# 555 voltage drop

#### GuruMeditation

Joined Jan 9, 2016
9
Hi All,

I've done a search and can't see a specific answer to my query so here goes...

I'm newish to electronics and am experimenting with a 555 timer IC in astable mode and powered by a 9v battery. I'm trying to calculate the ratings of the resistors needed for the LEDs it will be flashing but the output voltage at pin 3 seems wrong. I'm given to believe the voltage drop should be around 1.7v but when I took a reading with my multimeter while it was high, it was showing 8.6v. I then changed out the IC for another and that was showing 8.7v.

Thanks, Tony

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#### sailorjoe

Joined Jun 4, 2013
365
Yeah, it's right because you're using a 9 v battery. Why are you given to believe it should be 1.7 v?

#### Colin55

Joined Aug 27, 2015
519

#### GuruMeditation

Joined Jan 9, 2016
9
Yeah, it's right because you're using a 9 v battery. Why are you given to believe it should be 1.7 v?
Good question! I can't remember exactly where I first read that. Maybe I misunderstood something

#### GuruMeditation

Joined Jan 9, 2016
9

#### Colin55

Joined Aug 27, 2015
519
No. Where on the circuit are you taking the measurement

#### GuruMeditation

Joined Jan 9, 2016
9
Oh sorry! From between pin 3 and ground.

#### Papabravo

Joined Feb 24, 2006
21,306
IIRC the output on pin 3 should swing between Vcc minus a couple of tenths and GND. Sounds normal to me.

#### GuruMeditation

Joined Jan 9, 2016
9
Ok guys, thank you very much for your quick replies! I got a voltage I can work from now

Tony

#### wayneh

Joined Sep 9, 2010
17,504
FWIW, I believe some flavors of the 555 cannot get the output that close to Vcc, and you might have seen the 1.7V value as the max drop off of Vcc, for instance 7.3V output from a 9V source.

#### GuruMeditation

Joined Jan 9, 2016
9
FWIW, I believe some flavors of the 555 cannot get the output that close to Vcc, and you might have seen the 1.7V value as the max drop off of Vcc, for instance 7.3V output from a 9V source.
Very possibly. If I build any other projects using a 555 I'll power it up and check before proceeding with any component calculations.

#### GopherT

Joined Nov 23, 2012
8,009
FWIW, I believe some flavors of the 555 cannot get the output that close to Vcc, and you might have seen the 1.7V value as the max drop off of Vcc, for instance 7.3V output from a 9V source.
Very possibly. If I build any other projects using a 555 I'll power it up and check before proceeding with any component calculations.
The datasheet says that high level output is at least 13V on a 15V supply (lists 13.3 as typical). So there is your 1.7 V. Same for 5v supply. 3V minimum and 3.3V typical.

#### GuruMeditation

Joined Jan 9, 2016
9
Would it make a difference that the one's I'm using are ST & not TI?

#### GopherT

Joined Nov 23, 2012
8,009
Would it make a difference that the one's I'm using are ST & not TI?
Check the datasheet is my typical answer. Next time google manufacurer's name, apart number and the word datasheet.

Looks like it drops off more with more current. It doesn't show voltage with no load (0 amp current on pin 3).

The sT datasheets are terrible in general. They show 200 and 100 mA load data. Goes down to 12.7 at 200 mA load (from 15 v). Otherwise typical values mirror TI part data. It is likely near 15 v at no load.

#### GuruMeditation

Joined Jan 9, 2016
9
Thanks GopherT...same though occurred to me...found the datasheet but not read one before...ummm....

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#### GopherT

Joined Nov 23, 2012
8,009
Thanks GopherT...same though occurred to me...found the datasheet but not read one before...ummm....
Here is what you are looking for (high output voltage.

#### Colin55

Joined Aug 27, 2015
519
Yes, you lose about 3v when driving a load from pin3. The 555 is a very poor chip and we only use it because it costs 8 cents.
You can parallel pin 7 with 3 to gain about 0.8v.

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