DNA20 LED question

[warlordxxx]

Quick question for those in the know - This is not something i'm planning on doing but I have been told someone had done it and it lead me to question it in my own mind..

Will adding an LED to the output + and - from the battery make the wattage be off - I have it in my head that in theory it won't because the voltage supplied will head out to both the atomizer and the LED at the same time - However at the same time I also have rattling round amongst the cobwebs that there must be some resistance added to the circuit by lighting an LED at the same time, which in turn would throw off the resistance of the atomizer and therefore the voltage used to get the wattage required would be incorrect..

Please help my brain stop arguing with itself

[Breaktru]

Probably wouldn't see a difference.
A typical light emitting diode has an internal resistance of 12.5K. Temperature rise will lower resistance. I measured a typical Red LED at 53 degrees Fahrenheit (my garage temperature) . If you put it across the output which would be in parallel with for argument sake of a 1.5 ohm atty coil. Using ohms law, total resistance seen by the DNA would be  1.4998 ohms. Not significant enough to see a change in voltage/wattage.

The DNA measures the load resistance and calculates the output voltage using the wattage set on the display.

Does anyone see a problem with this analogy? I would like to hear from some members that may have a higher knowledge of electronic theory than I.

[warlordxxx]

Excellent thanks Dave - Also seeing a few folks with the battery weak come up on their board - I'm thinking that they have either used the wrong wiring and gone far too thin or their solder joints are bad or dry .. I'll be posting up my latest little project once it's finished

[CraigHB]

Breaktru has pointed out correctly that loading from the LED is insignificant compared to the atomizer, but here's the technical explanation if you're interested in that.

LEDs are current driven devices so they don't have resistance in the traditional sense.  Small changes in voltage can result in large changes in current flow.  This is why LEDs require bias resistors.  They do have an equivalent resistance but it's non-linear, it changes depending on current flow. 

You can derive an equivalent resistance by looking at voltage drop over current flow.  For LED indicator lights, voltage drop is typically around 2V for a current flow of 10mA which would be equivalent to a resistor of 200 Ohms.  The bias resistor is in there too and that would be around 100 Ohms for a 3V source.  Total equivalent resistance would be 300 Ohms, 2V over 10mA across the diode and 1V over 10mA across the bias resistor.

Of course, equivalent resistance is going to depend on the diode's forward drop characteristic, bias resistor, and source voltage, but for any indicator LED, it's not going to be hugely different than the example I made above.  Most are pretty close to 2V at 10mA for the bias and source voltage is typically close to 3V.

The resistance for an indicator LED is insignificant compared to the resistance for the atomizer so it should have little effect on accuracy.  You can also look at it in terms of current, 10mA compared to several Amps for an atomizer is insignificant.

It's still a good question since peripheral loading is definitely a consideration for any measurement circuit.

[warlordxxx]

Craig that was exactly what I was wanting to read - You sir are a living legend thank you very much for taking the time to write that out for me it's very much appreciated.

Now i'm going to post up another question lol

[CraigHB]

Welcome,

It's actually something you have to consider often when using a DMM or oscilloscope.  In some cases, the loading introduced by the measurement device can throw off readings considerably.  It's especially the case when measuring high frequency signals with a scope.  The impedance introduced by the scope leads and the scope's measurement circuitry can be a real problem.  This is why high quality scope leads are surprisingly expensive.

When measuring voltages with a DMM from high impedance sources, the impedance introduced by the DMM can throw off readings considerably.  This is why high quality DMMs have very high impedance, the higher the better.  A cheap DMM can throw out readings from sources with impedance as low as 10k Ohms.

[warlordxxx]

I had actually noticed that on the cheaper DMMs and wondered why - Craig never give up doing what you do bud it's wonderful to have folks like yourself and Dave around. Sometimes I ask these things to myself in my head and it can drive me a little nuts when I can't come to a conclusion ...

[CraigHB]

Don't feel bad about that, I've had the same trouble before, wondering why I wasn't getting expected readings, than I was like, doh, scope (or DMM) impedance.

[warlordxxx]

I don't feel bad Craig it's all part of the attraction for me - Problem solving in my own mind - If I come up with an answer that's great, but I don't have the EE background that you have, i'm pretty much going off of things that i've read or mapping out circuits in my head at 2am whilst trying to sleep. Yourself, Dave and Mamu (in fact i'm using a slightly adapted Mamu OKR circuit in the mod I use daily - Thanks Mamu for the inspiration) doing the things that you do pushes me to go further with my own modding. I've always been curious about the way things worked electrically in the same way I taught myself HTML for wanting to know how websites worked lol (even though HTML has now moved on it's a great base to start to recognize newer languages)

[CraigHB]

I tell you, the web has made this kind of thing so easy anymore.  If you have the interest, the info is right there.  I come from a time where if I wanted to know something, I had to make a trip to the library.  Gotta love the web.

