# LM7805

Discussion in 'General Electronics Discussion' started by wgs, May 3, 2012.

1. ### wgs

2
0
May 3, 2012
Hi Guys,

First post. I've built a 5VDC power supply using an LM7805 chip. I have a 1/2 amp 6.3 VAC transformer supplying the chip. I have a 3200MFD filter after the rectifier. I have .1 MFD on the input and output of the chip.

The problem: With a 10ma load the output voltage is 5.02 volts, great. With a 225ma load the output sucks down to 2.87 volts, not great. When this happens the input DC voltage to the chip is 7.75 VDC. Is that enough overhead to maintain output regulation?

Walt

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,481
2,830
Jan 21, 2010
Show us the circuit diagram you used, and possibly some photos of your constructed circuit as well.

Do you have a heatsink on the 7805? Is it warm at all?

3. ### Harald KappModeratorModerator

11,164
2,550
Nov 17, 2011
The LM7805 has a thermal resistance of 65 ° C/W (junction - air) if you use the TO-220 case. At your figures there is approx. (7.75 V-2.87V)*0.225mA=1.1W which translates into a junction temperature of 71 ° above ambient temperature. Even at 30 ° C ambient temperature this means the junction gets hot to ~ 100 °C. The fairchild datasheet goes up to 125 °C junction temperature so the LM7805 should not yet be shutting down (although a heat sink is a good idea anyway).

Are you sure you have the TO-220 variant, not the LM78L05 in a TO-92 package?

Also: have you looked at the input and output waveforms? Oscillating voltages can mock a completely different DC voltage if not measured correctly. Try another 100nF capacitor directly from the input pin of the regulator to GND. The large Elko you're using is no good for high frequency.

Harald

4. ### BobK

7,682
1,688
Jan 5, 2010
The 7805 has a dropout voltage of 2V, which means to get 5V out you need 7V in. Your 6.3V 1/2 A transformer + rectifier + capacitor will not provide 7V at any signifcant current. Think about switching to a low-dropout regulator like the LM2940, which has a .5V dropout.

Edited: Simulation shows that the voltage does drop below 7V when 250ma is drawn from the circuit, but only by a little. This is probably not the cuase of your problem.

Bob

Last edited: May 3, 2012
5. ### Wabajig

75
0
Apr 14, 2012
What is the VA of your transformer or wattage? Does it have 3 wires out of the transformer or 2 wires out?

6. ### wgs

2
0
May 3, 2012
Update

Thanks to all of you for your timely and knowledgeable replies.

I apologize! ........... when I posted my message I was tired and frustrated. I should have spent just a little more time checking the circuit before asking for help.

Some of the info I presented is wrong. I was looking at my drawings instead of the board itself. The transformer is not 6.3 volts - it's 5 VAC @ .22 A. The full load input voltage I quoted was a calculated value - the actual voltage sagged to 4.4 volts. So, my problem was not enough transformer capacity. I've replaced the transformer with a 6.3 VAC/.44A transformer. The N.L. DC input volts are 9.3 VDC and the output voltage holds solid at 5.02 VDC with a .225A load.

Thanks again,
Walt

7. ### Harald KappModeratorModerator

11,164
2,550
Nov 17, 2011
That's o.k. It sometimes so happens.

Harald

8. ### BobK

7,682
1,688
Jan 5, 2010
Trust but verify!

Bob