Author Topic: Wire Gauge  (Read 702 times)

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Offline VegaObscuraTopic starter

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Wire Gauge
« on: October 23, 2011, 10:31:44 PM »
I'm using a breadboard with an atmega8 and a UART module to control a lamp by using a relay.  Here's how the whole setup looks:

PC -> UART Module -> ATMEGA8 -> TIP120 Transistor -> 9V Relay -> Lamp

The whole setup works flawlessly.  But right now all of the wires are relatively short, so the lamp has to be right next to my PC.  I want to be able to control the lamp from across the room (or in another room).  I want to fix this by using longer wires going from the breadboard to the relay.  The relay is a 9V DC SPDT relay with contacts rated at 12A at 120V AC.  The lamp is a typical basic lamp with nothing fancy, and a fluorescent 13W bulb at 120V, so that should be 0.1A.  Yes I am going to be very careful not to touch the exposed 120V contacts or wires =)

Here's my concern.  The relay is supposed to have at least 9V to turn on.  It might flip at around 8.5, but it definitely will not turn on at 8.0V.  I'm worried that if I use longer wires, it might not be getting the full 9V by the time it gets to the relay.  My power supply is a universal AC adapter with a voltage select switch.  With the wires I'm using now the relay will turn on with the power supply set to the 9V setting, but the power supply can go up to 12V if needed.  The relay's coil resistance is 500ohm (I don't know if that matters or not).  Currently I am using 22 gauge solid core wire, and none of the wires are more than 6 inches long.

Would it work to use 22 gauge solid core wire to go approximately 10 feet?  Would this cause a voltage drop of more than 3V?  Also are there any other possible problems (or dangers) that I have overlooked?

Offline Soeren

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Re: Wire Gauge
« Reply #1 on: October 24, 2011, 12:39:54 PM »
Hi,

Would it work to use 22 gauge solid core wire to go approximately 10 feet?  Would this cause a voltage drop of more than 3V
It will work.
10 ft. of 22 gauge is around 0.3228 Ohm when the return wire is included (a distance of 10 ft. equals 20 ft. of wire)
That means you'll have 9*500/500.3228 = 8.994V

Assuming a voltage drop of 0.25V would be OK, the distance could be up to: (0.25/(8.75/500))/0.03228 = 442.6 ft.


Also are there any other possible problems (or dangers) that I have overlooked?
Not as long as you wire it so that pople don't trip in it ;D
But do mount the mains voltage to the relay in a secure fashion - RTV silicone could be blob'd over that side of the relay terminals, if it's not in a plastic box.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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