Hi All,

I am having difficulty with an op amp circuit that I am building.
I am using a LA6500 op amp attached in parallel with a 0.1ohm shunt resistor. See attached circuit design.
This is a 12V DC circuit.
Purpose of amplifier is to power an LED when there is current through the circuit and turn off when there is no current.
Amplifier works fine when there is current through the circuit.
Using the circuit attached. I am using Rf = 68000ohm and Rg as 8200ohm. Voa is coming from the circuit before the load.
When I break the circuit between the load and the shunt resistor(dropping voltage across the shunt resistor to 0) the op amp gives max output voltage.

Any help would be appreciated,

Thanks,

Peter

 

When the load is disconnected from the shunt, the amp input should still be connected to the shunt. If it was, I don't know what is going on.

 

hi,
Are saying when its connected like this edited circuit, it saturates the OPA.?
E

 

Yes, amp input is still connected to the shunt, for some reason Voa is transferring max voltage through op amp to Vo. Do op amps lose function and transmit all supply voltage at null or close to null signal voltages?

 

hi,
Are saying when its connected like this edited circuit, it saturates the OPA.?
E

Yes, correct

 

Do op amps lose function and transmit all supply voltage at null or close to null signal voltages?

No, they do not. In this case, if your op amp is good and if you've got it connected the way you think you've got it connected, the circuit absolutely, positively should do exactly what you wanted it to do.

But wiring errors do occur from time to time, so I'd say check your connections.

 

hi,
The datasheet for that LA6500 is not very helpful.
It should not saturate with a gain set up for ~9

E

 

hi st,
Can you post a photo image of the project, so that we can see the wiring around the LA6500 pins.?
E

 

I have checked my circuit multiple times and have had other people check my circuit as well, please see attached picture and let me know if you see anything wrong,

Thanks,

Peter

 

And you're sure that the other side of the shunt is still connected to the same ground that is used for the opamp?

Try operating the system with the load and then just using a wire to short out the shunt. What happens?

Grasping at straws, but you might move everything to different positions on the solderless breadboard.

 

Yes, op amp ground and shunt ground T in together, I thought it may be a breadboard issue but then why would the op amp work when the circuit has a load?

 

Yes, op amp ground and shunt ground T in together, I thought it may be a breadboard issue but then why would the op amp work when the circuit has a load?

My thoughts, too. As I said, grasping at straws at that point.

 

Hi All,

I am having difficulty with an op amp circuit that I am building.
I am using a LA6500 op amp attached in parallel with a 0.1ohm shunt resistor. See attached circuit design.
This is a 12V DC circuit.
Purpose of amplifier is to power an LED when there is current through the circuit and turn off when there is no current.
Amplifier works fine when there is current through the circuit.
Using the circuit attached. I am using Rf = 68000ohm and Rg as 8200ohm. Voa is coming from the circuit before the load.
When I break the circuit between the load and the shunt resistor(dropping voltage across the shunt resistor to 0) the op amp gives max output voltage.

Any help would be appreciated,

Thanks,

Peter

An important question- how much is your load? What is your load?

 

The opamp circuit is missing an important supply bypass capacitor between pin 5 and pin 3. It keeps the opamp from oscillation which lights the LED. Try 0.1uF with short leads plus 10uF.
It is also missing an RC zobel network from the output to ground as shown on the datasheet.

 

Yes, op amp ground and shunt ground T in together, I thought it may be a breadboard issue but then why would the op amp work when the circuit has a load?

When a shunt is used to monitor current the differential amplifier input connections are made right at the shunt. That may be part of the problem. AND, in the circuit shown, that resister from the other input to ground is certainly the other half of the differential input circuit. ALSO, it is entirely possible that the op-amp is not able to work all the way down to the negative supply terminal voltage. So check the data sheet for the input voltage range. That does matter.

 

The inputs of that opamp are PNP darlingtons like in an LM324 and their common mode input range includes the negative supply which is ground in that circuit.

I think the opamp might be a fake or mis-labeled one bought on ebay.

 

The inputs of that opamp are PNP darlingtons like in an LM324 and their common mode input range includes the negative supply which is ground in that circuit.

I think the opamp might be a fake or mis-labeled one bought on ebay.

The Rg as shown in the circuit drawing should be connected right at the shunt. Also, unless you need high frequency response, which I don't think you do, try a capacitor across Rf. Some op-amps will oscillate at several megahertz and that will cause all kinds of problems that are tricky to solve.

 

The positive saturation can be caused by overdriving the input negatively. This was a common problem with older opamps, and is mostly designed out of newer parts. The ON Semi datasheet "equivalent circuit" does not have enough detail to evaluate.

Audio power am chips and power opamp chips require extra-careful power supply decoupling. Some parts explicitly recommend tantalum caps, although yours does not. Add a 0.1 uF to 1.0 uF ceramic and a 10 uF to 100 uF electrolytic in parallel, as close as possible to the plus and minus power pins, with the shortest possible leads.

ak

 

Geez a 1 amp op amp to drive a LED? I don't think the LA6500 will operate as a DC amp in the single-ended mode.
SG

 

I don't think the LA6500 will operate as a DC amp in the single-ended mode.

Do you have any justification for that statement?