I left a reply in a Yahoo group about power supplies and another commenter left a reply that he used the LM78XX chips often for regulated power supplies, usually without a heatsink. I would like to relate some of the experiences I’ve had with these wee beasties.
Test Equipment I worked for a company that built microprocessor based test equipment that used a LM7805 in the power supply. We would burn them in over the weekend to make sure that if anything failed, it would happen before they left the production line. We had to send one of the techs to Jamaica where we had several of our boxes that were failing intermittently. It just so happened that he was from Jamaica, so he really wanted to go there to see his home. Anyway, the motherboard had a LM7805 without a heatsink freestanding on the board. The problem was solved by soldering some short pigtails onto the replacements and bolting the 7805 to the case. They overheated, apparently because in Jamaica, the sweatshops really are sweatshops, and the equipment and people were working in a hot environment. Also, the AC line voltage was not as well regulated as here in the U.S.
The 7805 chip would heat up, and the internal overtemp protection would shut down, lowering the voltage from 5V down to less than 4.75V which was the absolute minimum for the processor and other chips. When the 7805 started to shut down, the voltage drop across it increased – the unregulated input voltage minus 4.75V was more than the unregulated input voltage minus 5V – so the chip would get even hotter. Just shutting the box off for a few minutes would get it working again, only to have it go bananas later.
Old Modems I had a few modems, 1200 and 2400 baud, that we were still using until a few years ago. They would lock up and cause me to have to go out to a site and power them off. I found out what the problem was. They were overheating. The LM7805 regulator would get hot and shut down, and the low voltage would cause the rest of the board to go into an undervoltage confition. The case was solid plastic with no air circulation to the board inside. My solution was to drill several holes through the case above (and if needed, below) the 7805, and mount a small piece of aluminum to the 7805 chip. It added just enough heatsink to drop the chip temperature enough to prevent it from going into thermal shutdown. Another contributing factor was the AC adapter that came with the modem. The adapter is rated for 11VAC at 1 amp. But it actually puts out more like 14 volts AC when the load is less. So the rectified voltage may be more like 17 or more volts before the regulator. And that doesn’t include higher voltage when the AC line voltage goes above 120VAC. That can really add to the heat that the 7805 chip has to dissipate.
Crowbar Effect Apparently the overtemp shutdown has sort of a crowbar effect. The chip gets warm and the overtemp starts to shut it down. The voltage across the chip increases and the chip gets even hotter, and drives the overtemp into further shutdown. So my experiences with this versatile but somewhat finicky chip is to keep it cool, or else you will be asking for trouble later. The easiest way to keep it cool is to add a small heatsink. or bolt it to the PC board with a lot of copper under the chip. Also, sometimes the chip may be slightly out of tolerance and the shutdown temp will be lower than the usual, and cause thermal problems just with that chip, even though it works normally otherwise. The best thing to do is to replace the LM78XX chip.
I’ve also found that the heat dissipation can be offloaded from the LM78XX chip. One way is to put a resistor between the unregulated supply and the regulator chip. The chip still needs a 0.1uF bypass capacitor on the input. Another way is to put one or more 1N4003 rectifiers in series with the supply; each rectifier will drop about 0.8V. Three or four in series will offload a watt or more, and that may be enough to keep the LM78XX from going into overtemp. but the power is till being dissipated in the case, so the case will not be any cooler. It may be necessary to put a heatsink on the chip or some other way to get rid of the heat.
Newer Equipment Nowadays, hardly any new piece of equipment uses these LM78XX linear regulator chips. The newer stuff uses a switching regulator that dissipates far less power, and is much ‘greener’ – Energy Star rated.