Showing posts with label Transistor. Show all posts
Showing posts with label Transistor. Show all posts
Latest In Circuit Transistor Checker Using 555 timer
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This simple circuit has helped me out on many occasions. It is able to check transistors, in the circuit, down to 40 ohms across the collector-base or base-emitter junctions. It can also check the output power transistors on amplifier circuits.
Circuit operation is as follows. The 555 timer ( IC1 ) is set up as a 12hz multi vibrator. The output on pin 3 drives the 4027 flip-flop ( IC2). This flip-flop divides the input frequency by two and delivers complementary voltage outputs to pin 15 and 14. The outputs are connected to LED1 and LED2 through the current limiting resistor R3. The LED’s are arranged so that when the polarity across the circuit is one way only one LED will light and when the polarity reverses the other LED will light, therefore when no transistor is connected to the tester the LED’s will alternately flash. The IC2 outputs are also connected to resistors R4 and R5 with the junction of these two resistors connected to the base of the transistor being tested. With a good transistor connected to the tester, the transistor will turn on and produce a short across the LED pair. If a good NPN transistor is connected then LED1 will flash by itself and if a good PNP transistor is connected then LED2 will flash by itself. If the transistor is open both LED’s will flash and if the transistor is shorted then neither LED will flash.
From :: http://home.maine.rr.com/randylinscott/nov97.htm
10 000x With One Transistor Diagram Circuit
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For a collector follower with emitter resistor, you’ll often find that the gain per stage is no more than 10 to 50 times. The gain increases when the emitter resistor is omitted. Unfortunately, the distortion also increases. With a ubiquitous transistor such as the BC547B, the gain of the transistor is roughly equal to 40 times the collector current (Ic), provided the collector current is less than a few milliamps. This value is in theory equal to the expression q/KT, where q is the charge of the electron, K is Boltzmann’s constant and T is the temperature in Kelvin.
For simplicity, and assuming room temperature, we round this value to 40. For a single stage amplifier circuit with grounded emitter it holds that the gain Uout /Uin (for AC voltage) is in theory equal to SRc. As we observed before, the slope S is about 40Ic. From this follows that the gain is approximately equal to 40I cRc. What does this mean? In the first instance this leads to a very practical rule of thumb: that gain of a grounded emitter circuit amounts to 40·I c·Rc, which is equal to 40 times the voltage across the collector resistor.
If Ub is, for example, equal to 12 V and the collector is set to 5V, then we know, irrespective of the values of the resistors that the gain will be about 40R(12–5) = 280. Notable is the fact that in this way the gain can be very high in theory, by selecting a high power supply voltage. Such a voltage could be obtained from an isolating transformer from the mains. An isolating transformer can be made by connecting the secondaries of two transformers together, which results in a galvanically isolated mains voltage.Circuit diagram:
![]( "transistor")
That means, that with a mains voltage of 240 Veff there will be about 340 V DC after rectification and filtering. If in the amplifier circuit the power supply voltage is now 340 V and the collector voltage is 2 V, then the gain is in theory equal to 40 x (340–2). This is more than 13,500 times! However, there are a few drawbacks in practice. This is related to the output characteristic of the transistor. In practice, it turns out that the transistor does actually have an output resistor between collector and emitter.
This output resistance exists as a transistor parameter and is called ‘hoe’. In normal designs this parameter is of no consequence because it has no noticeable effect if the collector resistor is not large. When powering the amplifier from 340 V and setting the collector current to 1 mA, the collector resistor will have a value of 338 k. Whether the ‘hoe’-parameter has any influence depends in the type of transistor. We also note that with such high gains, the base-collector capacitance in particular will start to play a role.
As a consequence the input frequency may not be too high. For a higher bandwidth we will have to use a transistor with small Cbc, such as a BF494 or perhaps even an SHF transistor such as a BFR91A. We will have to adjust the value of the base resistor to the new hfe. The author has carried out measurements with a BC547B at a power supply voltage of 30 V. A value of 2 V was chosen for the collector voltage. Measurements confirm the rule of thumb. The gain was more than 1,000 times and the effects of ‘hoe’ and the base-collector capacitance were not noticeable because of the now much smaller collector resistor.
For simplicity, and assuming room temperature, we round this value to 40. For a single stage amplifier circuit with grounded emitter it holds that the gain Uout /Uin (for AC voltage) is in theory equal to SRc. As we observed before, the slope S is about 40Ic. From this follows that the gain is approximately equal to 40I cRc. What does this mean? In the first instance this leads to a very practical rule of thumb: that gain of a grounded emitter circuit amounts to 40·I c·Rc, which is equal to 40 times the voltage across the collector resistor.
