Showing posts with label 3. Show all posts
Showing posts with label 3. Show all posts
Wednesday, April 10, 2013
Universal Tester for 3 pin Devices
Most 3-terminal active components can be tested statically using just an ohmmeter. But when you have a lot of these devices to test, the procedure soon becomes boring. That’s where the idea came from to combine fast, easy testing for these types of device into a single instrument.
The unit described here enables you to test NPN and PNP bipolar transistors, N-or Pchannel FETs or MOSFETs, UJTs, triacs, and thyristors. Regardless of the type of device, the tests are non-destructive. Universal connectors allow testing of all package types, including SMDs (up to a point). The unit lets you change from one type of device to another in a trice. It avoids using a multi-pole switch, as they’re too expensive and hard to find.
Here’s how to build a versatile instrument at a ridiculously low cost. IC1 is a 4066 quad CMOS switch which will let us switch between bipolar transistors and FETs. LEDs D1–D4 tell us about the condition of the test device, when we press the ‘Test’ button. The 4066 can only handle a few milliamps, not enough for the other component types to be tested, hence the reason for using relay RE1. This 12 V relay offers two NO contacts. The first applies power to the UJT test circuit, the second applies it to the triac and thyristor test circuit.
Extensive testing has shown that the best way to test UJT transistors is to do so dynamically, with the help of a relaxation oscillator. Net-work R11/C1 sets the oscillator frequency to around 2 Hz. On pin B1 of the UJT we find a nice sawtooth, which is not of much interest to us here. However, pin B2 gives good but very short pulses. IC2, wired as a monostable, lengthens these pulses so they can be clearly seen via LED D5.
The relay’s second pole is going to drive the thyristor’ sortriac’s trigger pin. The value of R18 is a good compromise with respect to the varying trigger currents for this type of device. Resistor R17 is important, as the hold-ing current must be high enough for a triac; 250 mA is a good compromise. LED D6 tells you if the device is in good condition or not; but watch out, the test result must be con-firmed by briefly cutting the power in order to reset the triac.
On the web page for this article [1] you’ll find the author’s CAD files (PCB layout and front panel) along with some photos of his project. On the prototype, the LEDs and the ‘Test’ button were wired onto the copper side of the PCB. The six female connectors for the devices being tested were salvaged, but there are lots of models available on the market (the pitch is standard). The test cable crocodile clips must be as small as possible for testing SMD devices.
Source : http://www.ecircuitslab.com/2012/05/universal-tester-for-3-pin-devices.html
Tuesday, April 9, 2013
Cell Phone Battery Meter Circuit 3 6 Volt
This is a circuit for charger that is is a similar circuit to the above and provides a 4 LED bar graph indicating the voltage of a common 3.6 volt Lithium – Ion recharable cell phone battery. The reference voltage is provided by a TL431 programmable voltage source which is set to 3.9 volts where the TL431 connects to the 1K resistor. The lower reference for the LED at pin 14 is set with the 5K adjustable resistor. This is the figure of the circuit;
The programmed voltage of the TL431 is worked out with a voltage divider (10K 5.6K). The adjustment terminal or junction of the two resistors is always 2.5 volts. So, if we use a 10K resistor from the adjustment terminal to ground, the resistor current will be 2.5/10000 = 250uA. This same current flows through the upper resistor (5.6K) and produces a voltage drop of .00025 * 5600 = 1.4 volts. So the shunt regulated output voltage at the cathode of the TL431 will be 2.5 + 1.4, or 3.9 volts.
Working out the LED voltages, there are three 390 ohm resistors in series with another adjustable (5K) resistor at the bottom. Assuming the bottom resistor is set to 2K ohms, the total resistance is 390+390+390+2000 = 3170 ohms. So, the resistor current is the reference voltage (3.9) divided by the total resistance, or about 3.9/ (390 + 390 + 390 + 2000) equals 1.23 mA. This gives us about .00123*2000= 2.46 volts for the bottom LED, and about .00123*390 = .48 volts for each step above the bottom. So, the LEDs should light at steps of 2.46, 2.94, 3.42, and 3.9. A fully charged cell phone battery is about 4.2 volts. You can adjust the 5.6K resistor to set the top voltage higher or lower, and adjust the lower 5K resistor to set the bottom LED for the lowest voltage. But you do need a 6 to 12 volt or greater battery to power the circuit.
Friday, April 5, 2013
3 Transistor Audio Amplifier Circuit for 80 mW
This circuit is for amplifier 80 mW uses positive feedback to get a little more amplitude to the speaker. This is a simple design for a small amplifier. This has a disadvantage in that as the output moves positive, the drop across the 470 ohm resistor decreases which reduces the base current to the top NPN transistor. Thus the output cannot move all the way to the + supply because there wouldnt be any voltage across the 470 resistor and no base current to the NPN transistor. This is the figure of the circuit.
In this circuit, the 1K load resistor is tied to the speaker so that as the output moves negative, the voltage on the 1K resistor is reduced, which aids in turning off the top NPN transistor. When the output moves positive, the charge on the 470uF capacitor aids in turning on the top NPN transistor. The original circuit in the radio used a 300 ohm resistor where the 2 diodes are shown but I changed the resistor to 2 diodes so the amp would operate on lower voltages with less distortion. The transistors shown 2n3053 and 2n2905 are just parts I used for the other circuit above and could be smaller types. Most any small transistors can be used, but they should be capable of 100mA or more current. A 2N3904 or 2N3906 are probably a little small, but would work at low volume.
The 2 diodes generate a fairly constant bias voltage as the battery drains and reduces crossover distortion. But you should take care to insure the idle current is around 10 to 20 milliamps with no signal and the output transistors do not get hot under load. The circuit should work with a regular 8 ohm speaker, but the output power may be somewhat less. To optimize the operation, select a resistor where the 100K is shown to set the output voltage at 1/2 the supply voltage (4.5 volts). This resistor might be anything from 50K to 700K depending on the gain of the transistor used where the 3904 is shown.
In this circuit, the 1K load resistor is tied to the speaker so that as the output moves negative, the voltage on the 1K resistor is reduced, which aids in turning off the top NPN transistor. When the output moves positive, the charge on the 470uF capacitor aids in turning on the top NPN transistor. The original circuit in the radio used a 300 ohm resistor where the 2 diodes are shown but I changed the resistor to 2 diodes so the amp would operate on lower voltages with less distortion. The transistors shown 2n3053 and 2n2905 are just parts I used for the other circuit above and could be smaller types. Most any small transistors can be used, but they should be capable of 100mA or more current. A 2N3904 or 2N3906 are probably a little small, but would work at low volume.
The 2 diodes generate a fairly constant bias voltage as the battery drains and reduces crossover distortion. But you should take care to insure the idle current is around 10 to 20 milliamps with no signal and the output transistors do not get hot under load. The circuit should work with a regular 8 ohm speaker, but the output power may be somewhat less. To optimize the operation, select a resistor where the 100K is shown to set the output voltage at 1/2 the supply voltage (4.5 volts). This resistor might be anything from 50K to 700K depending on the gain of the transistor used where the 3904 is shown.
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