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  #21  
Old 12-29-2013, 04:24 AM
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I had simpler thing on my LED strobelight for bike - LiPo battery with 6 or 8V (dont remember, only remember it was higher than 5v) and LM358-based charger. PSU and battery were connected by diodes, to avoid current go wrong direction. Not efficient way, but it worked, and eliminates boost, as that one can mess with your sensetive thermocouple measurements
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  #22  
Old 12-30-2013, 06:12 AM
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All opamp based chargers i found was only constant current sources and that will not charge battery completely only to ~80% capacity But i think thats not problem here

So here is my idea not complete schematic:



two battery cells in series thats something around 8V
IC1A-current source(R1 sensing resistor)
IC1B-comparator with hysteresis to turn off charging when is battery full(it should close Q1 when it open Q2)
IC1C-comparator to turn battery off when is battery empty
D1,D2- iam not completely sure about D2 place is it ok? I placed it on input side because dont want voltage drop on output and they used it in datasheet too(fig 37)
Power-probably need to use separate opamps and power Opamp A,B from input and opamp C from regulator output

So idea is right or there is some fail?
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  #23  
Old 01-04-2014, 03:13 PM
RULE RULE is offline
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RRaimbo i've a couple of question that are connected to one of my next project and at your job too.

Why you chose MAX31855 IC for thermocouples, instead of use the ADC imput on the PIC microcontroller?

It's a bad ideaa read the voltage through the ADC PIC?
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  #24  
Old 01-04-2014, 08:09 PM
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thermocouples are not that easy to hook up to mcu

They are based on seebeck efect so change 1C results in ~40μV difference. You must amplify that to your ADC voltage resolution.
But that way you only get temp difference between hot and cold junction so next you need another temp sensor next to thermocouple connector (cold junction) to get absolute temperature.
Measuring negative temps get even more complicated you must add some offset to get rid off negative voltage and second problem is that linearity is gone.



So i would need precise opamp, high resolution adc and some temp sensor and that will cost more money,place on pcb and time than max31855.


Quote:
It's a bad ideaa read the voltage through the ADC PIC?
If resolution and sampling rate is enough for your application why not?
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  #25  
Old 01-12-2014, 03:24 PM
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Update

-Meterboard
-pcb routed need just update some small details


-Powerboard
-schematic


-orientational BOM

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  #26  
Old 02-21-2014, 11:00 AM
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Another update

-Meterboard

-upgraded ADC to external 16bit MCP3425
-mcu downgraded to 18F14K22
-BOM changed about ~+$1 but i expect that better accuracy will worth it









-Powerboard
-routed pcb hope its ok i did it first time
-still need to do proper silkscreen









-Case
-dark plexi with engraved C and V from inside that will be backlighted by led
-on top room to write note with white marker
-to hold more parts together some kind of clip that will hold two standoffs together
-i should learn how to use sketchup properly





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  #27  
Old 02-23-2014, 05:05 AM
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Interested in a temp monitor if/when they make it to market!
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  #28  
Old 05-22-2014, 01:36 AM
motek-18 motek-18 is offline
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Very cool and Good idea!!!
You sold?
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  #29  
Old 05-22-2014, 05:16 AM
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Sorry not finnished I dont like powering part. Need to rethink that. Not sure when i get to it again working on new stuff now.
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