Hy Simon822,
Here are the answers to your questions in post #19 and some basic electronic theory along with schematics of a high side (positive supply) referenced constant current generator and an inverted form, a low side (0V supply line) referenced constant current generator. Both of these circuits are widely used in electronics. R1 value has been changed to maximize the supply line voltage range.
(1) Answer to Questions
(1.1) Any of the transistors that I listed in previous posts will be suitable and will give equal performance, although most are plastic cases rather than metal can as shown in your picture of the constant current circuit. I would advise you to use a pair of BC546 NPN plastic case transistors.
(1.2) Schematics are posted below.
(1.3) The voltage range of the circuit posted below will be from 2V to the maximum allowable voltage, collector emitter (VCE), of the particular transistor used.
The maximum VCE is shown on the transistor's data sheet. For example if you use a 2N2222 transistor the maximum supply line voltage would be 30V. On the other hand, if you used a BC546 transistor the maximum supply line voltage would be 65V.
Bear in mind though that any voltage drop caused by the 0.005 Amps constant current flowing through the load fitted to the circuit will make the collector voltage of the second transistor, Q2, that much lower than the supply line voltage. The circuit will not operate correctly if Q2 collector voltage is less than 2V.
(2) Elementary Principles
(2.1) Electric Current
(2.1.1) An electric current is nothing more than a flow of electrons, billions of them. (measured in Amps)
Assume that the current flows from positive to negative (in practice electrons flow from negative to positive but do not concern yourself with this for the present)
(2.1.2) A voltage is the force that pushes the electrons through a circuit. (measured in Volts)
(2.1.3) Resistance tries to stop the electrons flowing. (measured in Ohms)
(2.1.4) The formula for finding the current flowing in a circuit is I = V/R (Ohm's law)
where:
I= current in amps
V= voltage in Volts
R= resistance in Ohms
For example if you had a 6 Volt battery and connected a 2 Ohm resistor across the battery, 6 Volts/2 Ohms = 3 Amps would flow through the resistor.
(2.2) Transistor
(2.2.1) A transistor has three terminals (leads): Collector (C), Base (B), and Emitter (E). The physical form of a transistor varies as does the position of the terminals, but the electrical performance is the same. In fact, the silicon chip inside a metal can transistor may be identical to the chip in a plastic case transistor.
There are thousands of transistor types but generally they fall into groups with similar performance (this is a gross simplification but useful to start with): small signal, medium power, high power.
(2.2.2) There are two polarities of transistors: NPN and PNP. For a given complimentary pair (NPN and PNP) the parameters are the same except that the polarity of all voltages and currents are reversed, including the supply line voltage.
(2.2.3) The basic operation of a transistor is that you feed a current into the base emitter circuit and a much larger current flows in the collector emitter circuit. Typically, the collector current would be 100 times the base current.
(2.2.4) When a silicon (normal) transistor is conducting current the voltage between its base and emitter (VBE) is 0.6V. Think of VBE as a battery. In practice VBE will vary with temperature and collector current. It will also vary slightly between two transistors of the same type. But in essence VBE is constant. This characteristic of silicon transistors is used in your initial circuit to generate a 0.005 Amp constant current.
(2.3) Accuracy
(2.3.1) There is no such thing as absolute accuracy in engineering. Every parameter: voltage, current, resistance, transitor current gain, has an accuracy. The major aim in electronics is to ensure that the accuracy of a circuit design meets the requirements of the application.
(2.3.2) Take resistors; they commonly come in +-10%, +-5%, +-2% and +-1% tolerance but you can get precision resistors that are +- 0.1% even +-0.001%.
(2.2.3) The objective of your circuit is to produce a constant current of 0.005 Amps but many factors will affect the absolute accuracy of that constant current: temperature, resistor tolerances, supply line voltage, VBE of Q2, etc etc. But for many applications the accuracy of your circuit will be adequate.
In some applications the important thing is that the current is constant and its absolute value is not important. Constant current means that the current flowing through the load will be the same whatever the load is, provided the collector voltage of transistor Q2 does not drop below 2V that is.
(3) Schematics
(3.1) Here are two schematics (drawn in EAGLE): the constant current generator circuit from your original post which has a positive supply line referenced load is on the right; both transistors have been changed and the value of resistor (R1) has been changed. An inverted constant current generator which has a zero Volt referenced load is shown on the left. Both circuits have identical performances.
(4) Notes
See Les' post #11 for a good description of how these circuits work
(5) Data Sheets
(5.1) Transistor NPN 2N2222 metal can
**broken link removed**
(5.2) Transistor PNP 2N2907A metal can (complement to 2N2222)
https://www.onsemi.com/pub_link/Collateral/2N2907A-D.PDF
(5.3) Transistor NPN BC546 plastic case
https://www.onsemi.com/pub_link/Collateral/BC546-D.PDF
(5.4) Transistor PNP BC556 plastic case (compliment to BC546)
- Need to locate a pair of transistors, prior commencing to apply knowledge acquired from this forum.
- Yes I could use some schematics on this simple project, (perhaps with new discovered specs, if possible).
