# A diode reduces the current

(revised December 15, 2005)

Current limitation, voltage reduction, LED connection

How do I limit a direct current? How do I reduce a DC voltage? How do I connect an LED? Often times the answers are so simple that they cannot be found on electronics pages.

Current limitation:

page 1

If low direct currents are to be limited, e.g. so that an LED is always equally bright at different voltages, the circuit with the BF 245 is ideal. (N-channel junction FET = junction FET = JFET)

Stress reduction:

If it is only a few milliamperes and the accuracy is not a major issue, voltages can be reduced simply by adding diodes. (also several diodes in series) In contrast to the connection of a (series) resistor, when a diode is connected upstream, the voltage does not change significantly with minor changes in the current.

A Schottky diode usually has a voltage drop of 0.2 volts in the direction of flow, a silicon diode 0.6 volts.

It is different when using Zener diodes. They are switched in the reverse direction. The voltage is reduced by the value that is printed on the Zener diode. (by the way: this is not the actual field of application of Zener diodes and the designation has nothing to do with the number "ten")

page 2

Conventional direct current limitation with 2 PNP transistors.

If the voltage is higher than 30 V, the BF 245 is not suitable. Then you use the circuit with NPN transistors. The BC546 can handle voltages of 80 volts. The circuit even works satisfactorily with 1.5 volts.

Sheet 3

Direct and alternating current limitation with the BF 245.

Generally:

The BF 245A is used for fixed currents of 2.5 mA, the BF 245B for 8 mA and the BF 245C for 20 mA. No other components are required. If the current is to be adjustable, all that is needed is a miniature trimmer 2.5 k-Ohm. The current limited in this way is almost constant from 2 - 30 volts. (In the case of the adjustable variant, this applies from approx. 6 volts, below this the current is recognizably voltage-dependent)

It is important that the maximum output of the BF 245 (0.3 watts) is not exceeded.

e.g. 20 mA

U = N: I = 0.3: 0.02 = 15 volts

or at 30 volts

I = N: U = 0.3: 30 = 0.01 A = 10 mA

Sheet 4

A current limit with LED voltage references. Very precise, with coarse and fine adjustment.

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Imprint:

© September / 2003 by HANS-DIETER TEUTEBERG • [email protected]

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