured at the intended operating frequency of the coil and with its intended capacitor. The coupling capacitor should by comparison be a much smaller value.

source : Elektor Circuit Collections 

Two Position Dimmer

This super-simple dimmer consists of only two components, and it can easily be built into a mains switch. If you do this, don’t forget to first switch off the associated branch circuit in the fuse box, since the mains voltage is always dangerous! The circuit does not need much explanation. When S1 is closed, the lamp works at full strength, and the position of S2 does not matter. When S1 is open and S2 is closed, the capacitor causes a voltage drop, so the lamp is dimmed. The power dissipation of the capacitor is practically zero, so the circuit does not generate any heat. The resistor prevents sparking when S2 is closed while S1 is already closed. The value of the capacitor can be matched to the power of the lamp to be dimmed; it should be between 2 and 6 µF. Be sure to use a class X2 capacitor. Also, don’t forget that thiscircuit works only with resistive (non-inductive) loads. Unpredictable things can happen with an inductive load!

source : Elektor Circuit Collections 2000-2014

±5-V Voltage Converter

A symmetrical ±5 V power supply is often needed for small, battery-operated operational amplifier projects and analogue circuits. An IC that can easily be used for this purpose is the National Semiconductor LM 2685. It contains a switched capacitor voltage doubler followed by a 5-V regulator. A voltage inverter integrated into the same IC, which also uses the switched-capacitor technique, runs from this output voltage. The external circuitry is limited to two pump capacitors and three electrolytic storage capacitors.

The IC can work with an input voltage between +2.85 V and +6.5 V, which makes it well suited for battery-operated equipment. The input voltage is first applied to a voltage doubler operating at 130 kHz. The external capacitor for this is  connected to pins 13 and 14. The output voltage of this doubler is filtered by capacitor C3, which is connected to pin 12. If the input voltage lies between +5.4 and +6.5 V, the voltage doubler switches off and passes the input voltage directly through to the following +5-V low-dropout regulator, which can deliver up to 50 mA. C4 is used as the output filter capacitor.

All that is necessary to generate the –5-V output voltage is to invert the +5-V voltage. This is done by a clocked power-MOS circuit that first charges capacitor C2, which is connected between pins 8 and 9, and then reverses its polarity. This chopped voltage must be filtered by C5 at the output. The unregulated –5 V output can supply up to 15 mA. The LM 2865 voltage converter IC also has a chip-enable input (CE) and two control inputs, SDP (shut down positive) and SDN (shut down negative). If CE is set Low, the entire IC is switched off (shut down), and its current consumption drops to typically 6 µA. The CE input can thus be used to switch the connected circuit on or off, without having to disconnect the battery. The SDP and SDN inputs can be used to switch the VPSW and VNSW outputs, respectively. These two pins are connected to the voltage outputs via two low-resistance CMOS switches. This allows the negative output to be separately switched off, whereby the voltage inverter is also switched off. Switching off with SDP not only opens the output switch but also stops the oscillator. There is thus no longer any input voltage for the –5 V inverter, so the –5 V output also drops out. The SDP and SDN inputs are set Low (< 0.8 V) for normal operation and High (>2.4 V) for switching off the associated voltage(s).

source : Elektor Circuit Collections 2000-2014

*SCAP' AVR Programmer

 By Michael Gaus (Germany)

Many newcomers to AVR programming would love to build their own low-cost programming device, but they face a chicken-and-egg problem: many of the designs themselves use an AVR microcontroller; this needs to be programmed, and so they first need to make a programmer...

This is where the SCAP (Serial Cheap AVR Programmer) can come in handy. It is a very simple programming device using a minimum of components, and it can be connected either directly to a PC's RS-232 interface or to a USB interface via a USB-to-RS232 converter.

The circuit includes a nine-way D-sub socket (K1) which can be connected to the PC's serial port or to the USB-to-RS232 converter. The circuit takes advantage of the internal protection diodes on the AVR's I/O pins to V cc and GND, and the two series resistors R1 and R2 are thus needed to limit the current flowing through these diodes. The values are chosen to keep this current below 1 mA. The RS-232 interface can be as high as ±15 V. At -15 V the AVR's internal protection diode to GND limits the voltage on the I/O pin to a minimum value of -0.7 V, while at +15 Vthe protection diode to V cc limits the pin voltage to a maximum value of V cc +0.7 V. Now, because the values of the series resistors R1 and R2 are relatively high, the charging and discharging of the AVR's input capacitance is considerably slower than if it had been driven directly by a push-pull stage, and this limits the maximum permissible frequency on SCK for reliable operation. The wiring of K2 corresponds to the standard six-way Atmel ISP connector.

The well-known open source program AVRDUDE is an essentially universal programming tool that can very easily be configured to work with SCAP: see [1] and [2]. The configuration file avrdude.conf needs to have the following section added to it:

This adds a new programmer called 'scap', which can then be selected as the device to be used for programming using the command-line option '-c scap'.

Even though the reset pin of the microcontroller is tied permanently to GND in the circuit, it must still be defined for AVRDUDE. If AVRDUDE fails to establish a connection with the AVR device to be programmed, the power to the device must be interrupted briefly to cause it to perform a power-on reset. Here is a sample command to invoke AVRDUDE. We have assumed that SCAP is connected to COM1 with an ATmega8 as the taret device (this corresponds to the commandline option '-p m8') and thatthe hex file to be programmed istest.hex.

avrdude 300 -

-P coml -p m8 -c scap U f lash : w : test . hex : i

The instruction to slow down SCK is specified by the command-line option '-i 300', which gives a delay of 300 jlls. This makes the programming operation rather slow. Depending on the type of interface used (normal RS-232 or a USB-to-RS-232 converter) it may be possible to reduce the delay value to as little as 50, which will make programming faster. If SCAP is being used just to solve the chicken-andegg problem mentioned at the start of this article, then programming speed will not be of any great concern

Internet Links

[1] AVRDUDE:

www.nongnu.org/avrdude/

[2] AVRDUDE version for Windows: www.mikrocontroller.net/ attachment/69851 /avrdude-5.1 0.zip

Power Controller for Convector Heaters

In Fall or Spring, the weather may be warm enough that we'd like to save money by shutting down the main heating system in our home and just use supplementary heating based on one or more electric convector heaters.

Even though these convectors are quite heavy consumers of electricity, this can be reduced by fitting a power controller between the heaters and the AC power outlet, which will affect the effective power consumption of the convectors.

The circuit diagram is based around the use of the emblematic NE555 IC, used here as an astable multivibrator with variable duty cycle (D = t high / T), but at a fixed operating frequency, given by:

f = 1 / (0.693 x PI x C6) = 0.0654 Hz

The duty cycle D of the signal at the output (pin 3) of IC2 will change depending on the position of the wiper of potentiometer PI :

• If the wiper is at mid-travel, the duty cycle D will be 0.5;

• If the wiper is at the +12 V end, the IC2 output signal is zero and hence D = 0;

• If the wiper position takes it down to the voltage on C6, IC2's output supplies a constant voltage of around 11 V and D = 1.

By way of transistor T1, IC2 drives two MOC3021 phototriacs (IC3 & IC4) which provide the isolation between the circuit's 'driver' section and the 'power' section, which is directly connected to the AC powerlines.

Each phototriac drives a power triac (TRI1 & TRI2). These two triacs are fitted in parallel and share the task of supplying the convector (R L ): one triac supplies the positive half-cycle while the other triac supplies the negative