Need Help Mod wiring - A few question on steping down voltage with resistor

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Rockproof

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Fellow moders help with a newb moder questions. I have reviewed the moder posts but am getting cross eyed on charts:confused:

If I wanted to step 7.4v down to 4.0-4.2v what ohm resistor would I use (using a 1.5 ohm atty...I'm willing to risk poping the attys)?

Does Radio Shack carry these (if so, what model/designation is on the package)?

Where/how is the resistor placed in the circuit?

When a resistor is placed in the circuit, every press of the power is still drawing the same amps as if you were running at the full 7.4v but now only delivering 4v as the resistor is consuming the extra power correct?

I want to make a mod that delivers a consistent 4-4.2v to the atty for the duration of the batts and can only think to do it simply with a resistor. I doubt a regulator is as simple as placing a resistor in the circuit but am open to ideas.

Thanks all.
 

Java_Az

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Not really possible to do with resistors off of a battery. Reason being is 7.4 is a nominal voltage. Fully charged two 3.7 nominal voltage batteries in series are 8.4 volts. As you use the batteries they go down in voltage. Protection usually cuts the batteries off at around 5.5 volts. So you have a 3 volt swing from fully charged batteries to depleted batteries. Resistoring it down is going to result is a similar swing in voltage.If you used 4.25 as the middle ground you could make it go from 5 volts fully charged to 3.5 volts Depleted battery by resistoring it down. Plus it will be very inefficient because the resistors will be consuming the extra current. ( extra current consumed = HEAT ) You would also have to use quite a few resistors because your going to need about 12 to 15 watts with a LR atomizer. Really your better off using a regulator , switching reg would be the most efficient.
 
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McDude

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For simplicity I would use a 5v regulator with a diode. The 5v regulator would give you... well.. 5v while a switch and a diode would step it down to 4.2'ish volts as an option. That way your not putting resistors in harms way (3amp circuit), or bleeding off precious amperage needlessly.

You can get 5 free samples of the 5v regulator from texas instruments by signing up. The part is UCC283T-5 or UCC383T-5.
 

Rockproof

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Thanks Java and McDude. Please remember noob here and no electrician to boot, I want to deliver a consistent 4.2v for life of batt, not 5v, so shouldn't use 5v regulator correct? A switch (?) with a diode (any recomendations) would be my best bet? I get the 5v regulator and diode (a certain diode?). How does that look in the circuit?
 

Rockproof

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Sorry guys, I confused myself, the above should have read:

Thanks Java and McDude. Please remember noob here and no electrician to boot, I want to deliver a consistent 4.2v for life of batt, not 5v, so shouldn't use 5v regulator correct? A switch (?) with a diode (any recomendations) would be my best bet? How does that look in the circuit?
 

Java_Az

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Sorry guys, I confused myself, the above should have read:

Thanks Java and McDude. Please remember noob here and no electrician to boot, I want to deliver a consistent 4.2v for life of batt, not 5v, so shouldn't use 5v regulator correct? A switch (?) with a diode (any recomendations) would be my best bet? How does that look in the circuit?

You can get UCC383T adj version and use a resistor to set it to 4.2 volts. The adjustable ones are also in the free sample program. I got a couple that should be here tomorrow i believe. The UCC383t have a really low dropout voltage which dropout is another thing to worry about, but those chips are about the best i have seen for the voltages we use. I will look at the data sheet in a bit and see what resistor it would take to get you to 4.2 volts. Need me at least one more cup of coffee before i start doing any math.
 

McDude

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You'd want the 5v regulator to give you a consistent output to work with, then a diode to shed some voltage (.8 or .5 I believe, I forget how much a diode drops). You can get a consistent 4.2'ish that way. I only mentioned a switch in case you wanted 5v and 4.2'ish volts as options on the same device, if you just want 4.2'ish, then no switch and just an inline diode.
 

McDude

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You can get UCC383T adj version and use a resistor to set it to 4.2 volts. The adjustable ones are also in the free sample program. I got a couple that should be here tomorrow i believe. The UCC383t have a really low dropout voltage which dropout is another thing to worry about, but those chips are about the best i have seen for the voltages we use. I will look at the data sheet in a bit and see what resistor it would take to get you to 4.2 volts. Need me at least one more cup of coffee before i start doing any math.

