# Power Supply and Ohm values Help....

Discussion in 'The Projects Forum' started by Needs2TinKer, May 1, 2016.

1. ### Needs2TinKer Thread Starter New Member

Apr 26, 2016
4
0
Hello all,

First and foremost I will say I'm new to the electronics field, I've only had real experience in machining and fabrication.
So apologies in advance for any silly mistakes. I have tried to immerse myself in as much as I can understand concerning electronics.
I just have a few questions in some areas that don't make sense and would appreciate any input

My project is a home built AC unit using rather basic means. Will my project be that efficient? I doubt it but I will have fun making it and learning
Ive taken into account of all parts I need but Im having a few issues with understanding some terms and wasn't able to find an answer that is clear to me.

So:
I'm going to be using a 12V 8.5Amp, 102 Watts DC power supply to power to 4 devices which require a total of 73.2 watts combined.
Ive taken into account the resistors I will be needed( If I've done my maths right) but when looking at resistors and their values, this additional info is part
and parcel of the resistor "1/2W Watt 0.5W Metal Film Resistor ±1%". From what I can gather it states the obvious of 1/2 a watt of power but I don't
understand how that applies so a device that may be using 12 V at 5amps for 60 watts ...........?
Please see the attached PDF where I have input details and done a very basic design of my per say circuit. Im just not sure if the "1/2W Watt 0.5W Metal
Film Resistor ±1%" values are the correct ones for this project

And any recommendations to parts to use would be greatly appreciated

Many thank.

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2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,155
772
Draw the circuit diagram out so we can see what you need.

3. ### Veracohr Well-Known Member

Jan 3, 2011
559
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For each resistor the power it will dissipate is P=I^2*R. Use a resistor with higher power rating.

4. ### Needs2TinKer Thread Starter New Member

Apr 26, 2016
4
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I've done the circuit as accurate as to how I would understand it to be. I have also added the aspect of a potentiometer to control fan speeds of the radiator fans. I do apologize for my lack of understanding and if the design is wrong, I'm currently finishing night shift and haven't really slept much. Any and all criticism is welcomed and I will endeavor to learn from your instructions and what I can learn on my own part.

Many thanks again.

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5. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,155
772
Don't understand why you want to use resistors, if the items run on 12v dc just put them on the supply...

6. ### Needs2TinKer Thread Starter New Member

Apr 26, 2016
4
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I would have thought that I would have needed to limit the current as the devices use less that 8.5amps and mainly 150ma to 350ma to 5/6 amps. (I wanted to limit the peltier to 5 amps opposed to 6amps)

Well if that's the case great, less fiddling about and would it be okay to use a 12v 8.5amp 102w DC supply to power devices that require a total of 98.4 watts or should I use one with more power? I did forget to add another fan motor........ Also what about a potentiometer to control the fan speeds ? would you be able to recommend anything on that note ?

Thanks again for the info, its much appreciated

7. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,155
772
Controlling fan speeds is done with pwm, most pc fans are dc brushless motors, and dont work well slowed down, personally i would use a psu with 50% more power.

8. ### eetech00 Active Member

Jun 8, 2013
682
116
What are the resistors for?

9. ### Needs2TinKer Thread Starter New Member

Apr 26, 2016
4
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So I'd be fine using the DC 12 v 8.5 amp to power the peltier at a working current: 4.3-4.6 A ? without it going over its max of 6 amp ?
If so would the use of a 2.4 ohm 1/2W Watt 0.5W Metal Film Resistor ±1% would be the correct one to limit it to 5 amps of current?

http://www.ebay.co.uk/itm/New-1-2W-...hash=item1ea8f94617:m:mNyLewwndV1vFLMfWyh2DHg

And the rest of the devices I can leave them as they are without any need or resistors and they should be fine. I was under the impression from what I have read that the resistors would limit the flow of current through the device. Or was I mistaken I just didnt want to over load the devices.