resistor question

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lmackattack

lmackattack

old school
I have a walthers motorized oil pump and have wired it to my DC transformer that also supplys power to all my layout lights. I have the lights on vairable DC to keep them at about 60%. The issue is the oil pump is spinning to fast even at 60% power. is there a way to slow it down with a resistor?:confused:

Thanks
 
Yes
go to radio shack and pick up a few different values. 10, 100, and 1000 ohm or one of their 1/2 watt value packs. They're cheap. You could also try a low value potentiometer that you can simply dial in.
 
ok thanks, one more question

does a 10 ohm resist more or less voltage then the 100ohm..just trying to understand resistors a little better.

Thanks again
 


The higher the value the more resistance, it actually allows less current to flow.

Like squeezing a garden hose the more you squeeze the less flow you get. The voltage is the pressure behind the flow, which stays the same at the tap, but because of ohms law (I=V/R I is current, V is Volts, R is resistance in the circuit) the current is reduced.

Ken.
 
Ken: Excellent analogy.
If I may add: when you measure voltage across a resistor, you are measuring a differential in this pressure (potential) from one side of the resistor to the other. This is what we call a voltage (pressure) drop. Using Kens analogy, it would be the same thing if you put a pressure gauge across the hose kink (parallel), connected to each side of the kink.

If you put a flow gauge inline with this hose (series) and measured the water flow amount, it would be the same as measuring current in a series ckt with the resistors.

Keep in mind that current is the electron flow (water) and voltage (pressure) is what moves the electrons. A resistor is a restriction to this flow of electrons and causes a pressure drop. More resistors (or higher value), lesser flow and more drops in pressure. These values will always add up to the voltage of the main supply, e.g. 12volt supply= 3v drop + 2v drop + 7v drop. The amount of total current from the main supply in this circuit is dependant on total resistance. (Only so much of it can flow through these restrictions.;))

Another example with LED's, where our big concern is limiting current:
12 volt power supply= 2volt (20milliamp) LED + 10volt drop resistor.
We must restrict our current to 20ma to protect the LED rating. We know that the resistor must drop the voltage that the LED doesn't or 10volts. To find the size resistor value, divide the resistor drop voltage (10) by the current rating of the LED (0.020) = 500. We need a 500 ohm resistor to restrict the current flow to the LED at 20milliamps. ;):)

ANY HELP?:D
 
Thanks again for todays lesson..

I think understand the basics of what its doing and it makes sence i just need to follow the math better to understand what is needed for the specific lightbulb,led,motor etc...

now next question. when you resist the current do you have to worry about heat build up in front of the resistor? you know the cause and affect laws?
 
Ken: Old habits are hard to get rid of. :D

Trent: The current restriction in a resistor does create heat and is rated in Watts...as a light bulb. There is not a problem providing the resistor is rated at the proper wattage....1/4, 1/2, etc. To figure the wattage size you have to take the known values and use one of the following formulas. It is always a good idea to double the calculated wattage as a safety margin.
For DC
voltage x current=watts
current squared x resistance= watts
voltage squared / resistance= watts

The heat will be at the resistor and can sometimes be HOT depending on the use. With small current values as with LEDs, this is seldom a problem. Incandescent lights or motors can use larger amounts of current and can generate a good bit of heat in the resistor. Proper shielding or placement can usually remove any danger from damaging the surrounding area.
 
Last edited by a moderator:
Well I went to Radio shack this afternoon and got the value pack that you suggested. Got the pump slowed way down after testing a few of them. it doesent look like a midevil catapult trying to knock down a castel anymore LOL

thanks again for all the help guys.

Trent
 


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