LM7824 | Electronic Project Circuits

Posts Tagged ‘LM7824’

Dual Power Supply +24V /-24V by IC 7824 + 7924

This supply gives both positive and negative outputs. Appropriate Fuses should be used to protect from fire hazard and overload of transformer. The Filter capacitor C1 4700uF has an impedance of Xc = 1 / (2 * 3.14 * f * C) which comes to 0.6 ohms at 50 Hz. The impedance of the load at 2A for 24V is R = V / I that is 12 Ohms which is more than 20 times the impedance of the capacitor at 50 Hz. That means less than 1 / 20 of ripple current will flow thru the load. The Regulator also reduces the ripple a little.
Read More Source:http://schematics.blogspot.com/2004/10/dual-power-supply.html

Be the first to comment - What do you think? Posted by admin - December 16, 2007 at 6:22 am

Categories: Power supply Tags: 24v power supply, dual power supply, IC 7924, LM7824

7824 Single Power Supply 25V 2A with IC 7824 and TIP2955

This circuit uses a PNP Power Transistor TIP2955, you can use any other according to your current and voltage requirement. Look at R2 a 10 Ohm resistor, when the current in your load to the power supply is less than 70mA the voltage across R2 is less than 10E * 70mA = 700mV right.

The base emitter junction of Q1 will be biased or turned on around 700mV, less than 700mV the transistor just does nothing. When the current in your load goes over 70mA the voltage across R2 goes above 700mV and a small base current Ib flows from emitter to base of Q1 turning on the transistor. Now a collector current Ic flows from emitter to collector and then to your load supplying the excess demand. The Ic = Ib * hfe where hfe or beta is the DC gain value.

Some transistors will have only AC gain specified which is lower than DC gain. TIP2955 has a gain of 20 so for an Ib of 50mA the Ic will be 1 Amp which saves the regulator from heating up or shutting down as the main current flows thru the transistor. Q1 should be provided with a good heatsink.

Source:http://schematics.blogspot.com/2004/10/single-power-supply.html