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[SOLVED] Op amp based open loop ckt not working with load (LM324)

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Y.li

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Hello,

I am making an op-amp ckt (LM 324 with 5v single supply). I am facing some problems in that. Pls. see if anyone can help.

The op-amp is connected in open loop which gets input at its non inverting terminal. When there is no input, there is an bias voltage of 0.3 v. To get zero output at this condition, the inverting terminal is biased with 0.5 v.

The ckt works ok in the unloaded condition. But when I connect a load to the o/p of this op-amp the ckt does not give the required output.

The load is an LED with 10K current limiting resistor and an i/p of a AND gate (with a10k grounding resitor) in parallel. When the non inverting pin is low, the output of the op amp should be low and the LED & AND gate o/p should be low. But that is not the case. Both the outputs remain high.

When I checked the voltage at the inverting and non inverting pin, both of them shift to 0.5 v (where as the voltage from the previous stage, connected to the non inverting terminal is zero). This is true if I change the bias voltage to any other voltage also; eg. 0.6 v.

I tried by inserting an op-amp buffer between the load and the output of op-amp. That didnt work.

Thanks,
Y. Li
 

barry

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Can you post a schematic. It's not totally clear how this is configured. What is driving your non-inverting input? Also, running the opamp with no feedback is not a good idea.
 

Y.li

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The non inverting input is driven by an op amp with -ve feedback. This op amp receives input from a current sensor. When there is output from the sensor, this op amp amplifies it, passes it to a RC filter which smoothens out the wave form. This smoothened waveform is given to the non inverting input.

The open loop op amp is used as a comparator. When there is input from the RC filter, it gives high output, when there is no input from RC filter (ie no input from the current sensor), it is to give 0 output. As there is a 0.3v offset voltage at the non inverting terminal (when there is no input from the RC filter), a 0.5v offset is given to the inverting terminal.
 

barry

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When you say 'when I connect a load to the o/p of this op-amp the ckt does not give the required output.' what is your "required" output? You've got a 5 volt opamp, which will give a nominal maximum output of 3.5V. Then you've got an LED which has a nominal forward voltage of 2V in series with a 10K resistor. This means you can expect about 150 microamps to flow through your LED. If you are expecting to see the LED illuminate, you better change your expectations, that is nowhere near enough current to get that LED to glow.
 

Y.li

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Thanks Barry ! I will try by reducing the LED resistance. BTW, when the output of the Op amp is expected to be high, the LEDs are glowing mildly (thats why I will try by reducing the LED resistance values). But when the op amp output is expected to be low (when the sensor o/p is low --> input op amp output is low --> RC filter o/p is low --> which drives the output opamp), the LEDs still glow (mildly) ! But then, this is worth a try. :)

Y .Li
 

barry

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How is the LED connected to the output, to ground or to the supply? If it's connected to the supply, you might be having a problem where you're not actually ever turning it off, since the opamp can't drive to the positive rail.
 

Y.li

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The LED is connected to gnd (output of op-amp to 10K res--> to positive terminal of led--> led -ive terminal to gnd)
 

barry

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Besides changing the 10K resistor, have you verified that the output is switching properly?
 

Y.li

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Yes, the o/p is switching properly... Anyways the problem was resolved .... The supply regulator was giving a lower than 5v (3.2v) ! changed it... things started working...

Thanks
Y. Li
 

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