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.
Sorry but looking at this circuit, (I think) there are glaring issues with this current regulator design. For a start, 78xx regulators can NOT have 350 volts across them whether in voltage or current mode. See the datasheet. So if the load is not high enough impedance/resistance, the 78xx will instantly let out it’s magic smoke, and might even start a fire. Or am I missing something??? Did you build and test this or just put it into design software? One of the BIGGEST mistakes people DESIGNING circuits make, is believing that what the design software says works, will actually work in real life. Semiconductors have all quirks / requirements that don’t seem to make it into the theoretical software design packages. Let’s take the 78xx regulator for example, you seem to have completely overlooked the heat dissipation requirements, are you aware of the necessity to have a minimum output current or the regulator can go crazy when there’s no or low load, you don’t seem to have specified bypass capacitors very close to the input and output leads, without them, the 78xx regulators have a habit of oscillating and self-destructing. Design software is only a GUIDE, NOT a reliable design tool. That’s why, in real life production processes, after creating a design, you build a series of product batches using a diversity of suppliers parts, and you thoroughly test each built unit in every way imaginable to ensure they work totally reliably. In the first build batches, it’s not uncommon to have a 20 to 30% unit failure rate until you work out the tolerance and quirks that need to be factored in to your final design. Notice that even major manufacturers have version upgrades from time to time to solve newly discovered quirks.
Thank you for your analysis! The circuit was probably taken from an old ETI magazine. However I cannot see how the 7805 IC could burn. Could you please clarify how the high input voltage could destroy the IC. As far as heat dissipation is concerned, definitely the BJT and the R1 would dissipate a lot of heat, just as any other linear regulator would do.
I never depend on simulators, i always use my mind simulation, using whatever knowledge I have gained so far, and then build it practically and check for real issues…
go to http://www.alldatasheets.com and search for LM7800 and LM78L05. The spec sheets tell you the max voltage is 35 volts. over that they might of might not survive. its nit the current alone, its voltage difference between the three terminals. some brands datasheets give deeper explanation of what they mean by max voltage. note also some give better explanation of the need for the fikter caps and minimum output current ti enable stable output.
Yes, I know that, but i cannot understand across which terminals these specifications are exceeded?
Input is clamped with 8V zener and current limited by R1.
Output pin has +5V which is applied to the BJT base, so that its emitter is around +4V…
Ground pin is grounded through the preset resistors and also it gets 4V from the BJT emitter.
So where is it exceeding?
I pulled an OH SUNG squirrel cage fan vent out of an old microwave/convection oven. It is rated at 120 V 60Hz. Other markings show 128/105W, and 1.09/0.87A. Cap 7uF “Thermally Protected”.
I live in an apartment building built in 1947 which has steam heat. My thought was to attach a passive thermostat to the circuit that energizes and starts the fan when the steam heat begins and de-engergize when it cools. By running this fan under the radiator I can significantly gain more heat when the steam heat is present.
The wire harness has five wires. There is a diagram which I can’t seem to post an image of here. However, it can be seen here in this ebay post: " rel="ugc">
I don’t understand why there are five wires (black, white, yellow, red, blue). If I simply want to apply 120V AC current, which is hot and which is neural? What are the other three wires for? Can I use them to my advantage, like maybe to control fan speed based on the thermostat’s state? In any event, a simple on/off based on a hot thermostat would be a great outcome. Thank you kindly for any insights you can offer.
Looking at the image, it seems the Blue wire is directly connected to one of the mains AC input supply, preferably the Neutral wire.
The 7uF capacitor is supposed to be attached between the Yellow and the Red wires.
Now the LIVE or the Phase of the 120V AC seems to be going through a relay or a thermostat switch to the black and the white wires.
You could initially try applying the 120V AC between the Blue and the Black wires through a 1 Amp FUSE and check the response.
Thank you, I will give it a try. From further research, I see the fan has a Hi and a Low setting.
Sure, no problem. For the H/L selection you can probably try an SPDT switch, or maybe the H/L switch is supposed to be an SPDT relay configured with the protection circuitry…
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.
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