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Topic: 8x8x8 multiplexed LED cube with an Arduino Mega 2560 (Read 41 times) previous topic - next topic

dc42


Lets use 20 mA x 64 LED as an example. Assuming its running on 5v, using constant current driver to sink, (that sounds like the direction he is heading), it should use up to 1.28A. With this design, he is only doing 12.5% duty cycle. How do we apply the math to figure out what specific specifications to look for, and ranges maybe? It seems like a simple table would be easy enough to make.


Let's look at power dissipation first. If we assume that the software works so that the 1/8 duty cycle won't be exceeded, and we go for 0.5W maximum power dissipation in the mosfet, then the dissipation while the mosfet is conducting can be up to 8 * 0.5W = 4W. So we need Rds(on) <= 4/(1.28 * 1.28) = 2.4 ohms.

However, the voltage drop would then be 2.4 * 1.28 = 3.1V, and we certainly can't tolerate that in a LED driver running from a 5V supply as there wouldn't be enough voltage left to drive the LEDs and series resistors. So in this case, maximum Rds(on) is determined by maximum allowable voltage drop. If we were not concerned about the LEDs getting dimmer as more are turned on, we might allow 0.5V, allowing Rds(on) of up to about 0.4 ohms.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

CrossRoads

I search for MOSFETs like this at digikey.com:

x-channel mosfet (P or N)
in stock
Fets-single
logic level
thru hole (for prototyping anyway)
sort by price (click on unit price)

scroll down past the 1,000 qty min buy parts to  the 1-lot, looking for a low Rds part,
which usually puts me at the start of the TO220 type parts.
Then I check to make sure the gate capacitance isn't ridiculous.

Right now, that search puts me at these two parts
p-channel - small list of parts, just 12 choices
http://www.digikey.com/product-detail/en/NDP6020P/NDP6020P-ND/1055922

n-channel, 18 pages of parts
after the price sort, right on the first page
http://www.digikey.com/product-detail/en/NTD5867NL-1G/NTD5867NL-1GOS-ND/2401422
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

Un4Seen

Very nice explanation dc42! Now I understand why I need the MOSFETS that I need.
I bought the NDP620Ps from Farnell in the end: http://ro.farnell.com/jsp/search/productdetail.jsp?sku=1017724

In the datahsheet we can see this:
"Features
-24 A, -20 V. RDS(ON) = 0.05 W @ VGS= -4.5 V.
                  RDS(ON) = 0.07W @ VGS= -2.7 V.
                  RDS(ON) = 0.075 W @ VGS= -2.5 V."

I hope that's good news. What confuses me is that you used Ohms for the unit of Rds(on) and the datahseet uses Watts. But I guess V=I*R and W=I*V, so W = V/R*V = R. W=R ?!

CrossRoads

Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

dc42

When I look at the datasheet, the characters that you describe as W are Greek omega-characters.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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