LM7805

[wgs]

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?

Your help will be appreciated.

Walt

 

[(*steve*)]

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?

 

[Harald Kapp]

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

 

[BobK]

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

 

[Wabajig]

What is the VA of your transformer or wattage? Does it have 3 wires out of the transformer or 2 wires out?

 

[wgs]

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

 

[Harald Kapp]

That's o.k. It sometimes so happens.
Glad it works now.

Harald

 

[BobK]

Trust but verify!

Bob