# Led Cube - Transistor

I am building a 8x8x8 LED cube. Using 9 shift registers: 8 for the 64 columns and 1 for the 8 layers/levels.

At any one time the max number of LEDs on will be 64. 64*.02A = 1.28A

I need 8 transistors to control the ground for the different layers what should I use?

I'd use logic-level mosfets, perhaps one of these http://www.rapidonline.com/Electronic-Components/TO-220-Logic-level-power-MOSFETs-N-Channel-77687 but there are lots of other suitable types. You can't realistically use a single transistor to switch 1.28A because you'd almost certainly need more than the 20mA you can reasonable get from an Arduino pin to drive the base. Assuming the LEDs are powered from 5v, I wouldn't recommend using power darlingtons because they have a high saturation voltage.

Hi dc42,

Can you explain this a bit? A source current on the base of 20mA and 1.28A is a gain of 64.

If I search on Digikey for NPN transistors where Ic >= 1.5A and gain is at least 70, I get pages of results (for example http://search.digikey.com/us/en/products/MPS651RLRAG/MPS651RLRAGOSCT-ND/1139913 ). Why wouldn't something like this work?

I'm still trying to figure transistors out so any advice is greatly appreciated.

Chris

I will be using a 5vdc 2A wallwart to power the LED cube and Arduino.

How do figure 64 on? I don't think you'd have more than 8 in one row (or column) of 1 layer.

[quote author=Chris Magagna link=topic=76363.msg577124#msg577124 date=1319331164] Can you explain this a bit? A source current on the base of 20mA and 1.28A is a gain of 64.

If I search on Digikey for NPN transistors where Ic >= 1.5A and gain is at least 70, I get pages of results (for example http://search.digikey.com/us/en/products/MPS651RLRAG/MPS651RLRAGOSCT-ND/1139913 ). Why wouldn't something like this work?

I'm still trying to figure transistors out so any advice is greatly appreciated. [/quote]

Hi Chris,

When you're using a transistor as a switch and you want to get a low voltage drop across the transistor when it is turned on, you need to drive it with somewhat more base current than its hfe would indicate. The voltage drop of a transistor when it is driven hard is called its Vce(sat). For that particular transistor, Vce(sat) is quoted as 0.5V at 2A collector current and 200mA base current (i.e. 10:1 not 75:1). When you drive it with 20mA base current and 1.28A collector current, the transistor will not be fully saturated and its voltage drop will be somewhat higher. Not only might it be too high for your application, but the power dissipated by the transistor in this condition is Vce * Ic plus a smaller contribution from the base current. The maximum power dissipation for that transistor is 625mW, so you need to keep Vce down to about 0.45v when it is passing 1.28A to avoid overheating the transistor.

As a rule of thumb, I use single transistors to switch up to 500mA and mosfets above that.

Awesome explanation. Thank you very much!

CrossRoads: How do figure 64 on? I don't think you'd have more than 8 in one row (or column) of 1 layer.

8 of my shift registers are connected the 64 columns and 1 register controls the layers. So if turn on all of the 64 columns and then tell the last register to light up layer 1 there will be 64 LEDs on.

Therefore 64*.02A = 1.28A so I suppose I need a FET, but I don't know which one.

No, that's only rated at 0.5A, also it's in an SMD package that makes it hard to hand-solder. Try this one: http://nl.mouser.com/ProductDetail/Fairchild-Semiconductor/FQP13N10L/?qs=sGAEpiMZZMvAvBNgSS9LqlXn9tn9He7s or http://nl.mouser.com/ProductDetail/Fairchild-Semiconductor/RFD3055LE/?qs=sGAEpiMZZMsX0CFs5rpLHm6WNML5U6p8.

8 of my shift registers are connected the 64 columns and 1 register controls the layers. So if turn on all of the 64 columns and then tell the last register to light up layer 1 there will be 64 LEDs on.

That doesn't sound like it would fit in with a multiplexing approach then.

CrossRoads: That doesn't sound like it would fit in with a multiplexing approach then.

I don't see what you mean - it appears to me that he's using 8 x 64 multiplexing, which I think is a sensible choice for this project.