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Small fixed current limiter PDF Print E-mail
Thursday, 23 October 2008 21:29

This circuit can be used for driving capacitive loads from a DC/DC converter, or as a driver for opto coupler inputs.

When used as a DC/DC converter load controller it can prevent the DC/DC converter from failing to start because of a too high capacitive load (in which case it basically sees a short and won't start). This can easilly happen when a board with too many decoupling capacitors is supplied from a small DC/DC switch converter.

When used as an input protection it can be used as below, driving an opto coupler. This gives a high input voltage range, something like 5V...30VDC, making it possible to drive the input from a 5V logic source or e.g a 24V PLC.

The first PNP transistor starts to conduct when the VBE voltage is >0.6V. This happens when the current trough the 30ohm resistor is 20mA. When it conducts it will increase the voltage on the second transistors base, effectively blocking it until a steady state occurs. The last series diode is a reverse polarity protection for the opto coupler, as the LED itself can not withstand a high reverse polarity. 

Current limiter

Last Updated on Friday, 24 October 2008 07:33