[C]hosen battery, it appears to be a 12v one. How, then, does your 9v source charge it? Or have you gone for a lower-voltage battery?
The 3V3 supply is generated by the TL431 shunt regulator (good spot of the labelling too, I'll fix that), and is also used as a stable reference for the "battery low" condition. I chose the shunt, rather than a linear regulator of voltage reference, as it's much more flexible if people want to use different batteries, for example, and all it takes is a change of resistors. The TL431 is incredibly cheap for its accuracy, and also very widely available unlike some niche regulators and references. The dropout voltage is inherently low as well, limited only by the current source (R1, in this case), which is a potential issue on battery power (not with an SLA, but I wanted the design to be as flexible as possible). The 3V3 is only drawing 10-15 mA (dependent on voltage), and that's something that can be optimised as a well if you're massively concerned about draw. My thinking was that the RPi draws at least 300 mA, so it's not a bit extra draw, especially with a large lead acid battery. Finally, it's inherently short circuit proof (although R1 may disagree with you!) and so is more robust, especially when not used with an RPi with fixed IO configurations.But to power the EEPROM you're using a shunt regulator, which is even less efficient than a normal linear regulator - is there a reason for this?
Good spot, thanks! IC3 is a TL431, so Vref = 2.495 V. This is set to stop the charge for an SLA at 25-30 C, again something I would like the end user to be able to change if they're using it at temperatures other than ambient.Or should the 910R and the LED be swapped?
Good suggestion - I hadn't thought of that, that would certainly allow a few extra mA to be trimmed off the shunt if required. Thanksmahjongg wrote:I get why you use the shunt regulator for a precision 3v3, but why power the eeprom with the precision voltage too, instead of the normal 3v3 voltage from the PI?
Thanks for the interesting step through of your design on your blog! It was enlightening, and also seems well-suited for my use case. Any news from the testing front?awjlogan wrote:Ok, the final part regarding the circuit design is now here Questions and suggestions welcome!
I've ordered some boards, so next up will be the build and testing.
Well, the boards arrived yesterday, so good timing! Very glad you liked the blog posts, hopefully I'll be able to get these made up in the next week and let you know!chmrr wrote:
Thanks for the interesting step through of your design on your blog! It was enlightening, and also seems well-suited for my use case. Any news from the testing front?
Oooh, shiny!awjlogan wrote:Well, the boards arrived yesterday, so good timing!
Heh, it's a trick of the light, honest Probably a decent 9 V supply, such as this one. That's pretty heavy duty, so up to the user really. The trickle charger is only 50 mA, so you can cut back to a 2 A supply without penalty.
By the way, what were you planning on using to provide the 9V supply from the mains?
What's the source of the 3A number? Even the B+ Pi only draws 2A max, right?awjlogan wrote: After going over the boards again, actually the part I'm most concerned about is the diode OR. At the full 3 A, that's going to be burning 1.8 W, which is probably a bit much for those little surface mount diodes, but will check when I finalise the parts and BOM>
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