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:

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.


Anonymous said...

Do you know of any circuits that abuse 78XX regulators for building HF transmitters? It seems to be possible, but i cannot find any reference, maybe you already experimented with it?

Ricardo - CT2GQV said...

Well, the 78 series I know are voltage regulators. They are used in hf transmitters (low power) but for power supply regulation... I don't think their frequency limit be suitable for HF, maybe as an very strange audio amplifier in push pull with the 79 series also....but don't think it will be good even for that.

Anonymous said...

Thanks Ricardo, I needed a very small AM sender, that can pick up a beep signal from about 100 meter with a simple am receiver. So I googled and read on talking about this.

Thats why I thought of using a voltage regulator for it, and pulse it between 500-1500 khz.

Maybe it would be possible to hack it with an adjustable power switch like the Thanks anyway and good luck with your projects :-)

Ricardo - CT2GQV said...

There are a lot of AM transmitters schematics around without the use of the 78XX series (that I know of), just google around, I am sure you will find something easy to make. Any simple oscillator will do the job for you. If it's just a "beep" you need look for an oscillator with logic dividers... for instance 4Mhz/8 equals's rock solid...and there's a lot of cristal values at those frequencies. Hope it helps.

Anonymous said...

thank you Ricardo for the suggestion