Resistor?

[Avast Marine]

Not sure if thats what i need or not. I have a 6 volt dc power supply and I want to change it to 4.5 volts. Can someone guide me in the direction I need to go please. TIA

 

[pvtschultz]

Yup, a resister will work. Otherwise you can get a coil of wire (try 25') and test for voltage. If the voltage is too low, cut off some of the wire. The resistance of copper wire is about proportional to length, but just keep trimming until you get to the length you want.

Otherwise:

V=IR
V=1.5 volts
if you know the current, solve for the resistance and get one that is close at Radio Shack. Test with a voltage test if you have sensitive equipment.

 

[Avast Marine]

The power supply says 6VDC 800 MA. It doesn't have to be exact, it is to turn a gearmotor and 6v is a little to fast. How do I solve the equation for resistance? Sorry if this is a very basic question.

 

[AZDesertRat]

You can buy a variable voltage DC power suply here for $5.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=46977
I use these on all my cooling fans to vary the speed.

 

[Avast Marine]

I already have the power supply though and would like it if I could use a $.10 part instead of buying a new one.

 

[pvtschultz]

R=V/I

Divide the voltage drop (1.5 volts) by the current of your gear motor.

Otherwise you could get a rotary dimmer switch from HD or Lowes which is basically a potentialmeter (variable resistor) so you can set the speed as fast or slow as you like.

 

[Avast Marine]

The gearmotor current is rated 175ma

so...

R=1.5/.175
R= 8.57

So I need a 8.5 ohm resistor?

 

[Avast Marine]

Is this math correct? Sorry this is foreign to me.

 

[pvtschultz]

Yup, looks good to me. Radio shack should have one, otherwise go to www.digikey.com and you can order one. Look for one rated for as close to 1-2 watts for accuracy, otherwise going up towards 5 watts shouldn't cause you any headaches.

 

[Scooter12ga]

8.5 ohms at 1-5 watts is going to be huge, isn't it? Think of a ceramic brick a few inches long by about 1/2" square at least, right?

 

[pvtschultz]

What!?!?!

Typical kila-ohm resisters are smaller than a AAA battery. Ohm level resisters are quite small and are typically a "wound wire" type that just uses a winding of very thin wire to produce the resistance.

Here is an ebay link, you can get 100 for $2, can't be very big.

http://cgi.ebay.com/8-2-Ohm-1-4-Wat...oryZ4664QQrdZ1QQssPageNameZWD1VQQcmdZViewItem

Another for a range that would really help you dial in the speed that you want. Again, a pot would also work by having a wide range of resistance at the turn of a knob, but will cost you more.
http://cgi.ebay.com/1-4-Watt-Leaded...ryZ73147QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

 

[Avast Marine]

Looky what I found.....

http://www.luxeonstar.com/resistor-calculator.php

 

[Pbrown3701]

It's the wattage on a resistor that determines the size, not the resistance value.

With that said, a 5 Watt resistor shouldn't be big at all...

 

[Avast Marine]

OK, I put a 1/4 watt 47 ohm resistor inline with the hot wire and it is giving me the speed I want. I happen to have 500 resistors laying around now and would like to add a 5mm LED to the circuit. How should I go about this? Thanks for all the help so far!

 

[Icefire]

your 1/4watts resistor will toast as your resistor now output 0.258 Watts, way too close. 1/2 Watts minimum.

You can put 4x 47ohm resistor to form a 1watts 47ohm resistor with 2x 2x47ohm in serie than wire them in paralelle.

The 5mm led depend of the kind, a 1.2v 10mA regular or 3.5v 20mA high power?

 

[Avast Marine]

So:

-r47--r47-
-r47--r47-

and next time I should buy a 1w 47ohm resistor?

 

[Avast Marine]

This is the LED I have:

http://www.allelectronics.com/cgi-bin/item/LED-1/340100/T_1_3_4_RED_LED_.html

 

[pvtschultz]

If it blows, just tie the ends together in parallel so that it goes:

........-R47-..........
---<............>-----
........-R47-..........


This way they'll share the same voltage but half the current.

Hmm...That didn't work.

Just lay them side by side and solder/tie the adjacent ends together. My little diagram didn't work.

 

[Avast Marine]

So now that I have the resistance in ohms figured out in the future how do I figure out the watts I need the resistor rated for. Previously you said 1-2 watts for accuracy. I want to do this right even if it means going back and buying a 1-2 watt 47 ohm resistor. Based on the information I have outlined how did you determine that 1-2 watts is what I needed? i don't want it to blow, I want several years service out of this

 

[pvtschultz]

W=V*I
Watts = Volts x Current

What I forgot about was the you'd have the same current, 0.175A, but now 4.5 V instead of 6 V.