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### Topic: op amp question (Read 4246 times)previous topic - next topic

#### jonisonvespaa

##### Feb 20, 2013, 05:45 pm
hi,
ive got a simple exercise im going to try, i need to control 2, 0  to 10v's using just 1 10k pot just wondering if my circuit will work i intend using a quad lm324, also i was just wondering if i could control all 4 op amps using just 1 10k pot? (bottom circuit same as top sorry)

thanks

#### afremont

#1
##### Feb 20, 2013, 06:49 pm
I'm thinking that will work ok, but what is your supply voltage?  Hope it's at least 12V.
Experience, it's what you get when you were expecting something else.

#### retrolefty

#2
##### Feb 20, 2013, 06:51 pm

I'm thinking that will work ok, but what is your supply voltage?  Hope it's at least 12V.

Yea, and probably won't output close to 0vdc without using a negative supply rail also.

Lefty

#### jonisonvespaa

#3
##### Feb 20, 2013, 07:04 pm
yes my supply will be 12vdc, why wont it go to 0v lefty?  whats the lowest v it will output?
thanks

#4
##### Feb 20, 2013, 07:19 pm
See page 4 - see the PNP transistor that connects the output to Gnd? The closest you will is the Vce across that transistor.
Could 3+mV depending on your operating conditions, multiplied by the gain of 2 you have set up (I believe Gain = 1 + R1/R2 for the noninverting amp as drawn).
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### retrolefty

#5
##### Feb 20, 2013, 07:20 pm

yes my supply will be 12vdc, why wont it go to 0v lefty?  whats the lowest v it will output?
thanks

You would have to look at the datasheet for the opamp for the value. Unless it's a 'rail-to-rail' opamp it's output voltage can not reach down all the way to zero voltage rail unless that rail is a negative voltage.

Lefty

#### Docedison

#6
##### Feb 20, 2013, 08:20 pmLast Edit: Feb 20, 2013, 08:25 pm by Docedison Reason: 1
An LM324 will go to 1.25 volts of either rail. If you require the posted schematic to go from 0 to 10V, try an LMC662 (Dual package) It has R/R Input (+/- .3V of supply rails) and Output. There is an LMC6684AIM which is a 12V part that has guarantee'd specs @ 10V. It "can" be accomplished with a '324 but it will require a +15 -5V supply to get the 0 and 10 v limits with a little margin.

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### jonisonvespaa

#7
##### Feb 20, 2013, 08:57 pm
LMC662 looks perfect for what im after thanks all very much for the help

#### dc42

#8
##### Feb 20, 2013, 09:08 pm
Whether it will work with an LM324 depends on the load. If the load is resistive between the output and ground, so that the LM324 doesn't need to sink any current, it should be OK down to 0V output.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

#### afremont

#9
##### Feb 20, 2013, 10:35 pm

I'm thinking that will work ok, but what is your supply voltage?  Hope it's at least 12V.

Yea, and probably won't output close to 0vdc without using a negative supply rail also.

Lefty

The inputs can go to -.3V according to the datasheet, it needs a couple of volts headroom on top though.  That's why I mentioned 12V.  What datasheet spec will tell me how close the output can get to 0V with a single supply?  Another poster mentioned 1.25V, that seems a little high to me.
Experience, it's what you get when you were expecting something else.

#### dc42

#10
##### Feb 20, 2013, 11:22 pm

What datasheet spec will tell me how close the output can get to 0V with a single supply?  Another poster mentioned 1.25V, that seems a little high to me.

Like I already said, the output can go right down to 0V provided you don't ask it to sink any current. That's why the first page of the datasheet http://www.ti.com/lit/ds/symlink/lm124-n.pdf says "Large output voltage swing 0V to V+ ? 1.5V". If you do ask it to sink current, you'll find the maximum sink current quoted at 2V and 0.2V output voltage on page 4.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

#### jonisonvespaa

#11
##### Feb 22, 2013, 06:38 pm
hi,
been looking through data sheets what figure am i looking for in terms of the max rail, and min rail voltages in op amps?
thanks

#### jonisonvespaa

#12
##### Feb 22, 2013, 06:39 pm

hi,
been looking through data sheets what figure am i looking for in terms of the max rail, and min rail voltages in op amps go to?
thanks

#### afremont

#13
##### Feb 22, 2013, 06:59 pm
Make your life easier and just use rail-to-rail op-amps.  The older parts have funky rules when it comes to getting close to the supply rails.  You'll see the rail-to-rail terminology in the datasheet when it applies.  Then you just need to look at supply voltage and input voltage numbers then.  I have a bunch of Microchip MCP6021 and MCP6022 op-amps in my tinker box.  They have a maximum operating voltage of 5V though.  The MCP6023 has a /CS line for power saving.
Experience, it's what you get when you were expecting something else.

#### fm

#14
##### Feb 22, 2013, 08:42 pm
You should remove the caps on the output of the op-amps too. These may cause oscillation problems making the output unstable.

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