Resistor Wattage

[AtomSoft]

hey guys i wanted to know how to calc the type of resistor needed for certain things.

I have notice a lot of resistors burning up from stupid ol' me not putting the right resistor on it.

If i have a circuit that uses 5v @ 1A what kind of resistor should i use ? like i have seen .25, .5, .75, 1watt resistors.

I also notice in ohms law images :

P = VI
POWER = VOLTAGE X CURRENT

5Watt = 5v X 1A

Would i need a 5 watt resistor if 1 amp is to flow through it without burning up?

 

[MikeMl]

...
POWER = VOLTAGE X CURRENT

5Watt = 5v X 1A

Would i need a 5 watt resistor if 1 amp is to flow through it without burning up?

Nope, you would want to have some safety factor, so you would need the next larger wattage, likely 10W!

 

[AtomSoft]

Am i using wrong formula? Should i use:

(V*V) / R = P
(25) / 100 = .25W

 

[MikeMl]

Am i using wrong formula? Should i use:

(V*V) / R = P
(25) / 100 = .25W

If you have a resistor with 5V across its ends with 1A flowing through it, then it is dissipating R*I = 5*1 = 5W.

If you have a 100Ω resistor with 5V across it, then it is dissipating E^2/R = 25/100 = 0.25W.

Two different situations...

 

[AtomSoft]

let me better explain what im trying to do. I am using a AMS1117 voltage regulator but for some reason it smokes up even had one blow. Im trying to create a ADJUSTABLE 3v-12v 1A supply... 1A or less really

the most this IC can output is 1A so i dare not stress 900mA on it. I want this for basic small powering devices

Depending on what the circuit is pulling maybe 500mA. What size (watt) resistors should i be using?

 

[AtomSoft]

I think this was my issue before tho:

Electronic Equiptment - DIY LM317 power supply

If you notice most Schematics are like this or maybe more basic but R2 is usually a POT. The problem i see there is. the ADJ is not supposed to be shorted to GND.. so if you are using a POT and make a full turn you will soon hit 0ohm hence a GND. Thats why mines blew perhaps. Do you thing it would be wise to place a 10ohm or so from ground to R2 essentially making a R3 below R2

 

[MikeMl]

Please post a schematic of what you are doing.

 

[MikeMl]

I think this was my issue before tho:

Electronic Equiptment - DIY LM317 power supply

If you notice most Schematics are like this or maybe more basic but R2 is usually a POT. The problem i see there is. the ADJ is not supposed to be shorted to GND.. so if you are using a POT and make a full turn you will soon hit 0ohm hence a GND. Thats why mines blew perhaps. Do you thing it would be wise to place a 10ohm or so from ground to R2 essentially making a R3 below R2

It is perfectly ok to make R2 zero Ohms. In that case, the LM317 regulator will output a voltage of 1.25V, the minimum it can do. If the output is adjusted to 12V, the 240 Ω resistor will have 1.25V across it and R2 will have 12-1.25 = 10.75V across it. The current flow is E/R = 1.25/240 = 5.2mA.

The same current flows through R2 (plus a tiny bit more), so that means that R2 will be dissipating P =I*E = 10.75*0.0052 = 56mW.

 

[AtomSoft]

you told me to post a schematic and i think i figured out what i was doing wrong heh... ill post old and new one..

 

[AtomSoft]

look at yellow circle the pins from POT:

 

[MikeMl]

The OLD one is correct. Slightly better would be to connect the wiper to the top terminal. The NEW one will not allow the voltage to be varied.

 

[AtomSoft]

how about this:

 

[MikeMl]

Wont work as a voltage regulator. Why are you trying to reinvent the wheel? Use **broken link removed**.

 

[AtomSoft]

hey im not reinventing the wheel and im tired of people using that old term. It kinda gets old. I found info here:
12 Volt 30 Amp PSU
**broken link removed**

Im using a AMS1117 where its almost the same.

Both those are wrong then?

 

[MikeMl]

No, both circuits return the bottom end of the adjustable resistor to the negative rail, not to the unregulated positive input like you showed in NEWER!

 

[AtomSoft]

http://english.cxem.net/power/power8.gif

Doesnt connect to gnd.. anyway is this better:
LM317 Regulator Circuit

 

[RCinFLA]

You have to pay attention to the dissipation on the regulator.

You said you want 3v to 12 v adjustable supply at 1 amp output. The regulator you are using is a low drop out version of LM319. Where a LM319 needs about 2.5 v input to output this regulator only needs about 1.5 v delta.

What package are you using? The SOT223 package does not have good heat sinking capability. The SOT252 is better but you have to solder down the tab and have good heat conduction to heat sink.

You will likely be better off to use a TO220 version of LM317 even though it has higher dropout voltage delta.

If you want 12 vdc then you need at least about 15 vdc on input with LM317. With 15v-12v = 3 vdc drop across regulator you need to dissipate 3vdc * 1 amp or 3 watts via heat sink. This is a pretty good sized heat sink.

Now when you are at 3 vdc output you have 15 vdc - 3 vdc = 12 vdc across regulator. Now there is 12vdc * 1 amp = 12 watts to dissiplate. This requires a very large heat sink. Something in range of 5 x 8 inches.

Play with heat sink size, monitoring temp with your thumb. Burning your thumb can be a learning experience on heat sink size requirements.

 

[audioguru]

The pot must have one terminal connected to ground like shown.

 

[AtomSoft]

im blind and for my defense was tired lol. Thanks a ton. I think ill try something today.

 

[AtomSoft]

Wow works great so far.. I plan to get the LM350 i here its great!! in the TO-220 of course heh.. Thanks again. As you can see i need to just slow it down and look at things more often. Thanks and sorry for being a pain in the butt.

Here are a couple of pics of the TEST BOARD (which i will still use):