7805 Regulator input voltage?

Question

I recently posted a question about the regulator 7805 (http://www.radioshack.com/product/index.jsp?productId=2062599&cp=&sr=1&kw=regulator&origkw=regulator&parentPage=search) and I was wondering if 6v input would provide a 5v output. The answer I got was no. They said that this regulator operates from 7.5 volts to 20 volts to produce 5 volts. I was wondering, since I can't use 6 volts, if I were to just attach two 6 volts in series to produce 12 volts, would this create an output of 5 volts? If so, about how many amps would it put out as well?

Answer 1

You can avoid those problems (have to use 6V batteries in series) by using a LDO (low drop out) regulator. Try to check LM2940 (http://www.national.com/mpf/LM/LM2940.ht...

It is pin to pin compatible to 7805 and the best thing is you can use 6V battery to get regulated 5V output with 1A max current.

note: 7805 -> 3 terminals +5V regulators are well known to many designers and electronics enthusiasts. But LDOs are newer technology to improve (lower) input and output voltage difference and also help to lower the power dissipation on the regulator itself.

I hope that helps.

Answer 2

yes, you can. Any voltage over 7.5 volts will work.

Amount of current depends on the package, the heat sink, and the voltage drop.

For example, 1 amp with a 12 volt supply means 7 watts, and a small-medium heat sink would be needed.

0.5 amps is 3.5 watts, and a small heat sink would be required.

0.25 amps would probably be ok with no heat sink.

Follow the data sheet for bypass caps.

There is a possibility that you may get a unit with lower than normal Vbias, and at lower currents, it may work with 6 volts. But the spec is 7.5 volts, look at the data sheet.

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Answer 3

Noooooooooooooooooooooo

theres another regulator for 12 v

Answer 4

The 7805 regulator is commonly used to provide a Steady 5V DC supply typically for TTL circuits. It is often used when the supply is AC mains transformed down to about 9 V AC rectified to DC then the 7805 voltage regulator provides a steady 5 volts DC as long as the input is 5V DC or more. I believe your original answer to be incorrect. Build the circuit with a 6V DC supply and try it if it doesn,t work increase the voltage

The problem with AC mains is that it does fluctuate depending on load conditions therefore the reduced AC voltage will also fluctuate so it is best practise to have a higher than required AC  voltage then use a rectifier/voltage regulator.  If you are using 6V DC input from batteries then the 7805 Regulator should work OK. It is good for up to 35 V DC input.

From the tech specs on your own link the max current is 1 Amp

Answer 5

So long as the two 6V sources are completely independant and isolated from each other and from earth ground, hooking them in series to get 12V, then regulating it to 5V will work.  They must be capable of 1A minimum, and be set to close-to-identical limit current (of about 1A or slightly less).

If the power supplies' negative outputs are 'grounded', then you will be shorting out the higher 6V supply when you hook it up to the lower one.

Two 6V batteries will work.

Even if you took your 6V power supply and put a 1.5V D cell in series, it will work.

As for the current, the 7805 is good for about 1 Amp max.

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