Constant Current Circuit
Re: Constant Current Circuit
by admin Thu Nov 14, 2019 6:56 pm
I think you will find there are several reasons - sorry for the late response.
The amplified diode also has a poor temperature coefficient. The greater the amplified voltage, so the greater is the temperature/voltage shift.
A simple silicon transistor B-E junction has about +2.5mV per degree celcius. The 2:1 amplified diode has about 5mV per °C.
Another point is that the voltage across this amplified diode is not as constant as a "normal" diode. A normal diode is not always 0.7V, it does vary logarythmically with current.
One thing you can do, instead of using an amplified diode, try using an LED. I found that they are more stable than conventional diodes when used as voltage regulators. Only disadvantage I have found is that they are light-sensitive, so you can get light-to-current modulation on your constant-current generator, if you do not keep the LED in darkness, or paint it. But ordinary glass diodes are also very sensitive to light.
BR Harry - EA/SM0VPO
The amplified diode also has a poor temperature coefficient. The greater the amplified voltage, so the greater is the temperature/voltage shift.
A simple silicon transistor B-E junction has about +2.5mV per degree celcius. The 2:1 amplified diode has about 5mV per °C.
Another point is that the voltage across this amplified diode is not as constant as a "normal" diode. A normal diode is not always 0.7V, it does vary logarythmically with current.
One thing you can do, instead of using an amplified diode, try using an LED. I found that they are more stable than conventional diodes when used as voltage regulators. Only disadvantage I have found is that they are light-sensitive, so you can get light-to-current modulation on your constant-current generator, if you do not keep the LED in darkness, or paint it. But ordinary glass diodes are also very sensitive to light.
BR Harry - EA/SM0VPO
Last edited by Admin on Thu Nov 14, 2019 7:57 pm; edited 1 time in total (Reason for editing : Korrecting spolling errorz hand adding more werdz)
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Constant Current Circuit
by The Student Prince Fri Sep 20, 2019 10:28 am
Here's another constant current circuit but I've yet to see it used:
It uses the 'amplified diode' configuration (often used in push-pull amplifier outputs).
The output current (AM1) is given by VbeR1/(R2R3).
T3 can be replaced by an NPN (R1 and R2 need to be swapped - either on the circuit or the equation).
It does use a couple of resistors more than the more usual two transistor version but I wonder if there's another reason why it's not used. Any ideas?
It uses the 'amplified diode' configuration (often used in push-pull amplifier outputs).
The output current (AM1) is given by VbeR1/(R2R3).
T3 can be replaced by an NPN (R1 and R2 need to be swapped - either on the circuit or the equation).
It does use a couple of resistors more than the more usual two transistor version but I wonder if there's another reason why it's not used. Any ideas?
The Student Prince- Guest
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