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Not sure if current opamp module has enough gain #1
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Thanks for the feedback. This is a preliminary experimental design and hasn't actually been tested, It's probably worthwhile to consider increasing the current sense resistor Currents of 16A or so are not unusual in some high-power domestic Basically, as you understand, the offset and gain in both the voltage and There is also an issue with the capacitive-dropper transformerless power On Mon, May 27, 2013 at 11:14 AM, Roger Clark notifications@github.comwrote:
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Hi Luke, OK. I hadn't considered 110V e.g US and possibly Japan (which I think can be even lower) I'm actually going to build the current detector and have bought some 10 milli Ohm resistors (from Jaycar). So I was going to use different values for the feedback side of the opamp - probably 27k for the voltage divider and 180k for the negative feedback resistor. I'd also considered the transformerless PSU, as there are issues with using this with the new EU directive on standby power, which now needs to be max 1W and most transformeless units take 2W or 3W due to their inefficiency. Hence for my tests I'm going to use the internals of a 5V USB switch mode charger. BTW. I'm not using an XBee either, I'm using a RFM12B at the moment but I'm hoping to switch to Bluetooth 4.0 Low Energy as soon as the open source designs of PCB with antenna become available. FYI. I'm well aware of the dangers of working with 240V and switch mode PSU's etc ;-) |
Great, sounds really cool, let me know how you go :) |
Hi Luke, I thought I'd give you an update on whether the circuit worked. Unfortunately, I couldn't get reliable readings for current, from the Arduino. At the moment I've had to stop because I don't have any way to work out why this is occurring. I strongly suspect that its a problem of noise, being amplified by the opamp. This could be supply noise from my switching 5v (USB) PSU, that I modified to output 3.3V, or possibly mains borne noise. I'll need to get hold of a fairly substantial isolating transformer so that I can connect my scope to the inputs to the ATMega to see whats happening, but don't have the $ spare to spend on this project at the moment. |
Hi Luke,
I'm not an expert, but I modeled the current amplifier in LTSpice and it doesn't appear to have enough gain.
You are using a 0.005 Ohm shunt resistor, so at 10A this will give you 0.05V AC on the SHUNT input to that module.
The feedback 13k / 22k loop appears to give a gain of approx 2, hence the output is 0.1V AC offset to 1.65V
But you appear to need around 1.5V AC offset at 1.65V to get decent resolution on the ADC in the micro-controller.
You appear to need to replace the 13k with something more like 330K to get enough gain.
Also. In LTSpice, the input cap C2 appears to be a bit too low to get full transfer of the voltage across the shunt.
It looks like 330n would be a more appropriate value.
And... I think you need to put something in to prevent the input of the opamp being destroyed by power spikes. (I've blown several opto isolators on mains circuits due to spikes with other equipment turning on and off).
Adding a resistor on the input to C2 may be enough (1k seems to be OK).
Or you could also have 2 back to back diodes after the resistor.
But, as I've said, I'm not an expert, so I may be talking rubbish
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