Posted on Friday, May 6, 2011 • Category: Test and Measurement
"ESR" stands for equivalent series resistance. ESR is one of the characteristics that defines the performance of an electrolytic capacitor. Low ESR is highly desirable in a capacitor as any ripple current through the capacitor causes the capacitor to heat up due to the resistive loses. This heating accelerates the demise of the capacitor by drying out the electrolyte at an ever increasing rate. Over the lifetime of a capacitor, it is not uncommon for the ESR to increase by a factor of 10 to 30 times or even go open circuit. Typical lifetime ratings for electrolytics are 2000-15000 hours and are very dependant on ambient operating temperature. As the ESR increases, the filtering operation of the capacitor becomes impaired and eventually the circuit fails to operate correctly.
Why are ESR Meters so Useful?
A typical capacitor checker measures the capacity (usually in micro farads) of the test capacitor. Some advanced units also test for leakage current. Most of these testers require that the capacitor be removed from the circuit. Unless the capacitor has totally failed, they will not detect a high ESR value. In a typical circuit, there may be 10's or 100's of capacitors. Having to remove each one for testing is very tedious and there is a great risk of damaging circuit boards. This tester uses a low voltage ( 250mv ) high frequency (150khz) A/C current to read the ESR of a capacitor in the circuit. The in circuit testing is possible because of the low voltage used for obtaining the measurement. The voltage is low enough that solid state devices in the surrounding circuitry are not activated and do not affect the low resistance reading we are attempting to obtain. A lot of capacitor checkers will be damaged if you happen to test a charged capacitor. This circuit is A/C coupled and will withstand up to 400vdc of charge on a capacitor (but watch your fingers!). The ESR checker will not detect shorted capacitors as they will read with a very low ESR value. If you are trouble shooting a circuit, you will have to use several instruments including your nose, voltmeter and oscilloscope to locate all the possible failure modes. My experience has found that the ESR meter catches about 95% of capacitor problems and potential problems.
Posted on Thursday, May 5, 2011 • Category: Stepper Motors
Stepper motors are everywhere in electronics these days. There are two main types of stepper motors:
1. Bipolar motors. These have two coils and are controlled by changing the direction of the current flow through the coils in the proper sequence. These motors have only four wires and cannot be connected to this kit. See our Kit 1406 for a Bipolar Stepper driver Kit.
2. Unipolar motors. These have two center-tapped coils which are treated as four coils. These motors can have five, six or eight wires. Five-wire motors have the two center-taps commoned internally and brought out as one wire (Fig 1). Six-wire motors bring out each center-tap separately. The two center-taps need to be commoned externally (Fig 2). Eight-wire motors bring out both ends of each coil. The four “center-taps” are joined externally to form one wire. In each case the center-tap(s) are connected to a positive motor power supply. Unipolar motors may be connect as bipolar ones by not using the ‘+’ wires.
A stepper motor has no brushes or contacts. It is basically a synchronous motor with the magnetic field electronically switched to rotate the armature magnet around.
The Internet is where to get all the explanation about steppers. Just google ‘stepper motor’ and you will find tens of sites. In particular, look for ‘Jones on Stepper motors’ (it comes up top of the list when I did it just now) and read it. If you look at the other references you will find that the circuit in this kit has been around for many years in various forms. The latest publication was in Silicon Chip, 5/2002, and I have based this circuit on it.
Posted on Thursday, May 5, 2011 • Category: Headphone Amplifiers
If you built our 20W Class-A Stereo Amplifier described last year, you will be aware that it lacks a headphone socket. Similarly, many hifi valve amplifiers also lack a headphone socket, the assumption being that a true hifi enthusiast will want to listen via good-quality loudspeakers.
A headphone output was not included in the Class-A Stereo Amplifier because it would degrade its superb audio performance. Both the wiring paths and the general circuit layout are critical factors in the design and any changes, however slight, can cause big changes in the signal-to-noise ratio and harmonic distortion figures of the amplifier.
Click for larger image
Fig.1: the Stereo Headphone Adaptor connects between your stereo amplifier and the loud-speakers and can drive two pairs of headphones.
If you do want to listen via headphones, a far better option is to build the simple Stereo Headphone Adaptor presented here. It connects directly to the amplifier’s speaker terminals and switches the loudspeakers and stereo headphone sockets using two DPDT (double-pole, double-throw) relays, so there’s no chance of it degrading the audio performance.
Posted on Thursday, May 5, 2011 • Category: Sensors
I needed to replace two old, unreliable thermostats for controlling the heating and cooling for a large garden shed.
Commercial basic digital thermostats are available quite cheaply, but some lack the ability to control heavy loads or have the extra features that I require for saving energy when the door is often left open or to indicate temperature being out of range etc.
I like the PIC18F1320 microcontroller used in my previous project - so decided to use it again in a very similar design to drive three multiplexed LED displays and read the temperature from a Dallas/Maxim DS18x20 "1-Wire" digital sensor.
Posted on Wednesday, May 4, 2011 • Category: Timer Circuits
Here is a 60 minute countdown timer that can be used as an exposure timer for UV light boxes, photography, egg timer, and many other projects where counting or delay is necessary. The heart of the countdown timer is PIC16F84A chip and 4 digit character LED display. The relay is energized after the count down timer goes down from specified minute and second until zero, and can both turn devices on or off. See the link for details and schematic.
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