Op amp help

Using Arduino General Electronics
#1

Hello, I’m trying to use an op amp in my project and I’ve tried using an LM324, and a few others and they just dont seem to work for me. Obviously I’m doing something wrong, but its not obvious to me. I’ve looked at plenty of implemented circuits and watched a bunch of youtube videos explaining them, but for me to apply it, it aint working.

As a starting point, I just want to hook it up and watch it work.

I’ve hooked up a TL082 op amp, I have a voltage divider with a potentiometer as my input voltage (so I can watch the gain in action), and I have R2 set at 1k ohms and R1 set at 8.2k ohms, which if Im doing my match right, should give me a gain of ~9.

Attached is a picture of my circuit in use. No matter what I set the potentiometer to, I still am getting a constant reading of 4v. Even if I just straight up hook the Non-inverting input to ground, still gives me 4v.

I would greatly appreciate any assitance in helping me getting such a simple circuit to work. I’ve been banging my head for a while now.

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#2

Are you using a split ( +- )power supply?

#3

No, just 5v in the V+ and 0v on the V-

#4

I think you will need to bias the amp TL082 for a single ended supply. http://www.ti.com/lit/an/sboa059/sboa059.pdf You could try a Norton amp like the LM3900 for single ended supplies. http://www.ti.com/lit/ds/slos059/slos059.pdf

#5

Or you could use one that was designed specifically for 5V systems:

LT1215

The LT1215/LT1216 will operate on any supply greater than 2.5V and less than 36V total. These amplifiers are specified on single 3.3V, single 5V and ±15V supplies, and only require 5mA of quiescent supply current per amplifier. The inputs can be driven beyond the supplies without damage or phase reversal of the output. The minimum output drive is 30mA, ideal for driving low impedance loads.

Post a photo of a hand drawn schematic of the circuit you built.

#6

LT1215 looks good LM3900 is ~45 cents though.

#7

LT1215 looks good LM3900 is ~45 cents though.

My first boss always used to tell me "There's no substitute for quality, Bob..."

#8

Hi, Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png or pdf?

You can use op-amps with single supply, you just have to be aware that you will not get any negative outputs.

Tom..... :)

#9

It looks like a non-inverting circuit with a gain of two (two 10k resistors?).

The TL082 hasn't a chance in hell of working, it will not work within several volts of either rail, its designed for +/-15V rails.

The LM324 should work from about 0V to 3V, so you should see something with that. Best to find a true rail-to-rail opamp designed for low voltage though. That means beyond rail-to-rail on the inputs and rail to rail on the outputs.

No opamp can take the output quite to the rail (and you must load it very lightly to get close).

#10

What output voltage are you looking for? an opamp can't amplify beyond the voltage you give it (and then also minus a voltage drop) So if you're giving your opamp 5V to work with, getting up to 4V on the output sounds pretty correct :). If you want to amplify a signal to say 20V, you need to connect the opamp to a 21V supply :).

#11

MarkT:
Best to find a true rail-to-rail opamp designed for low voltage though.

No opamp can take the output quite to the rail (and you must load it very lightly
to get close).

near rail-to-rail
the closer you get to a real rail to rail the more expensive,

#12

The TL082 minimum Supply Voltage is 7V.

#13

You must, I repeat MUST properly terminate any unused Op Amp sections. If you don't, the unused section has floating inputs and it may go into oscillation, or it may periodically go into oscillation, or go into a latchup mode that prevents the other sections from working properly.

Do NOT simply tie the inputs to ground. That still leaves the output in an undetermined state.

Do this:

More in-depth information about this: http://www.maximintegrated.com/en/app-notes/index.mvp/id/1957

#14

I am successfully using some LMC6484 op amps. These are rail to rail units and have guaranteed operation at 3, 5 and 15V. The data sheets are readily available online.

#15

Hi,

Referring back to your first pictured circuit, you need to put 1K in the wire going from the pot wiper to the op-amp.

Op-Amps do their subtracting using currents, the theory referred to is Virtual Earths, you need the extra resistor so the the virtual earth can be maintained.

Tom… :slight_smile:

When you are talking about a circuit can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png or pdf?

#16

steve_beek: I am successfully using some LMC6484 op amps. These are rail to rail units and have guaranteed operation at 3, 5 and 15V. The data sheets are readily available online.

I'll second that suggestion; the LMC6484 quad, and the LMC6482 dual, are good general-purpose "jelly-bean" opamps. They're inexpensive and feature rail-to-rail inputs and outputs, good input characteristics, decent bandwidth (pretty much like an LM324), and don't draw a lot of supply current. They're excellent for low-voltage, single-supply designs. They're CMOS, though, so keep in mind that they're somewhat susceptible to ESD, and they can't take more than about 16 volts total supply voltage or they go "pop!" and emit smoke.

Another good opamp, especially for low-voltage work, is Linear Technology's LT1490A (http://www.linear.com/product/LT1490A). It's a dual, bipolar (i.e., not CMOS), micropower opamp that will work off power supplies from 2 volts to 44 volts (+/- 1V to +/- 22V) and uses only 40 microamps supply current per amplifier, making it great for battery-powered gadgets. It's somewhat expensive, though; Digi-Key unit price is $3.77 US.

As BillHo mentioned, the TL082 simply won't do the job you need it to do, being an "old-style" opamp designed to operate off +/- 15V supplies (+/- 5V minimum). Also, neither the inputs nor the output will go rail-to-rail.

#17