Need electronics help

[allcart]

I'll start by saying that I don't understand electrical theory, so I need to pick your brains.

Ok, so I have made a couple of dewstraps to run of a 12V 7AH battery.

One strap has 10 x 620 ohm 1/2 watt resistors in parallel.

other has 10 x 330 ohm 1/2 watt resistors in parallel.

Q. which will provide more heat? and what will be the wattage of each strap?

Q. If I add a resistor, does it increase the wattage and heat, or reduce it?

I appologise for my lack of understanding, but some things we know and some things we don't.

allan

[Auntystatic]

The 330's will provide more heat as they have a lower resistance=higher current= more heat. Each resistor provides its own heat spot so there would not be an over all heat increase by adding more resistors, it would lengthen the dew strap though, each resistor has to be spaced equally along the chain, adding a resistor will draw more power so will increase the wattage..

The 620 ohm will use 0.19A @12v = 2.28W

The 330 ohm will use 0.36A @12v = 4.32W

[allcart]

Thanks Danielle, very helpful.

so if I understand this, the 620 ohm strap will produce less heat due to the higher resistance and lower wattage, but will not drain the battery as quickly.

The 330 ohm strap produces more heat due to lower resistance for the same number of resistors in the chain, and produces more wattage etc, but will drain the battery quicker.

My goal is to make a dewstrap that produces slightly more heat than I need, but can be regulated with a dew controller.

just not sure how much heat I actually need yet.

[squeaky]

The resistors are in parallel, so Kirchoffs Law applies here...

1/R = 1/r1 + 1/r2 + 1/r3....

So the net resistance of each of your networks is 62ohms and 33ohms.

Ohm's Law then gives you the current drawn by each:

V = I x R

12 = I x 62

and

12 = I x 33

So the current is 12/62 and 12/33 ( 194mA and 360mA)

Wattage is W = I x V

So 0.194 x 12 = 2.328 for the 620 ohm

and 0.360 x 12 = 4.32 for the 330 ohm

Your 7AH battery is going to see you comfortably through a night's viewing even at the higher current. Roughly 20 hours worth there.

[allcart]

Thanks Richard, I may get to understand this stuff yet.

[squeaky]

I haven't calculated a network resistance with Kirchoff's Law for over thirty years!

Erm... hang on... yep, more than thirty. I was actually calculating the resistance of the windings in a two speed star delta two pole motor. The company concerned had bought it and the starter controller second hand and the wires between them had been cut and not labelled. They didn't know how to connect it, nor how to work it out - so they sent me a list of the resistances between each of the twelve terminals on the motor. Those were NET resistances of all the various different paths through the six windings - so the equations for that one were a bit of a challenge.

I sent them my solution and they rang back a couple of hours later. The first time they tried it they blew the breaker. Turned out that their lad had connected two cables wrong. They put that right and everything worked like a charm. Directions and speed were spot on. Phew!

[allcart]

See! all your years of learning wasn't wasted after all. Its just that my learning was on a different curve.

[squeaky]

Aye, we all know different things.

There's a big difference between "ignorant" as in "not educated in a subject" and being thick. I don't think I've ever met anybody who hasn't been able to tell me something I don't already know.

And I STILL don't qualify for Mastermind

Or even weakest link

[Photosbykev]

You know enough about a subject if you can avoid getting into trouble through ignorance and know enough to ask the right questions without an ego getting in the way

Sent from my iPhone using Tapatalk. Blame Apple for the typos and me for the content

[squeaky]

Yep

The guy who taught me that lesson many years ago was a bloke in my local. Thick as two short planks. Spoke like a right oik, didn't finish school and for a bet couldn't have told you if South America was North or South of the equator (largely ) if his life depended on it.

Then one day he started talking about his hobby, keeping tropical fish, and all of a sudden I'm hearing about the different types of water hardness, nitrogen fixing, gram positive bacteria... and I thought to myself Blimey! Just shows you shouldn't judge a book by its cover.

[Gina]

Very true I'm of the opinion that most people can do almost anything if they put their mind to it. It's a matter of dedication and wanting to do it.

And IMV you're never too old to learn. Not that I'm old really - just been living for 70 years I find it amazing But age is just a number! I'm still very much learning and loving it and I love teaching too - passing on anything I learn to other people.

[squeaky]

Very true I'm of the opinion that most people can do almost anything if they put their mind to it. It's a matter of dedication and wanting to do it.

And IMV you're never too old to learn. Not that I'm old really - just been living for 70 years I find it amazing But age is just a number! I'm still very much learning and loving it and I love teaching too - passing on anything I learn to other people.

Aye, agreed - though I haven't been around long enough to accumulate quite as much wisdom as you...

[Gina]