some questions and the mod's i've made so far....

[justinadams]

First up i have a question for the gods of this forum. i'm working on a micro controlled OKL2 mod with an oled screen and 3 buttons (up, down, fire). I have most of the circuitry worked out so far, but i can't seem to figure out how to do the OHM meter. all of the ohm meter designs i've found only seem to work with resistances above 100ohm's. i'm using an ATMEGA328-AU microcontroller. any help would greatly appreciated.

now for the fun part. the mods i've built so far.
first is my kinda ghetto Naos Raptor mod. is powered two 4000mah lipo batteries in series.




second is my first OKL2 mod with buttons for adjustment. two 2000mah lipo batteries in series.

[Visus]

Wow talk about horseshoeing

very non purdy but look uber functional

Good luck with your mega, thats some wire spaghetti there;  I just posted about the new oled screens on the market cant remember which post lol...
Some sharp new guys on the block bright as the sun..

[justinadams]

Wow talk about horseshoeing

very non purdy but look uber functional

Good luck with your mega, thats some wire spaghetti there;  I just posted about the new oled screens on the market cant remember which post lol...
Some sharp new guys on the block bright as the sun..

lol, i know my wiring is terrible, that's why i'm working on a chip/board so it'll be a bit neater and easier to fit in the case.
this is oled i will be testing once it gets here.http://www.ebay.com/itm/191013150020?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

[Visus]

Thats a sharp display
the post  I made you can order samples and play around with those too...

Found it
Near last post..
http://breaktru.com/smf/index.php/topic,1216.150.html

[justinadams]

sweet. and it's color.

[Claviger]

This should have the relevant data you need. Just adapt the circuit to fit your application.

http://forum.arduino.cc/index.php/topic,21614.0.html

Will allow ohm reading for very low resistances.

[justinadams]

Ok cool thanks. I will try that.

[CraigHB]

If you're already measuring current, then you can just divide the known voltage by the known current to get resistance.  You can do it on demand by pulsing a voltage on the atomizer and taking a current reading.  The time it takes to perform the operation is going to depend on converter startup.  Most don't take more than a few milliseconds so it should be an apparently instantaneous operation for the user.

I didn't read that whole thread on that Arduino forum, just glanced over it, but reading low resistances without measuring current gets a bit tricky.  If you want high resolution when measuring low resistances with a divider, you need to set it up with similar resistance which means higher power requirements and larger parts in your measurement circuit.

For example, if you want to measure a resistance of 100 mOhms with high resolution, you'd want to set up a voltage divider with a resistor that is close to that probably no more than 200 mOhms.  You're going have large currents in your measurement circuit and all the baggage that comes with that.  On the other hand, you lose resolution when measuring high resistances so there's a limitation there.  You have to select a resistor value that's middle of the road for the range you want. 

One consolation is that you don't need parts rated for those currents continuously, just ones that have surge current capability within tolerance.  However, you'll need to make sure your parts can handle currents for the time it takes the converter to stabilize, usually within 10ms, but check your converter's data sheet for startup times.

In general your accuracy is going to be highly dependant on the voltage and resistance you involve in your measurements.  You'll want to monitor applied voltage with an ADC input to your MCU and you'll want to use a resistor with at most 1% tolerance.  You'll need to switch the resistor in and out of the circuit with a MOSFET so you'll want one with low "on" resistance to minimize error.  Though whatever error is introduced, you can always calibrate in code to compensate for it.

Another way to measure resistances is with a constant current source instead of a constant voltage source, but that doesn't make sense for our application since we already have a constant voltage source in our power converter.  You would need a bunch more parts instead of just a resistor and MOSFET.

BTW, be sure to filter your ADC inputs.  The output from the power converter is noisy and you'll get jumpy readings.  An RC circuit with a 100nF capacitor and 1k resistor is usually sufficient for filtering AD inputs.  It's what I use normally.  You'll probably need a divider on your ADC input anyway to get output sensing within range of the MCU's supply voltage so just pop a 100nF capacitor on there between the MCU ADC pin and ground and you'll be good.

[justinadams]

Ok. Wow. Thanks. Lots of information. I hope I can apply it properly. Again thanks so much.

[CraigHB]

Welcome, if you're already using Arduino and comfortable with it, should be pretty routine stuff.  The calculations you need to do in code are the simple multiplication and division for a divider circuit.  Even so, I highly recommend you incorporate an ammeter into your mod.  Measuring current is kind of a necessity for these high power applications and it makes measuring resistance a trivial task. 

Ammeter circuits can be a bit involved but there are single chip solutions that can make it a painless prospect.  Breaktru can recommend a part for you.  I do it with a differential amplifier which is kind of the hard way, but it has it's advantages, namely adjustable gain.  Though the single chip solutions use a Hall effect transducer which is much more efficient.

[justinadams]

alright cool. i will definitely look into that.

[CraigHB]

I wanted to look at the Hall effect sensors again at some point anyway so here's the part that I think Dave has used;

http://www.allegromicro.com/~/media/Files/Datasheets/ACS712-Datasheet.ashx

[Breaktru]

I wanted to look at the Hall effect sensors again at some point anyway so here's the part that I think Dave has used;

http://www.allegromicro.com/~/media/Files/Datasheets/ACS712-Datasheet.ashx

Yes, that's it