Mod Box Question...

[TommyDee]

This is a question on functionality when you are reading watts.
I have a COV Volt Mini that I am cutting my teeth on for mod boxes.
I wired it up to a halogen bulb to get an ohms reading.
What do you suppose happened?

I am using a 35W halogen bulb.
It reads ~0.4ohms cold.
I hit the button and it glows.
I hit the button again and it glows much brighter and the ohms now read 1 ohm.
I can keep doing this until it hits 3 ohms and major brightness.
All while limiting output to 20W.
That doesn't seem useful for impedances that change resistance significantly as they heat up.
Is this just the nature of the Volt Mini or is this the way watts mode works for all mod boxes?

 

[KeroZen]

I think some use the live resistance reading and proper current limiting and some just offer a fake Power mode just to display a Watts figure to the user for convenience.

Does your mod have a way to lock down the resistance value?

 

[TommyDee]

As to locking ohms; not that I can tell as this has limited options.

This one takes an initial resistance reading and adjusts voltage and current to meet that wattage value. Therefore, no, there is no real-time monitoring of resistance.

Turns out halogen bulbs operate at 3 ohms at temp.

However, for a little brute of a unit, this does make for one nice little sub-ohm meter at a budget price! It suffers two other afflictions for my purposes however...
1) It has an auto-timeout on power-up limiting log heater use, and 2) it's limited to 0.2 ohm minimum. Might be able to run a Muad Dib but the launch box comes in at just under 0.06 ohms.

 

[blokenoname]

The filament used in halogen bulbs has rather unique properties resistance wise, as its cold resistance is quite miniscule, while the hot resistance can be ten times that much or more. 35w bulb will read at ~3ohm hot. Yes.

 

[TommyDee]

Do most current mod boxes maintain actual watts even if the resistance changes?

 

[blokenoname]

Do most current mod boxes maintain actual watts even if the resistance changes?

That's the general idea. A mod is after all, just a fancy buck/boost converter and acts like one. In power mode, the mod should detect resistance changes and auto adjust voltage output to achieve the set wattage. Coils must not be locked for that, of course.

 

[KeroZen]

I think the locking thing is just for the baseline reading. You lock it when you are certain that the cold resistance reading is close to the actual value, and afterwards it's immune to room temperature change as the mod will use that baseline value to perform TC on (so the starting point can change due to fluctuations and weather but the end computed target will stay the same, and this is what matters)