In any case, a formal education helps for the really advanced stuff, like AC theory, but for 90% of the stuff we do with mod building, a grade school educaton in math is sufficient.  Even so, I've always felt anything you can learn in school, you can learn on your own if you have the motivation and discipline.  School just puts a fire under your ass.

[warlordxxx]

Couldn't agree more Craig I got the education I needed as per the Math part however the terminology is what throws me the most - With each mod i've made rather than just follow a tutorial and make the mod i've looked at each individual component and looked at what it does - How it does it - Why it's needed - What it's characteristics are etc etc Then there's the other side of the coin what happens if you use a lesser value than recommended and what happens if you use a higher value - Is the component needed or is it just going to add power drain and take away any real world usefulness just to accomplish something that can be done another way with more efficiency.

[CraigHB]

I think half of any vocation is just knowing the terminology.  It comes quick though.  Just a matter of working with the stuff and becoming familiar with it.

Certain terms carry a lot of information behind them.  Impedance is one of those terms.  It involves AC theory which is an advanced topic.  The good news is that impedance becomes simply resistance when working with DC and since we almost exclusively work with DC when building mods, it's not something you have to consider much.

[warlordxxx]

Lol my dad had one of the old analog impedence meters - Looked something like this http://www.prosoundweb.com/images/uploads/OpenPBImpedanceTest.jpg always brings that image into my head whenever I see it mentioned ..

I'd also like to add to this a very big thank you to yourself Craig as well as Dave and Mamu for the time and effort you all put into this. I've been making mods for a couple of years now but I couldn't have got as far as I have so quickly without the explanations, tutorials, info and advice you all put out there. It's bound to get frustrating from time to time with people PMing you asking the same questions that you've just answered for someone else so from myself I would like to say thank you, you are all very much appreciated ..

[CraigHB]

You're welcome, I actually don't get pestered that much.  I've had a few PMs from people asking questions, but not so often as to complain about it.  That's just me though.  Dave probably has something different to say seeing as how he's posted a ~ton~ of practical stuff on mod building.

[Breaktru]

Yeah I do get a lot of PM questions. I try to encourage the sender to also post their questions so others can learn from the answer/question. Most are reluctant to do so for fear of looking dumb. This forum is here for all skill levels. Sharing is the name of the game. I do not consider any question dumb. I'm sure that there are many others looking for the same answer. I myself still have a lot to learn. TGFC (Thank God For Craig)

[studiovape]

While on the subject of LEDs, can I run two leds in series ( to reduce voltage without needing a resistor) and piggy back the switching onto the remote microswitch i will be running for the DNA 20 fire button,
so basically using the un boosted/bucked voltage of the 3.7v battery in the mod to run the leds at the same time as firing the DNA20 chip without interfering with the DNA20s loading etc.
I know the leds will get a little dimmer as the battery drains, but this is not an issue for my project.
Maybe a dual pole micro switch  if there is such a thing would be better?

[warlordxxx]

Studio good question but i'm pretty sure the DNA reacts the same as the vamo on the switch - The switching section is made so as to pass very little current through the switch (the one on board is tactile) so will probably start somewhere in the 3V ish range when pressed it will drop down to .8 ish V so there won't be enough going through it to power an LED - I can't confirm the numbers but i'm pretty sure that's the reaction when the button is pressed as there isn't that much voltage actually needed through the switch.

[CraigHB]

The DNA20 would have a regulator for the MCU so there would actually be something around 3V on the trigger switch regardless of battery voltage.  Also, there would be a fairly high resistance inline with the switch, 10 to 100k.  So no, you wouldn't be able to power an LED dirctly from the switch.  Though, you could slave a small MOSFET to the switch that controls an LED connected directly to the battery.  You could even put a small linear regulator inline to keep the LED brightness constant.

It's possible to run LEDs in series, no issue with that.  Though you will need at least 5V to bias them proberly which would be a problem for a mod powered by a single cell.  For a mod that runs dual cells, it would not be an issue.

You generally need a regulated source to power LEDs.  There's a pretty big difference in brightness over a differential of 1 Volt which is what you'd see with a single cell fully charged versus fully discharged.  It depends on the bias resistor though.  The higher the resistance, the less brightness fluctuates over changes in voltage.  You would have less issue with that for an LED powered by series cells versus a single cell.   The bias resistor for series cells would have a much larger value.

[studiovape]

Thanks Craig, great food for thought
Sv