Friday, May 28, 2010
78XX on reverse polarity
78XX and 79XX series voltage regulators are one of the most used components in small electronics equipment....the are, like the Titanic, and acording to the datasheet "...essentially indestructible..." except in the case of reverese polarity in the input or an Iceberg in the path for the Titanic case...
I think I found this for the first time this week, normaly I don't place polarity protection in my circuits because "I know what I am doing" but decided to test the "indestructible" assumption this week, I was in the shack for a small amount of time yesterday and connected the vhf receiver for a quick test, I saw that I was connecting power the wrong way but decided to see what appens.... well... at least one of the 78XX (LM7805) went dead, I just hope the MC3361, powered by it, survived. The 7809 connecting the LM386, audio amp, now has an output of 3v... go figure... will make some more tests when time allows.
Meanwhile, remember, look always at the datasheets and don't try anything that's not there!
Wikipedia (!?) entry on 78XX: http://en.wikipedia.org/wiki/78xx
By the way, it's easy to an rf signal bypass the 78XX series regulators so it's advisable to place some decoupling capacitors at the input and output. If that's not enough, change power "signal" routing and place some rf chokes in series with the power line.