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.\u00a0 I would like to relate some of the experiences I’ve had with these wee beasties.<\/p>\n
Test Equipment<\/strong> \u00a0 I worked for a company that built microprocessor based test equipment that used a LM7805 in the power supply.\u00a0 We would burn them in over the weekend to make sure that if anything failed, it would happen before they left the production line.\u00a0 We had to send one of the techs to Jamaica where we had several of our boxes\u00a0 that were failing intermittently.\u00a0 It just so happened that he was from Jamaica, so he really wanted to go there to see his home.\u00a0 Anyway, the motherboard had a LM7805 without a heatsink freestanding on the board.\u00a0 The problem was solved by soldering some short pigtails onto the replacements and bolting the 7805 to the case.\u00a0 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.<\/p>\n 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\u00a0 for the processor and other chips.\u00a0 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.\u00a0 Just shutting the box off for a few minutes would get it working again, only to have it go bananas later.<\/p>\n Old Modems<\/strong>\u00a0\u00a0 I had a few modems, 1200 and 2400 baud, that we were still using until a few years ago.\u00a0 They would lock up and cause me to have to go out to a site and power them off.\u00a0 I found out what the problem was.\u00a0 They were overheating.\u00a0 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.\u00a0 The case was solid plastic with no air circulation to the board inside.\u00a0 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.\u00a0 It added just enough heatsink to drop the chip temperature enough to prevent it from going into thermal shutdown.\u00a0 Another contributing factor was the AC adapter that came with the modem.\u00a0 The adapter is rated for 11VAC at 1 amp.\u00a0 But it actually puts out more like 14 volts AC when the load is less.\u00a0 So the rectified voltage may be more like 17 or more volts before the regulator.\u00a0 And that doesn’t include higher voltage when the AC line voltage goes above 120VAC.\u00a0 That can really add to the heat that the 7805 chip has to dissipate.<\/p>\n Crowbar Effect<\/strong> \u00a0 Apparently the overtemp shutdown has sort of a crowbar effect.\u00a0 The chip gets warm and the overtemp starts to shut it down.\u00a0 The voltage across the chip increases<\/em> and the chip gets even hotter, and drives the overtemp into further shutdown.\u00a0 So my experiences with this versatile but somewhat finicky chip is to keep it cool, or else you will be asking for trouble later.\u00a0 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.\u00a0 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.\u00a0 The best thing to do is to replace the LM78XX chip.<\/p>\n I’ve also found that the heat dissipation can be offloaded from the LM78XX chip.\u00a0 One way is to put a resistor between the unregulated supply and the regulator chip.\u00a0 The chip still needs a 0.1uF bypass capacitor on the input.\u00a0 Another way is to put one or more 1N4003 rectifiers in series with the supply; each rectifier will drop about 0.8V.\u00a0\u00a0 Three or four in series will offload a watt or more, and that may be enough to keep the LM78XX from going into overtemp.\u00a0 but the power is till being dissipated in the case, so the case will not be any cooler.\u00a0 It may be necessary to put a heatsink on the chip or some other way to get rid of the heat.<\/p>\n Newer Equipment<\/strong>\u00a0\u00a0 Nowadays, hardly any new piece of equipment uses these LM78XX linear regulator chips.\u00a0 The newer stuff uses a switching regulator that dissipates far less power, and is much ‘greener’ – Energy Star rated.<\/p>\n","protected":false},"excerpt":{"rendered":" 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.\u00a0 I would like to relate some of the experiences I’ve had with these wee beasties. Test Equipment \u00a0 I worked for a company <\/p>\n (Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-1397","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/1397","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1397"}],"version-history":[{"count":15,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/1397\/revisions"}],"predecessor-version":[{"id":1705,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/1397\/revisions\/1705"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1397"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1397"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1397"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}