- Have forgot to ask fellow members as to the range of voltage.
- Instance given of the top of my head was 22.5 Volts! Would there be a range from lets say 12 Volts - 24 Volts? With same specs above?
Here are the answers to your questions in post #19 and some basic electronic theory along with schematics of a high side (positive supply) referenced constant current generator and an inverted form, a low side (0V supply line) referenced constant current generator. Both of these circuits are widely used in electronics. R1 value has been changed to maximize the supply line voltage range.
(1) Answer to Questions
(1.1) Any of the transistors that I listed in previous posts will be suitable and will give equal performance, although most are plastic cases rather than metal can as shown in your picture of the constant current circuit. I would advise you to use a pair of BC546 NPN plastic case transistors.
(1.2) Schematics are posted below.
(1.3) The voltage range of the circuit posted below will be from 2V to the maximum allowable voltage, collector emitter (VCE), of the particular transistor used.
The maximum VCE is shown on the transistor's data sheet. For example if you use a 2N2222 transistor the maximum supply line voltage would be 30V. On the other hand, if you used a BC546 transistor the maximum supply line voltage would be 65V.
Bear in mind though that any voltage drop caused by the 0.005 Amps constant current flowing through the load fitted to the circuit will make the collector voltage of the second transistor, Q2, that much lower than the supply line voltage. The circuit will not operate correctly if Q2 collector voltage is less than 2V.
(2) Elementary Principles
(2.1) Electric Current
(2.1.1) An electric current is nothing more than a flow of electrons, billions of them. (measured in Amps)
Assume that the current flows from positive to negative (in practice electrons flow from negative to positive but do not concern yourself with this for the present)
(2.1.2) A voltage is the force that pushes the electrons through a circuit. (measured in Volts)
(2.1.3) Resistance tries to stop the electrons flowing. (measured in Ohms)
(2.1.4) The formula for finding the current flowing in a circuit is I = V/R (Ohm's law)
where:
I= current in amps
V= voltage in Volts
R= resistance in Ohms
For example if you had a 6 Volt battery and connected a 2 Ohm resistor across the battery, 6 Volts/2 Ohms = 3 Amps would flow through the resistor.
(2.2) Transistor
(2.2.1) A transistor has three terminals (leads): Collector (C), Base (B), and Emitter (E). The physical form of a transistor varies as does the position of the terminals, but the electrical performance is the same. In fact, the silicon chip inside a metal can transistor may be identical to the chip in a plastic case transistor.
There are thousands of transistor types but generally they fall into groups with similar performance (this is a gross simplification but useful to start with): small signal, medium power, high power.
(2.2.2) There are two polarities of transistors: NPN and PNP. For a given complimentary pair (NPN and PNP) the parameters are the same except that the polarity of all voltages and currents are reversed, including the supply line voltage.
(2.2.3) The basic operation of a transistor is that you feed a current into the base emitter circuit and a much larger current flows in the collector emitter circuit. Typically, the collector current would be 100 times the base current.
(2.2.4) When a silicon (normal) transistor is conducting current the voltage between its base and emitter (VBE) is 0.6V. Think of VBE as a battery. In practice VBE will vary with temperature and collector current. It will also vary slightly between two transistors of the same type. But in essence VBE is constant. This characteristic of silicon transistors is used in your initial circuit to generate a 0.005 Amp constant current.
(2.3) Accuracy
(2.3.1) There is no such thing as absolute accuracy in engineering. Every parameter: voltage, current, resistance, transitor current gain, has an accuracy. The major aim in electronics is to ensure that the accuracy of a circuit design meets the requirements of the application.
(2.3.2) Take resistors; they commonly come in +-10%, +-5%, +-2% and +-1% tolerance but you can get precision resistors that are +- 0.1% even +-0.001%.
(2.2.3) The objective of your circuit is to produce a constant current of 0.005 Amps but many factors will affect the absolute accuracy of that constant current: temperature, resistor tolerances, supply line voltage, VBE of Q2, etc etc. But for many applications the accuracy of your circuit will be adequate.
In some applications the important thing is that the current is constant and its absolute value is not important. Constant current means that the current flowing through the load will be the same whatever the load is, provided the collector voltage of transistor Q2 does not drop below 2V that is.
(3) Schematics
(3.1) Here are two schematics (drawn in EAGLE): the constant current generator circuit from your original post which has a positive supply line referenced load is on the right; both transistors have been changed and the value of resistor (R1) has been changed. An inverted constant current generator which has a zero Volt referenced load is shown on the left. Both circuits have identical performances.
(4) Notes
See Les' post #11 for a good description of how these circuits work
(5) Data Sheets
(5.1) Transistor NPN 2N2222 metal can
**broken link removed**
(5.2) Transistor PNP 2N2907A metal can (complement to 2N2222)
https://www.onsemi.com/pub_link/Collateral/2N2907A-D.PDF
(5.3) Transistor NPN BC546 plastic case
https://www.onsemi.com/pub_link/Collateral/BC546-D.PDF
(5.4) Transistor PNP BC556 plastic case (compliment to BC546)
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