my new project

[shandy27]

After being on here for a couple of years, I thought I would share my thoughts for a new project I am going to start working on, AT the minute it is just in the theory stage so I thought I would contribute from start to hopefully finished mod.  The heart and soul of the mod is going to be a boost board based on a linear technology controller. The board will handle a maximum current of 10 amps and voltage will be 6 volts max, there will be lcd display showing watts, resistance, voltage and current eventually as I learn more about coding I will program it to regulate wattage, I am off course assuming that I will be able to do that with software.  The power will be coming from two 5000mah batteries  in parallel with a rating of 40c.  At the minute I am currently looking for software that I can simulate the all the circuits before building them.  I am steering towards a pic controller but I have an arduino that I could use. Hopefully I will be able to give you regular updates.
bye for now

[doobedoobedo]

Its going to be big then. 2 x 5000 mAh LiPos - last all week and then some.

[shandy27]

thats if I ever grt past the theory, this is one of my favourite parts, doing all the maths and planning everything.  My thinking was if I use the 10000mah battery pack rated at 40c then hopefully the boost board won't stress the batteries  to much.

[doobedoobedo]

You won't need 40C cells. For 10000mAh 10A is 1C. Even at full power it would last practically forever .

[shandy27]

I could  go for the 20c lipos then and that would bring the size down a bit and still give me plenty of power. 

[doobedoobedo]

Yes that would still give you plenty of headroom and a very flat discharge curve.

[Tameron]

Sounds interesting. I will be watching out for updates 

[shandy27]

I changed my mind with the board it is now going to be a stepdown cc/cv power supply, it will running 4 sony vtc4 batteries in a 2s2p configuration. I have now decided to use an arduino as I have an uno board, so it saves having to buy a new development board.  my thinking at the minutr is I'm going to use the arduino to measure voltage, current and resistance and displayit on a 16x2  lcd, voltage will be controlled by a digital potentiometer wired to the arduino, I was hoping to include a menu to choose either variable voltage or wattage, I know it is possible is calculate voltage and current using resistance and power so I was hoping that I would be able to do this with the arduino, if anyone can tell me if it is that would be great,anyway using the power setting inputed and the resistance of the coil I'm hoping that I will be able to adjust the voltage and current to get a regulated wattage output. if my thinking is flawed let me know I would appreciate you letting me know.

[CraigHB]

I know it is possible is calculate voltage and current using resistance and power so I was hoping that I would be able to do this with the arduino, if anyone can tell me if it is that would be great,

You need to measure power and resistance using voltage and current.  Alternately you can incorporate an ohmmeter and take a resistance check prior to energizing the atomizer.  In that case, you can compute power that way.  In any case, you have to measure current and voltage is already known so it's easiest to calculate the rest from that.

There's a couple different ways to measure current.  You can use a chip dedicated to that purpose or use a current sense resistor and differential amplifier.  For the current sensing chips, they can output an analog or digital signal depending on the chip.  You can interface with the Arduino using either one of the digital buses or an analog input.

There's tons of documentation on the net on how to build an ammeter using an op-amp in differential mode and a current sense resistor.  Some chips are available designed for that purpose that incorporate the op-amp and external components required, but it's really not a big deal to use a generic op-amp.

Either method has its up sides and down sides so it depends on what you want.  The current sensing chips are probably easiest to use, but they suffer lack of flexibility and can be sensitive to electrical noise.  The difference amplifiers are much more flexible and are more immune to noise, but they have to be calibrated for every build due to tolerances within the op-amp and other components.

[shandy27]

thanks for the information I was hoping if I knew resistance and the power needed I would have been use some sort of calculation and set the resistance on rsense and fb pins using digital potentiometers to set current and voltage.

[CraigHB]

It's possible you could use an ohmmeter (which is electrically much more complicated than an ammeter) and skip current checking altogether.  However, there's some rather ugly engineering considerations with that.  For one, you need an ohmmeter that has very good accuracy at low resistances.  That means a bigger number of higher precision, more expensive components.  Also, you have no way of protecting the electronics from an over-load condition that may occur some time after a resistance check is made.

In general, I'd say it's a bad idea to forgo the measurement of current for an advanced design.  Even for a variable voltage device that does not need to make power calculations, you really want to keep an eye on current demand from the atomizer.  As we use relatively high currents with high energy batteries to power an atomizer, there are a number of safety concerns that go along with that.

[shandy27]

sounds like my thinking is seriously flawed, the way it was planned was to have the user input wattage wanted and the arduino measure the resistance before it is powered up, if I divided the inputed wattage by the resistance and then got the square root of than number it gave me the amp needed and then set the digital pot to the required resitance on rsense pin and something similar for fb to set the digital pot for fb pin to set the resistance for the voltage required, I was planning on using an ina209 chip for current and voltage monitoring .  I guess I'm going completly the wrong way for a variable wattage device.

[CraigHB]

There's always more than one way to do something in electronics.  Often it's not a matter of what's better, but a matter of preference.  If you want you can incorporate an ohmmeter and take a resistance check prior to powering the atomizer.  From that you can calculate required voltage for a given power output.  It's not a bad way to accomplish the objective, just a different way. 

The point I'm trying to make is that regardless of how you determine output, an ammeter should be incorporated even if transparent to the user.  If for nothing else, to monitor the health of the system.  That being the case, an ohmmeter becomes somewhat redundant since you can solve all variables with voltage and amperage.  Also, an ammeter is a simple device to implement, an ohmmeter is not.

Even with only an ammeter you can still do a resistance check prior to powering the atomizer.  It's just a matter of pulsing a known minimal voltage and taking a current reading.  So, you can still do on-demand resistance checking with just an ammeter.

[shandy27]

is there many benefits to using say an ad623 instrumentation amp and shunt resistor instead of a hall effect sensor like the acs712.

[CraigHB]

Like I said already, there's advantages and disadvantages in using a shunt and amplifier over a hall effect sensor.  The current sensing chips that rely on a hall effect sensor are generally limited in the available range.  So, if you have to select an upper limit on the chip that may be greater than the limit you have on your design.  You can lose a lot of resolution in your meter that way.  The shunt and amp has the range you select based on the resistors you choose to set the gain.

As the sensing chips rely on a hall effect, they can be sensitive to environmental electrical noise.  So, you may have issues if your converter is physically too close to the sensor.  Since shunt sensing simply amplifys the voltage drop over a resistor, it doesn't have that issue.

The big down side with a shunt is they have to calibrated if you want any real accuracy.  Also, shunts are more lossy than hall effect transducers.  The amplifier will have some offset and there will be error in the resistor values you select for gain.  You can use an "instrumentation" amp like the one you mentioned, but there's really no reason to do that and it's somewhat limiting since the gain is set by the factory.  It's also more expensive.  A general purpose op-amp is all you need, they're inexpensive and common.

In any case, whether you go with a shunt or a hall effect sensor is entirely up to you as the designer.

[shandy27]

thanks for the help. I'll go for the acs712 for now this is going to be my first arduino project so the less calibration there is to do the better. I've been looking through tons of datasheets doing all sorts of calculations and to be honest it's great but thats all useless if it doesn't work, but I'm hoping to have a schematic for the mod up in a couple of days.

[CraigHB]

Sounds like a good plan.  The hall effect chips ~are~ nice.  I've debated using them myself.  Well, best of luck with it and hope it works out well for you.

[CloudedScreen]

well, shandy, you have certainly piqued my curiosity. I am eager to see how your design turns out. best of luck