Hi,
Electronics is a hobby of mine and everything I know is from self learning and experimentation. I mostly like to program mcu's.
I've soldered a new circuit using 2 separate power supplies: 1 from a 5v battery and the other from usb. For each, I use a 1n4148 diode for protection. The cathode of each diode goes to the input of a 3.3v voltage regulator. 100uF filter caps are installed at the input and output pins of the regulator
The annoyance: when I connect the usb cable, the "ON" LED (connected to the output pin of the regulator) starts to blink and the measured voltage at the diode cathode is ~3.5V (the USB voltage at the diode anode is ~4.8V). If I connect the 5v battery, the LED is stable. It really annoyed me so I removed the diode from the usb connection.
Maybe the voltage drop of the diode is too much and should be replaced by a Schottky diode?
The forward voltage drop is specified as 1V for a 1N4148
A schottky diode can be as low as 0.31V
Could make a difference feeding into a 3.3V regulator.
I think I should check the dropout voltage of the regulator because the battery supplies 5.1V the most and the regulator output is a stable 3.26V in this instance. For a difference of only 0.3 in power supply I don't get lower output but unstable and variable.
Sounds like a plan.
xaxxx:
I've soldered a new circuit using 2 separate power supplies: 1 from a 5v battery and the other from usb. For each, I use a 1n4148 diode for protection. The cathode of each diode goes to the input of a 3.3v voltage regulator.
"Protection" ? Whither the anode of each?
A 1N4148 is a small-signal diode, <= 20mA.
My guess is not pushing current the "wrong way" if connecting both a battery and USB.
I think using a Schottky is the right option -- make sure to get a high-current one, in the range of 1A.
Also, I think the 3.3V regulator should be an LDO kind, ideally with a drop of no more than 1.2V at 3.3V/500mA out.
You are correct jwatte and thanks for the advice. I think I will skip the diode since I guess I only needed it if both the battery and USB cable are connected at once; I'll just have to be careful.
USB connectors are all over the place.
Hack a cable and connect the power wires to a battery snap, or alligator clips, or...
Afrotechmods described in a YouTube video a way of having protection with a much lower voltage drop (and lower power dissipation). Basically it is this:
Of course, the input power doesn't have to be 8.4V, just enough to turn on the P-channel MOSFET (use a logic level one I would suggest). In my particular case I was using it to protect my robot car from a LiPo battery pack being connected the wrong way.
Depending on the current drawn, and the RDS(on) of the MOSFET, the voltage drop can be very low (eg. 0.1V).
That is pretty slick! It's a little more expensive than a Schottky, but also performs arbitrarily well. (RDSon can be in the low milliohms with the right device.)
1N4148 forward current 200mA, repetitive forward peak current 450mA. Check your facts before you post.
[those are abs max rating though, but 150mA or so shouldn't ever cause problems, and that's plenty for a lot of applications]
P-channel is nice in this case.
Can also consider that in most cases Vf is Helping to lower voltage into the linear regulator and any 'improvement' just means the linear regulator has to dissipate that much more.
Great stuff guys, thanks.
Nick, the video helped me understand mosfets a lot better, thanks.