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Topic: Testing a Voltage Regulator (Read 2131 times)previous topic - next topic

marian42

Feb 06, 2013, 07:14 pm
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

#1
Feb 06, 2013, 07:28 pm
You bought a negative voltage regulator. Look for a 7805 instead of the 7905

lax123

#2
Feb 06, 2013, 07:38 pm
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 pmLast 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

#4
Feb 06, 2013, 08:12 pm
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

#5
Feb 06, 2013, 08:19 pm
Will an LDO regulator be necessary for a 7.2V battery? What happens if the 7805 is supplied less than 7V?

KeithRB

#6
Feb 06, 2013, 08:24 pm
It will stop regulating at some point, and the output voltage will go down.

marian42

#7
Feb 06, 2013, 08:28 pm
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

#8
Feb 06, 2013, 08:39 pm
Only if you use an independent voltage reference, if the reference is relative to the supply you will never see any change!

marian42

#9
Feb 06, 2013, 08:42 pm
So how can I read the battery level with a microcontroller?

KeithRB

#10
Feb 06, 2013, 09:05 pm
http://arduino.cc/en/Reference/AnalogReference

marian42

#11
Feb 07, 2013, 12:12 am
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

#12
Feb 07, 2013, 10:15 am
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.

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

marian42

#13
Feb 07, 2013, 01:38 pm
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

#14
Feb 07, 2013, 11:37 pm
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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