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Topic: Testing a Voltage Regulator (Read 1 time) previous topic - next topic

marian42

Hi,

I want to test a volatage regulator.

I applied a voltage betweet Vin and GND and measured between Vout and GND. I measured no voltage and the regulator got hot.
What did I do wrong?

How can I regulate 7.2V to 5V?

bobthebanana

You bought a negative voltage regulator. Look for a 7805 instead of the 7905

lax123

Quote
How can I regulate 7.2V to 5V?

why would you do that with a Negative voltage regulator when it says in the datasheet: DC input voltage: - 5 to - 18 V, and it already told you its a negative one in the second line of the datasheet?

7805 should work, it has a voltage dropout of 2V, so if u get below 7V at the input it wont be able to regulate to 5V.

marian42

#3
Feb 06, 2013, 07:51 pm Last Edit: Feb 06, 2013, 07:57 pm by marian42 Reason: 1
7805 should work, it has a voltage dropout of 2V, so if u get below 7V at the input it wont be able to regulate to 5V.

Are there regulators with a smaller dropout?
The 7.2V comes out of a battery pack. It will probably drop when it is almost discharged. Can I use the 7805 anyway?

If the regulator is correct, can I just connect the Vin and Vout to it or do I need a further circuit like this:

KeithRB

The capacitors are there to prvent oscillations and smooth out fluctuations, use them. If you want to input less than 7 V, look up low dropout "LDO" regulators.

marian42

Will an LDO regulator be necessary for a 7.2V battery? What happens if the 7805 is supplied less than 7V?

KeithRB

It will stop regulating at some point, and the output voltage will go down.

marian42

So it will output less than 5V if the input is for example 6V?

Could I use an analog input of a microcontroller to find out if this point is reached?

KeithRB

Only if you use an independent voltage reference, if the reference is relative to the supply you will never see any change!

marian42

So how can I read the battery level with a microcontroller?

KeithRB

http://arduino.cc/en/Reference/AnalogReference

marian42

So I guess I need to use an internal reference.
But how can I get a 0-8V to 0-1.1V? Would it make sense to analog read a voltage divider with 10k and 2k? I think this would scale it down to about 1.3V
By the way, will this Arduino function also work on an ATtiny?

lax123

you can do it with a voltage divider.

If your max is 7,2 why not use 7,5 instead of 8V for calculating the divider, increases accuracy.
And i think you should bring it down to 1.1V and not 1.3V. Your reference will be mapped as 0-1.1V/1024 for analog read.


About ATtiny, this looks promising:
http://hlt.media.mit.edu/?p=1695
Havent tried but would surely be nice and save a lot of money for simple projects

marian42

I just measured 7.6V at the battery and its not even completely charged.

Programmimg the ATtiny is no problem, I was just wondering if the AnalogReference works the same on it.

teddyz

If you have the voltage divider connected, it will of course drain the battery a bit. A total resistance of 50 kOhm is down to 0,15mA.

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