It makes perfect sense to me. It's just a function of the maximum voltage the board car produce (9V in the case of the DNA200). That means above 0.405 ohms, it's impossible for the board to make 200 watts. With a TC wire such as nickel, they take the expected resistance increase into account when they give the value above which you can't make 200 watts anymore, so with nickel you have to start out at or below about 0.2 ohms cold in order to be sure you can still make 200 watts when the coil is up to full operating temperature (i.e. when its resistance has doubled).
Does any dna base its wattage on cold resistance or live resistance? When I first heard about Replay and Evolv was saying it is ok to use nickel under wattage mode and everyone else says to never do this.
Well after thinking about it, I figure that nickel should be ok to use in wattage mode if the mod only reacts to cold resistance and not to live resistance. This is what DNAs seems to be doing is my guess. The DNAs doesn't totally ignore live resistance of course since it reacts on this when it comes to monitoring temperature. DNAs doesn't seem to be alone either. As I tested others and most of them appear to base wattage on cold resistance too. Although not all of them. Some seems to react to live resistance only.
The DNA board continually monitors the resistance during use and provides the set wattage (in wattage mode)
Most other boards just use the cold resistance to determine output.
Most other boards just use the cold resistance to determine output.
Ok, I understand that but I was talking about the relationship between voltage and current at less than max watts. For example when using my dna75 it will use the full battery potential (aporox 4.15v) when set to 75w with a .12 ohm build but if I turn down the wattage to 25w it bucks the battery voltage down to approx 2.2v. Why? It doesn't have to. Wouldn't it be more efficient to use full battery potential at all times?
75w / 4.15V = 18A
25w / 2.2v = 11.3A
Why not 25w / 4.15v = 6A instead?
Evolve boards track live battery voltage so it would be possible to use the highest battery potential available at all times. Perhaps they ran out of EEPROM bytes?
Seriously though, this has always been a curiosity to me with dna boards. Perhaps the dc-dc converters run more efficiently at lower voltage outputs, not sure.
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That's horrible! Running nickel in wattage mode means the voltage will keep climbing while the resistance does as well until something gives. But why doesn't nickel do this while using wattage to set up Replay?
On a dna the voltage will increase as the res increases to MAINTAIN your set wattage all the way from cold res to heated res.
Because the only thing the board has can actually control is its output voltage. The amount of current that's drawn from the board is determined by that voltage and the resistance of the coil at any given instance. The only way that the board can limit current is to reduce its output voltage, because the resistance of the coil is whatever it is at any given moment.
Of course you could install a higher resistance coil if you wanted to vape 25w at 6A, but the board can't do that for you.
So what? If you set it to X watts, you'll get X watts, unless/until voltage required to produce X watts is more than the board can provide, in which case you'll get less than X watts. DNAs generally top out at 8 or 9V, right? It's not like that's gonnna electrocute anyone.
What makes you think it doesn't?
Ohms law. Course you can cheat and go with averaging by way of pulse width modulation but the rattlesnake can rear it's ugly head.
Because the only thing the board has can actually control is its output voltage. The amount of current that's drawn from the board is determined by that voltage and the resistance of the coil at any given instance. The only way that the board can limit current is to reduce its output voltage, because the resistance of the coil is whatever it is at any given moment.
Of course you could install a higher resistance coil if you wanted to vape 25w at 6A, but the board can't do that for you.
Well that's an answer I can accept if indeed the board has no current regulation. Early on when I was reading on the Evolv dna200 threads I got the impression that the boards not only had current limiting but current regulation as well. I probably misunderstood.
PWM would also make it more difficult to monitor the amount of current in real time.
Semantics. Current regulation requires a moving voltage based on the load resistance and current which is what they do.
I did that unintentionally once when I grabbed the wrong type of 3x32 twisted wire.
It never fired at all because the mech in question had a fuse.
It has current limiting, but it does that by monitoring current and controlling output voltage to keep the current at or below the limit.
I would speculate that it's much easier, and practical in small electronics, to regulate power based on voltage rather than current.
This Is What Nickel Looks Like Under Wattage Mode
That isn't what I want to see with nickel. The wire just gets hotter and hotter. I guess to do it right, you need a VV mod. That way the voltage won't climb and the wire won't get hotter and hotter.
That isn't what I want to see with nickel. The wire just gets hotter and hotter. I guess to do it right, you need a VV mod. That way the voltage won't climb and the wire won't get hotter and hotter.
The same is true of any other wire at constant wattage, right up to the point where it reaches an equilibrium point, where the wattage being supplied to the coil is being dissipated somehow. The material that the wire is made of has an almost negligible effect on that equilibrium point.
Yeah, what rossum said, at some point an equilibrium will be reached in wattage mode, vv mode or on a mech mod. Wires that have a little tcr like ss316 are great for mechs. You get a little boost in power at first and then as the coil heats up the res rises and kind of automatically throttles back the power. Makes for much faster firing without getting too hot with the right build.
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