I’m a structural engineer, so this is a bit out of my element. I’ve tried to put together my first concept for an Arduino shield, and would appreciate feedback. I’d like to know if it’s “workable” before I try to figure out the board layout in Eagle.
It’s a dual power supply that can be monitored by an attached Arduino board. That board would then communicate status to the main project board. I have two Max1555’s connected to batteries, USB (for main charging) and two solar panels (3.6V, 210ma max). I followed the “mightymintyboost” concept. I tap off the battery voltage to two analog input pins on the Arduino.
3.7V Lipo battery power then feed two PNP transistors. If I have those hooked up right, they should conduct when the base is LOW, so they’ll automatically both be on. Then I can use a digital pin on the Arduino to turn one of them off. I could “exercise” the batteries that way, or individually try to monitor their amp-hours etc.
Finally, the combined power supply, now at about 3.3V, goes into two parallel LT1302 voltage boosters. I’m uncertain how those will work in parallel, but they’re 8-dip packages so I’m more comfortable with them. Their output is then 5V, and I’m hoping to be able to get 1A total out of it.
jwells:
… would appreciate feedback … LT1302 voltage boosters. I’m uncertain how those will work in parallel …
Looks pretty good.
As far as I can tell from reading the data sheets, the LT1302 regulators should properly current share when they are in parallel, two of them supporting a load of more than 1 A.
Forgive me for pointing out a few things that I noticed:
The MAX1555 datasheet recommends a 100 KOhm pull-up on pin 3, but I don’t see it on the schematic. (Is there a pull-up on the Arduino?)
The schematic seems to directly connect “7.2 V” solar cells to the MAX1555, but the MAX1555 datasheet seems to say its highest working voltage is 7.0 V. Would slapping a $0.05 worth of 7.0 V or 6.8 V Zener diode across each solar cell be excessive counter-productive paranoia?
I suspect that in one special case – solar cells disconnected, both batteries are connected and drained, the left USB “peripheral socket” connected into a computer USB host port – that some computer motherboards may think this device is pulling “too much” power, and turn off power to that USB port.
In one special case – at least least one solar cell connected, and its corresponding battery unplugged, and the Arduino selects that side to power the voltage boost – it seems possible for “7.2 V” to make it all the way to the “5 V” output. Would adding a few components to guarantee the output is no more than 5 V be excessive counter-productive paranoia?
Are the USB data lines on the left USB “peripheral socket” supposed to be pulled-up or pulled down?
Are the USB data lines on the right USB “host charger socket” supposed to be pulled-up or pulled down?
The schematic says merely “LT1302”, but its feedback is connected in a way that only works with the “LT1302-5”; it doesn’t have the 2 resistors needed for the “LT1302”. (one of my pet peeves: manufacturers who sell 2 similar things, where the full name of one thing is a substring of the full name of the other thing).
Forgive you? Wow! Thanks for taking the time to look through all this. Those are very educational comments. I appreciate it.
I think I’ve decided to use a MAX1703 boost IC instead of the two LT1302’s. That’s a surface-mount chip, but since the MAX1555 also is surface-mount, I may as well go for it. I’ve also discovered that I should use pull-down resistors on the base of the two transistor switches.
I’ve got (6) solar cells per battery. Each is 3.6V and 70 mA. I’m thinking about two sets in series of three in parallel, meaning I’d get a maximum of 7.2 open circuit volts and 210 mA. I need to put them together to see what I get in reality. I’m estimating it’s half of that. But your advice about protection is great.
Again, thanks for the insight on so many issues that I would have not considered.