The presented circuit offers an adjustable constant current source capable of reaching up to 60mA, operating at voltages as high as 350V.
Circuit Description
The input stage, consisting of resistor R1 and zener diode ZD1, delivers 8V to the regulator (a 7805), maintaining this voltage relative to its reference line.
This voltage level ensures sufficient power for the regulator's operation while minimizing excessive dissipation.
Function of the 7805 Regulator
The regulator's objective is to uphold a 5V potential difference between its output and common terminals, which is also applied to the base-emitter loop of transistor Q1.
Q1, configured as a Darlington device, exhibits a base-emitter voltage of approximately 1.5V when activated.
In case the load connected to the circuit's output causes the collector current to rise, the voltage developed across resistors R2 and RV1 will also increase.
Consequently, the voltage at the emitter of the transistor rises in relation to the 7805's common terminal.
During the conduction state of the transistor, its base voltage is compelled to increase.
Since the 7805 regulator maintains a constant 5V between its output and common terminals, the voltage drop across R3 will decrease, resulting in reduced base current.
This reduction counteracts the initial surge in collector current.
A similar mechanism comes into play when the collector current attempts to decrease.
The above working allows the circuit to behave like a high voltage constant current source.
Adjustable Current Feature
Facilitating current adjustment, the variable resistor RV1 is incorporated, while fixed resistor R2 serves to cap the maximum available current.
To handle power dissipation, a 3W wirewound type is recommended for the variable resistor.
Function of Darlington Transistor
The adoption of a Darlington transistor stems from its higher gain, leading to more consistent current flow.
The specific transistor used in this configuration boasts a rating of 400V and 150W at 25°C, ensuring safe operation within its specified limits.
Nonetheless, when a current demand surpasses a few milliamps, it's advisable to equip the Darlington transistor with a heat sink to mitigate potential temperature-related issues.
If desired, adjustments to the circuit values can be made to accommodate different voltage and current requirements, provided that the component ratings are not exceeded.
Are you certain about that 15K resistor? At 60 ma, that’s going to drop 900 volts! What is its purpose, other than to limit current?
Also, why the “0V”? If one end on the zener is grounded, this won’t work, will it? The 78L05 will just put out 5 volts relative to ground, right?
I want to use this as an anode load for a 12AU7 or 6SN7 vacuum tube at about 10 ma current.
You are right, the circuit has mistakes, but the 15k is OK, although it might get super hot.
First of all the ground of the 78L05 must be connected to the emitter of Q1 and not directly to the ground line.
Moreover, the the 78L05 can be actually replaced with another BJT which will allow even higher value resistors to be used for R1. Higher resistor value would mean lower operating current and lower dissipation.
Here’s an example circuit which you can try instead of the above design. Please replace the transistors appropriately with higher voltage rated transistors.
https://www.homemade-circuits.com/wp-content/uploads/2020/01/Darlington-current-limiter.jpg