Calculating Ohms

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ClintOHenry

New Member
I bought a 16v 60ma AC motor. Using a needed voltage drop of 104v, I come up with about 1700 Ohm resistor. Could someone check to see if I did that right?
 

dave1905

Active Member
I bought a 16v 60ma AC motor. Using a needed voltage drop of 104v, I come up with about 1700 Ohm resistor. Could someone check to see if I did that right?
I wonder how many hundred degrees that resistor will heat up to. Sort of a motorized electric range.
 

Red Oak & Western

Active Member
1700 +/- ohms would give an appropriate voltage drop but would consume 6-1/4 watts, with a temperature rise of 40 degrees C above ambient. A small transformer would be a much better choice.
 

gregc

Apprentice Modeler
I bought a 16v 60ma AC motor. Using a needed voltage drop of 104v, I come up with about 1700 Ohm resistor. Could someone check to see if I did that right?
it's one thing to limit the current thru an LED with a resistor, but not typically done with a motor which has a dynamic load.

why not find a supply, a wall wart, that is closer to the operating voltage of the motor and equal or greater current?
 

Iron Horseman

Well-Known Member
I bought a 16v 60ma AC motor. Using a needed voltage drop of 104v, I come up with about 1700 Ohm resistor. Could someone check to see if I did that right?
Your calculations are correct 1734 ohms... BUT -->

it's one thing to limit the current thru an LED with a resistor, but not typically done with a motor which has a dynamic load.
I agree, this is not a good plan. A motor is not a resistance load. It is an inductance load and so the value of a resistor would really need to vary as the motor demands more or less current with a load. Do we know if 60ma is the stall current, starting current, free spinning, load of x, or what?
 
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wvg_ca

Active Member
if the 60ma is run current, then with a resistor of that value it would quite likely not start, as start current is normally higher than run current
 

railBuilderDhd

Active Member
it's one thing to limit the current thru an LED with a resistor, but not typically done with a motor which has a dynamic load.

why not find a supply, a wall wart, that is closer to the operating voltage of the motor and equal or greater current?
When selecting a power supply And based on what is said here the operating voltage can be a greater current then the motor current. How much more current is acceptable when selecting a power supply?

Dave
 

wvg_ca

Active Member
for the motor that you mentioned in your first post, 16V AC, and probably 120ma should be okay, to allow for greater start current .. anything a bit more than that, even up to !A will work, as the motor will only draw the current it needs, and a higher current capacity isn't harmful
 

railBuilderDhd

Active Member
for the motor that you mentioned in your first post, 16V AC, and probably 120ma should be okay, to allow for greater start current .. anything a bit more than that, even up to !A will work, as the motor will only draw the current it needs, and a higher current capacity isn't harmful
So, just to be clear on this if I have a power supply say from an old laptop that has 16 or even 19 volts I could use it for a 12 volts motor? Would that be true for only AC or would DC motors be the same? Also, I'm not really clear what you are saying "...even up to !A..."? Is the more important number the ma used for the motor and released by the power supply?

Not doing anything at this point but just thinking if I could use these old laptop power supplies then I wouldn't need to spend more at WalMart.

Thanks,
Dave
 

wvg_ca

Active Member
Well, not really, if a motor is rated at 12V, driving it with a higher voltage than rated may damage the motor ..
You ideally want supply voltage equal to rated motor voltage [or less], and supply current equal to motor rated current [or more] ..
First of all, is there any reason you want the motor to run at "maximum" speed??
If the answer is no .. then a small LM317 based adjustable power module [off eBay for a couple of dollars] will allow you adjust motor speed by adjusting the voltage supplied to it, and you can use a higher voltage laptop supply, but this will work for DC motors only ..for AC it's easier just to find a wall wart with an AC voltage output that will drive the AC motor a a speed that is acceptable to you
Just out of curiosity, what do you intend to do with this 16V AC 60ma motor?
 

railBuilderDhd

Active Member
TIME OUT - I've started a new thread to carry on the discussion about the laptop power supplies and its use with layout accessories. This thread can be found at http://www.modelrailroadforums.com/...ccessories-on-your-layout&p=438497#post438497.
ClintOHenry had a question about Ohms and my secondary question although somewhat related was starting to take over the discussion as well as confuse what’s being asked here.
wvg_ca the questions about the laptop power supply and the 16V AC 60ma motor are not related.
If you would like to follow the power supply question and related discussion please switch over to http://www.modelrailroadforums.com/...ccessories-on-your-layout&p=438497#post438497 or follow with O-Henry’s question about Ohms here.
Hope this keeps the discussions clear and others from not feeling like a steam roller just crashed your thread.

Thank you.
Dave
 




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