| Electronics > Beginners |
| LM358 in Consant Current Circuit Problem |
| << < (5/9) > >> |
| TheInfernoMan:
So now I redesigned my schematic and followed the simple one. Would this circuit work now ? ??? If, then I would begin solder it to a piece of perfboard. Attached is the new schematic. What was ment with a supply bypass capacitor? (C3 is also attached to the Opamp, is this the bypass ?) Thanks in advance :) |
| Yansi:
No it won't work at all, as you are confusing the inverting and non-inverting inputs of the opamps. Have a thorough look at the orolo's schematic. Also, the buffer amp is absolutely of no use there. |
| TheInfernoMan:
Oh sorry, you are right, obviously. Now I changed the Input. What do you mean with the buffer amp? (Do you mean U1.2? It is also in the simple schematic...) |
| Yansi:
Yes, U1.2. That's a buffer, voltage follower or whatever you call it. It does not help with anything in the circuit, only makes it worse: The noninverting input of U1.1 is already high impedance, why buffering it again? The real opamps have offset voltage. That means a voltage, that is like "added" to the real input voltage. It is usualy a few mV and drifts with temperature. The useless buffer only adds error to the signal it is buffering, as it's output voltage equals the input one, plus the offset and its temperature drift. As you already work with small voltages in the circuits (let's say hundred mV or a volt), the few mV offset voltages both from the buffer and the current regulating opamp U1.1 adds significant errors in the circuits precision. Thats because the offset voltages are becoming significant compared to the operating voltages in the circuit. The useless buffer is lowering the precision of the circuit. That will show up as the current setpoint of your CC load being more temperature dependent and not tracking the setpoint voltage (from the potentiometer) exactly. You can also verify simply with some precision voltmeter: Measure the voltage on the pin 5 to ground, and then the voltage on the shunt. In theory, these should be absolutely the same. But there will be a difference of few mV (say up to 10mV). And this voltage error will be also dependent on the temperature of the opamp. If your shunt is being 0.2 ohms, the 10mV error equals 50mA error in the load current. Also considering that LM358 has a typical offset drift of 7uV/K up to 20uV/K, that means if you raise the temperature of the opamp for 10°C, you add up to 200uV error each amplifier, resulting in 0.4mV error. Your load current will change up to 0.4mV/0.2ohm = 2mA per 10°C temperature change. Might not be of any problem here, but may be a big trouble in other applications, so better being aware of this. (not mentioning we have not counted for many other aspects, like input offset currents) |
| Richard Head:
An NPN bipolar transistor is a better choice in this application. |
| Navigation |
| Message Index |
| Next page |
| Previous page |