If Ub is, for example, equal to 12 V and the collector is set to 5V, then we know, irrespective of the values of the resistors that the gain will be about 40R(12–5) = 280. Notable is the fact that in this way the gain can be very high in theory, by selecting a high power supply voltage. Such a voltage could be obtained from an isolating transformer from the mains. An isolating transformer can be made by connecting the secondaries of two transformers together, which results in a galvanically isolated mains voltage.Circuit diagram:
That means, that with a mains voltage of 240 Veff there will be about 340 V DC after rectification and filtering. If in the amplifier circuit the power supply voltage is now 340 V and the collector voltage is 2 V, then the gain is in theory equal to 40 x (340–2). This is more than 13,500 times! However, there are a few drawbacks in practice. This is related to the output characteristic of the transistor. In practice, it turns out that the transistor does actually have an output resistor between collector and emitter.
This output resistance exists as a transistor parameter and is called ‘hoe’. In normal designs this parameter is of no consequence because it has no noticeable effect if the collector resistor is not large. When powering the amplifier from 340 V and setting the collector current to 1 mA, the collector resistor will have a value of 338 k. Whether the ‘hoe’-parameter has any influence depends in the type of transistor. We also note that with such high gains, the base-collector capacitance in particular will start to play a role.
As a consequence the input frequency may not be too high. For a higher bandwidth we will have to use a transistor with small Cbc, such as a BF494 or perhaps even an SHF transistor such as a BFR91A. We will have to adjust the value of the base resistor to the new hfe. The author has carried out measurements with a BC547B at a power supply voltage of 30 V. A value of 2 V was chosen for the collector voltage. Measurements confirm the rule of thumb. The gain was more than 1,000 times and the effects of ‘hoe’ and the base-collector capacitance were not noticeable because of the now much smaller collector resistor.
Author: Gert Baars
Copyright: Elektor Electronics
Copyright: Elektor Electronics
One Transistor Code Lock
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This is of course the simplest electronic code lock circuit one can make. The circuit uses one transistor, a relay and few passive components. The simplicity does not have any influence on the performance and this circuit works really fine. The circuit is nothing but a simple transistor switch with a relay at its collector as load. Five switches (S0 to S4) arranged in series with the current limiting resistor R2 is connected across the base of the transistor and positive supply rail.
Circuit diagram:
![]( "One")
Notes:
Another five switches (S5 to S9) arranged in parallel is connected across the base of the transistor and ground. The transistor Q1 will be ON and relay will be activated only if all the switches S0 to S4 are ON and S5 to S9 are OFF. Arrange these switches in a shuffled manner on the panel and that it. The relay will be ON only if the switches S0 to S9 are either OFF or ON in the correct combination. The device to be controlled using the lock circuit can be connected through the relay terminals. Transformer T1, bridge D1, capacitor C1 forms the power supply section of the circuit. Diode D2 is a freewheeling diode. Resistor R1 ensures that the transistor Q1 is OFF when there is no connection between its base and positive supply rail.
Notes:
- This circuit can be assembled on a Vero board.
- Switch S1 is the lock’s power switch.
- The no of switches can be increased to make it hard to guess the combination.
- Transistor 2N2222 is not very critical here. Any low or medium power NPN transistor will do the job.
Schematic diagram for the One Transistor FM Radio with Improved Audio Gain
Schematic diagram for the One Transistor FM Radio with Improved Audio Gain
One Transistor FM Radio with improved audio gain.
Some wiring notes:
Unless you have experience with super-regenerative radios, I highly recommend using the FAR Circuits printed circuit board.
Connect the two sections of the variable capacitor (C3) in series to linearize the tuning somewhat. That is, use the connections on either end of C3 and dont use the middle lead.
L2, the RF choke should not be near a ground. The same is true for L1. Capacitance to ground will disturb the feedback.
The gain is just enough to drive an earphone. If you live too far away from radio stations, you might have trouble hearing one. There is no option here for an external antenna (that would require and extra transistor).
You can drive a speaker if you add an external audio amplifier.
If you want a little more audio gain, or you cannot locate a TL431CLP chip, you can use some other audio amplifier in the circuit where pins 1 and 2 of D1 normally connect. You can use an LM386 or a TDA7052 audio amplifier. Quasar DIY project kit #3027 is a complete TDA7052 audio amplifier kit and it works fine in this application.
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Some wiring notes:
Unless you have experience with super-regenerative radios, I highly recommend using the FAR Circuits printed circuit board.
Connect the two sections of the variable capacitor (C3) in series to linearize the tuning somewhat. That is, use the connections on either end of C3 and dont use the middle lead.
L2, the RF choke should not be near a ground. The same is true for L1. Capacitance to ground will disturb the feedback.
The gain is just enough to drive an earphone. If you live too far away from radio stations, you might have trouble hearing one. There is no option here for an external antenna (that would require and extra transistor).
You can drive a speaker if you add an external audio amplifier.
If you want a little more audio gain, or you cannot locate a TL431CLP chip, you can use some other audio amplifier in the circuit where pins 1 and 2 of D1 normally connect. You can use an LM386 or a TDA7052 audio amplifier. Quasar DIY project kit #3027 is a complete TDA7052 audio amplifier kit and it works fine in this application.