Totally agree, but it may be a bit more electronics than the OP wants to deal with - but really not. Just one extra pin for the resistor to set the regulator's output (no more complicated than figuring out where to put a diode). Could get fancy and even put a POT in there, but I think that's definitely going beyond just a desired 4.2.
 

McDude

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BTW, just thought - there's a really cheap way of becoming familiar with at least a linear 5v regulator and that's creating a circuit using wire, solder, and the 7805 regulator from Radio Shack. That regulator only costs maybe $3 bucks and is perfect for "learning" on. Note though, that it is not rated for the required 3amps (only 1amp rated). It's a cheap/easy way to become hands-on familiar with wiring up a regulator while waiting for the good stuff to arrive from texas instruments.
 

Rockproof

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Long and short of what I want to do is build a copper with (2) 18350's and have them deliver a consistent 4-4.2v to the atty. I have already built my first copper (thumb below) with a single 18650 and I just love that "sweet spot" from 4.2v peak down to about 3.8v. At 3.8v I pull the batt, replace with fresh, and recharge. I want to build a mod that keeps me in the sweet spot as long as possible.
 

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McDude

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Long and short of what I want to do is build a copper with (2) 18350's and have them deliver a consistent 4-4.2v to the atty. I have already built my first copper (thumb below) with a single 18650 and I just love that "sweet spot" from 4.2v peak down to about 3.8v. At 3.8v I pull the batt, replace with fresh, and recharge. I want to build a mod that keeps me in the sweet spot as long as possible.

Yup, understand. To get that precise "sweet spot" there is, unfortunately, no shortcut other than become familiar with wiring up regulators (that's their job). If you really want a very specific voltage then 5v regulator using a diode to drop a little voltage, or a variable volt regulator using a resistor to set the output (that is the most precise way - using a resistor on a variable volt). There's simply no way to get precise using raw battery output w/out a regulator of some kind.
 

Rockproof

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Roger that McDude. Ok so in your opinion, using (2) 18350's in a copper, the best route is?? The parts I need are??? Once I know the parts, I can get familiar with the wiring. I know how to solder (not great as I have very shaky hands) and basic wiring (I did alot of DIY in my house including central air), so think I can do this with the proper parts.

Thanks again all for the feedback and help!
 

McDude

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Roger that McDude. Ok so in your opinion, using (2) 18350's in a copper, the best route is?? The parts I need are??? Once I know the parts, I can get familiar with the wiring. I know how to solder (not great as I have very shaky hands) and basic wiring (I did alot of DIY in my house including central air), so think I can do this with the proper parts.

Thanks again all for the feedback and help!

I can help you out with a schematic once you get the part or if you want to practice on a7805 from radio shack (which I highly recommend doing btw, just once to get a feel for the task). Also for shakey hands, get a set of "helping hands" from radio shack as well ($15 bucks) - it's a setup with two little arms with alligator clamps and a decent size magifying glass.

Really, any two batteries with a decent maH (850+) will last all day. I use two 15400's (AA sized batteries @ 900mah) and I vape for 2 days straight on new batteries. Whatever batteries fits the container you want to use are best.
 

McDude

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I couldn't do anything without helping hands. I tried the cheap set from Harbor Freight to start with and quickly moved to the Radio Shack one. The set from HF has a small base, horrible articulation on the arms (always having to fiddle with the adjustments) and the magnifying glass is small - the thing tips over more than not. The Radio Shack hands are very nice to deal with - little adjustment, better design and - most importantly - don't tip over constantly like a drunken sailor.
 

McDude

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Question: Why not just use a LM317 with xx/xx value resistors to get xx volt. (and a 3.0 amp button)? Would that work and be easy? Ratshack sells em too........

LM317 / LM338 / LM350 Voltage and Current Regulator Calculators


You certainly could use that device, although the guaranteed amperage is a bit low. What you describe is no different really, just using a different device. For the UCC286 and UCC383 adj chips from TI you'd do exactly the same, but the LM317 is probably easier to get and cheaper (discounting the TI free samples).

Note the amps - not sure if "guarenteed" means nominal or max or ?? but it's what the datasheet had.

# Guaranteed max. 0.01%/V line regulation (LM317A)
# Guaranteed max. 0.3% load regulation (LM117)
# Guaranteed 1.5A output current
# Adjustable output down to 1.2V
# Current limit constant with temperature
# P + Product Enhancement tested
# 80 dB ripple rejection
# Output is short-circuit protected
 
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