Current Limit | Electronic Project Circuits

Posts Tagged ‘current limit’

PC Power 12 V Current Limiter

PC Power 12 V Current LimiterTo safely power something from your PC, the attached loads have to be limited to a safe current in order to avoid the chance that a short circuit might cause inadvertent shutdown and damage to your data. This circuit can be used to safely bring out the 12V line from your computer to supply a limited amount of current to power external gadgets.

With normal loads, the power transistor is in saturation and so the regulated +12V line is directly available (minus only the Vcesat of the PNP transistor). If the output is accidentally shorted, the npn transistor turns off and removes the base drive of this transistor.

This circuit is intended for protection from short circuits. It might be possible to apply a load that brings the series transistor out of saturation without removing base drive to it: such a situation will cause heating and might cause it to fail short-circuit. Those who wish to draw significant current from this circuit will be well advised to check that the transistor is indeed in saturation with their intended load.

That said, this is a simple way to draw a few tens of milliamps at 12 V from your PC without danger to either load or the PC.

Link Source http://www.instructables.com

Be the first to comment - What do you think? Posted by admin - July 23, 2007 at 5:38 pm

Categories: Circuit com, Power supply Tags: 12V, Circuit com, current limit, pc power supply circuit

L200 Current Limiting Power Supply

This is a 1-amp variable-voltage PSU. It adjusts from about 3v to 24v: and has the added feature that you can limit the maximum output current. This is invaluable when (for example) you power-up a project for the first time or soak-test a piece of equipment.

SW3 is the on/off switch. It also lets you choose between the output with the current limit and the one without. SW2 provides a selection of three different limits. You can increase or decrease this number if you wish. The limits are fixed by R4, R5 & R6. They are set at 10mA, 25mA & 65mA respectively; but you can choose whatever limits you like. If you try to draw a current above the limit you’ve selected, the output voltage will fall. Thus, the voltmeter indicates when the load on the output is excessive.

The housing should be well ventilated. The heatsink is a folded strip of aluminium about 2mm thick, 18cm long and 6cm tall. SW1 allows you to choose between the (3v to 12v) and (3v to 24v) outputs. This reduces the power the heatsink has to dissipate when the output voltage is low.

The drawing of the front panel is intended mainly as a wiring diagram – you can choose your own layout. The pin spacing of the L200C doesn’t suit the stripboard; but with a little persuasion it can be made to fit. Since the limiting resistors may have a relatively low value, a few ohms between the contacts of the wafer-switch will have a significant effect. If (with time) you find that the limits have fallen, use a spray-cleaner on the switch.