Need some VV mod help!

[Old_Trekee]

Exactly right Green Mtn. The parts list also show Tantalum capacitors, but the data sheet specifically says no Tantalum on the input side and only to be used on the output side if going to be used below -20C.

As for the size of the caps, we are only using a max of 8.4v input, so 10v is fine as is anything over 10v. Using over 100uf is over kill and will only increase the size of the caps making fitting into the box a bit more difficult.

I want as much space as possible. Saw that pump mod for injecting e-juice in the side of a carto or atty. Thinking about adding that in too, so space is important to me.

That's why they call it modding because everybody has different ideas about what is best for themselves. Just trying to help out clarifying a few things.

 

[Green Mtn Vpr]

That's what I love about ECF... The sharing of thoughts, experience, opinions, and even mistakes to help each other. Much appreciated!!

 

[Old_Trekee]

Another thought Green Mtn. I'm using a thumbwheel potentiometer and cutting a slot in the case for external voltage adjustment. Problem was trying to find one at 200 ohms. Found one on Digi-key that is 1k ohms and using a 250 ohm fixed resistor in parallel will work the same as a 200 ohm. You can mount the variable and the fixed on a separate board and connect to the other board with 2 wires where the variable in the schematic is supposed to go. Makes mounting more flexible.

 

[CraigHB]

It's always better to use a capacitor with a higher voltage rating. It makes the circuit more tolerant to voltage transients and less likely to fail due to heavy ripple currents (electrolytics can fail due to wear or stress). However, the trade off is size and sometimes cost. A 35V aluminum electrolytic is about double the size of a 10V aluminum electrolytic in the same capacitance. Typically, you want a minimum 20% margin between the rating and the worst case voltage. 10V is the lowest you can safely go with our application, but 16v, 25V, or 35V will be better for durability, if the space is there. The data sheet recommends what it does because it's what their engineers selected for their reference design and conformance testing. Their recommendations account for the module's maximum input/output voltages and they use a liberal tolerance.

 

[Green Mtn Vpr]

I had decided on the 16v just to stay safely outside of the 80% max margin...but just to clarify as there was much varying on this, can both be Tantalum caps to keep size down? Or should I use electrolytic? Room is premium, but I dont want to hurt performance etc either

 

[CraigHB]

The data sheet recommends against using tantalums on the input side. Ceramics and aluminum electrolytics are actually a little more effective at filtering ripple than tantalums are. The main benefit with tantalums is they are much smaller. If space is a problem, you can use an aluminum electrolytic on the input and a tantalum on the output. Though people have used tantalums on both sides without issue so it's probably okay.

You have to remember the recommendations in the data sheet are based on a much tighter tolerance for ripple voltage. Were not particularly concerned about that since we're only driving an atomizer. Normally, this module is expected to power other electronics which are sensitive to that sort of thing. We only need to keep ripple within tolerance of the regulator module itself, which seems to be pretty liberal.

 

[WillyB]

Well in the future if folks are concerned about caps just go with Murata's OKR-T/6-W12-C.

Same pin-out, same resistor/voltage chart, same efficiency, basically just swap out the TI.

From the datasheet.

Recommended Output Filtering

The converter will achieve its rated output ripple and noise with no additional
external capacitor

OKR-T/6-W12-C 6A/30W

And with a 1.5Ω LR atty at full bore, 6V, you'll only be at ~67% of it's rated amps.

 

[CraigHB]

Good point. It's really a much better part because of that. It's smaller too.

 

[CapeCAD]

Good point. It's really a much better part because of that. It's smaller too.

There are other advantages besides not needing the extra caps:
Higher efficiency (as much as 4% higher in the 1.5-4 Amp vaping range)
Accepts full Vin on the enable pin - External mosfet is not needed for proper control (>6V)
Pull-down resistor can be used to shut down converter, connect directly to Vin to turn on. Allows use of NO or NC switches.
Pins are robust and do not break easily when bending