# LED Beer Pong Table Upadate / Question

Discussion in 'The Projects Forum' started by Turkish, Jun 21, 2010.

1. ### Turkish Thread Starter New Member

May 16, 2010
24
0
A while ago I posted about my LED Beer Pong Table I've made some progress and at the bottom of this post are some update pics. I have to switch the leds with my arduino mega and i would like to know if you think my mosfet I've selected is adequate heres a link http://www.newark.com/jsp/search/productdetail.jsp?SKU=89K1814
Ill be switching sets of 10,12 and (1) set of 16 leds
will this mosfet be able to handel that load?
will the arduino be able to switch the mosfet?

(yes I know there are some lights out there fixed now)

Last edited: Jun 22, 2010
2. ### sceadwian New Member

Jun 1, 2009
499
37
10 12 and 16 LEDs in series or a series/parallel configuration? What is the load? You mentioned this is in reference to a previous post but don't provide any reference.

3. ### Turkish Thread Starter New Member

May 16, 2010
24
0
oh sorry here is a diagram of the circuits

Last edited: Jun 22, 2010
4. ### Turkish Thread Starter New Member

May 16, 2010
24
0
so... can anybody comfirm i"ve selected the right ones?

5. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
The original thread is here:
http://forum.allaboutcircuits.com/showthread.php?t=38494

The 2N7000 is easily driven from a microcontroller that is operating from a 5v supply.

I suggest not attempting to sink more than 100mA current from the LED arrays using the 2N7000. That is up to 5 strings of 20mA LEDs in parallel.

Do not use the wizard's "solution 0" unless you like having problems.

The way to calculate LEDs in series goes like this:

1) Determine your supply voltage (yours is 12v)

2) Subtract 1v or around 8% from your supply voltage, whichever is greater. (in your case, 1v)

3) Divide the remaining supply voltage by the typical Vf of a single LED. If not an integer result, discard the fractional portion. That is the maximum number of LEDs you can have in a series string.

4) Calculate the current limiting resistors:
Rlimit >= (Vsupply - (Vf_LED * step_3_result)) / Desired_Current
A table of standard resistance values is here: http://www.logwell.com/tech/components/resistor_values.html (Bookmark that page)
Use the E24 values (green columns). Select a standard value that is >= the result you obtained from the calculation.

5) Calculate the power requirement of the resistor:
P = Current_Desired^2 * Rlimit * 2 (we double the power requirement for reliability)
Use resistors with >= the wattage rating of the above result.

6. ### Turkish Thread Starter New Member

May 16, 2010
24
0
wait it says the 2N7000 can handel 350ma continuous and 1.4a pulsed(what I'm doing) even if I was using it for a continuous load why would I under rate it by 250ma? Also I already tested solution 0's cuircuit and it worked fine, what type of problems do you mean? will they happen later on?

BTW Thanks for your help wookie and retched you guys really have helped me out alot with this project.

Dec 5, 2009
5,201
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8. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
You can't believe what you read on a vendors' website. They make mistakes all the time. You have to look at the manufacturer's datasheet. Look at the maximum continuous current rating, and divide by two. That way you will have more margin for error, and the odds of your project working reliably improve considerably.

As LEDs warm up, their Vf (forward voltage) decreases. If they are being powered from a voltage regulated supply (implying low impedance), as their Vf deccreases, the current through them will increase. This will cause them to heat up more, and their Vf to decrease yet even more. It's called "thermal runaway".

If you use fixed resistors in series with that 1v/8% voltage drop like I said earlier, when the Vf of the LEDs decrease, the resistor takes the heat instead of the LEDs.

9. ### Turkish Thread Starter New Member

May 16, 2010
24
0
ok I read the wrong thing do you think you can point me in the right direction to a mosfet that will be able to switch up to 16 leds at a time and still be controlled by the arduino board? Also just double checking but the n-channel mosfet is always off unless switched on right?