# Stable current source from 5 μA to 20 mA

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The author needed a stable current source for debugging circuits based on bipolar transistors, which, as you know, are controlled by current. An important requirement for it is the isolation of the common wire of the device from the common wire of the device being debugged, so the power source had to be taken offline. The built-in four-digit microammeter with automatic switching of limits allows to slightly reduce the amount of equipment simultaneously placed on the experimenter's table.

The idea of the circuit is taken from here . Actually, the stable current source is arranged as follows:

The resistance of the resistor R1 is uncritical, it is only necessary that the base current of the transistor T1 fully open it. The current transfer coefficient of the BC559C is about 500, the upper limit of current regulation at the source is 20 mA, which means that 200 μA through the base is more than enough. A resistor of 10 kOhm will provide about 1 mA at 10 V, in principle, you can increase it even up to 50 kOhm.

Transistors T1 and T2 should be the same, but at high currents, the parameters of T1 will still “float” a little due to a little heat.

The current supplied by the device to the external circuit is determined by the total resistance of the resistors R3 - R5. Their functions: R3 - current limitation in case both resistor variables are turned “to zero”, R4 - precise current control, R5 - coarse. The current is calculated by the formula I = 0.7 / (R3 + R4 + R5), therefore, for example, if you take the resistor R3 with a resistance of 27 Ohms, the upper limit of the current adjustment will be 0.7 / 27 = 25.9mA. In practice, it turned out 21.6 mA, since the voltage drop across the T2 transistor turned out to be less - about 0.6 V. The

complete circuit of the device:

"Krona" feeds a stable current source, two AAA elements - a four-digit microammeter. Therefore, the power switch is taken with two normally open groups of contacts. Switch S1 allows you to disconnect the upper terminal and short-circuit the current source to configure it in advance, before connecting to the debugged circuit.

The parameters in practice were as follows: the maximum current is 21.6 mA, the maximum current with a “rough” regulator turned “to zero” is 0.3 mA, and the minimum is 4.7 μA. True, the built-in microammeter less than 10 μA does not show, so an external one may sometimes be required. The set current remains almost unchanged when the voltage on the external circuit changes from 0 to 8 V.

The microammeter is made of a multimeter with automatic switching of limits JT-033A manufactured by SHENZHEN JINGTENGWEI INDUSTRY CO., LTD: the mode switch is removed, jumpers are soldered instead, forcing it to always work in the current measurement mode.

The location of the components in the case is as follows:

Jim made a circuit simulation in Falstad, the author reworked it a bit to display more parameters, it turned out:

```
$ 1 0.000005 7.619785657297057 65 5 50
t 224 240 176 240 0 -1 0.6771607865907852 -0.5873050244463638 500
t 256 272 304 272 0 -1 1.8738439949380101 -0.6771607865907852 500
r 176 304 176 400 0 10000
v 80 288 80 192 0 0 40 9 0 0 0.5
w 176 304 176 272 3
w 176 272 176 256 0
w 176 224 176 32 1
w 176 32 80 32 0
w 80 32 80 192 0
w 80 288 80 400 0
w 80 400 176 400 3
w 176 400 304 400 0
w 304 336 304 288 3
w 304 240 224 240 1
174 304 128 352 48 0 5000 0.9950000000000001 Resistance
w 176 32 304 32 2
w 304 256 304 240 0
w 304 240 304 208 2
w 304 128 336 128 0
w 352 80 352 128 0
w 352 128 336 128 0
w 256 272 176 272 1
w 304 128 304 208 1
r 304 336 304 400 0 250
```

Simulation result:

And here is the simulation result when the resistance of the resistor R1 is 100 kOhm: