Hello, I'm designing a simple electronic load and here are my specs:
0 to 5 amps continuous (accuracy of 1mA for at least 0 to 4A range, 10mA is acceptable for the 4 to 5 A range)
0 to 30 volts (accuracy of 5mV for 0 to 5 volts range, 10mV for 5 to 30 volt is acceptable)
Maximum power is gonna be capped to 100 watt as I will be using a CPU cooler which are usually rated for 95 watts but can handle 100 watts easily.
I am at a predicament here: on one hand, I found this bad boy which is a 1% initial accuracy (can be calibrated in software to basically get close to 0 error), 30 watt 1\$\Omega\$ resistor:
https://www.mouser.co.uk/ProductDetail/Bourns/PWR221T-30-1R00F?qs=sGAEpiMZZMtlubZbdhIBIPNKsuXHpjQqm7P7fPg44Qo%3DThis would do just fine and if I use 12 bit DACs and ADCs with a 4.096V voltage reference, I can get the desired accuracy and use a x2 gain amplifier for the 4 to 5 A range and a similar method to divide by 10 to do the voltage monitoring (this would enable me to do upto 40 volts as well).
There's also the added benefit of the resistor carrying the majority of the load and the mosfet can run a lot cooler. but this means that I need to add another mosfet as I plan to have the ability to "short" the input as well. The resistor would also get really toasty at about 105 degrees at 5A load due to it's thermal resistance. I could use 2 , 2\$\Omega\$ resistors in parallel to dissipate the heat more evenly but it would increase the cost.12 bit DACs and ADCs are also quite expensive and I plan to sell this unit.
on the other hand I can drop the value of the resistor to 10m\$\Omega\$ and use divide by 100 op amps or even more (to use the 8 bit adc of the microcontroller itself),but then I need to use precision op amps and a split supply to get the full desired range with the desired accuracy which is gonna drive up the cost again and I'm also not really sure about the gain errors of a /100 or larger op amp ... it can turn out to be pretty horrific and inaccurate. The mosfet also needs to be beefed up considerably which can ruin all the gains in other areas.I also probably need to use a negative rail for op amps anyways as I'm not sure about their minimum voltage (even on precision op amps) and it's gonna be bad if the load can't go below lets say 50mA due to that offset.
It's really frustrating and I'm really lost as what approach to take to keep reasonable margins for selling it, as well as keeping it at the 40 to 50 GBP selling price. As this is the first time for me trying to make a commercial product, I don't know how much interest there is for a such device with the mentioned ratings. It would be fairly accurate and reasonably high power, but it would be a mid range product which is probably going to be avoided by the hobbyist and professionals at the same time. If you have specific part recommendations for the ADC, DAC and Vref, I'm open to suggestions. I would also love to gauge the interest in the device so feel free to comment about it. (It will have constant power, constant voltage, constant resistance, transient mode, internal resistance testing, data logging, automatic battery type detection (alkaline,lipo,NiMH, etc) and possibly even wifi or usb functionality to be used on a PC (I'm making a decent user interface using java. it won't be a basic useless app as I've studied computing science and know how frustrating a poorly designed interface is). It could also be used as a fairly accurate ammeter in the "short" input mode and a decent voltmeter as well). All in all, I think it has a really nice set of features which are mostly achieved in the software so they won't add any cost to the basic electronic load. I'm thinking about having different models with optional parts that might cost more, so people can choose if they want that high of accuracy, data logging capability, wifi / usb interface to pc which are obviously gonna cost more.
Thanks for your time and looking forward to your replies