(PAUSED): This topic is currently on hold as I source necessary hardware.
I need to connect a Wi-Fi/RS232 module to a low-powered monitoring device, and for maximum portability, I'd like to power the Wi-Fi module – which runs off 3.3v at a peak of 250mA – from the 3.7v/1200mAh Li-Po battery built into the device itself.
To do so, I first need to drop the voltage from 3.7v to 3.3v in a way that can tolerate 250mA. I figure a series resistor should be sufficient, since I'm just running off a battery and not an unregulated power source – but what value resistance should I use?
In addition, the Wi-Fi module's own low power/standby state draws 8mA, which is relatively low but still noticeable, and would drain the battery over time. I've managed to unearth some undocumented info on how to tweak a couple of unused sense inputs that wire straight into the device's CPU (one via a 100Ω series resistor, the other via a 330Ω one) so they work as output lines, and I'd like to use one of these to switch power to the module on/off (possibly via a transistor or something…?). [I need to verify this, but I'm 99% sure the output before the resistors is standard 3.3v TTL.]
Everything needs to fit inside a compartment 50x54mm in size, approximately 4-5mm thick.
My non-SMD soldering skills are passable, but I've never worked with SMD components, so I need non-SMD solutions.
I also need an solution that radiates virtually zero RFI/EMI due to the electromagnetically sensitive medical environment the device will be used in, which won't tolerate even low levels of continuous EMI/RFI radiation particularly well (although short Wi-Fi bursts will be fine).
Best Answer
A series resistor will not provide a good power supply for the WiFi module, as the voltage drop will vary depending on the current consumption of the module, which will vary.
Instead look at voltage regulator chips, "low-dropout" variety. Some are equipped with an enable line and sleep mode.