Warning newbie questions MR16 basic build for 12v DC

[Pyrotech]

I've a couple of boards on the way, so I've been reading up.  I intend to power the boards from 12v battery packs (4 x 12v 1,2AH), with the supply regulated.  My neighbour is in the security business and is a source an almost un-limited supply of the batteries.

Looking through the build manual it states :

Three of the .1uf capacitors need to be put on the places above the chips marked for them now. Install the 4.7k ohm resistor in the marked location above the big chip if you are going to use the mosfets for high current.


So am I correct that I do not need 4.7k ohm resistor for 12v DC.

Then   it states:

"You can see the row of paired rectangles that run across the center of the board just above the location for the 16 transistors. Each pair needs a 3k resistor and a .1 uf capacitor added to them as you can see in the above picture if you are using the transistors for high current operation.

If you are not using them and running the board as a low current constant current driver leave them out. "

I leave out the 3k resistors and the .1 uf capacitors for 12v DC ?

Finally this bit confuses me, 

"Three of the .1uf capacitors need to be put on the places above the chips marked for them now. Install the 4.7k ohm resistor in the marked location above the big chip if you are going to use the mosfets for high current.

If on the other hand you plan to jumper the mosfets where the ovals marks are and leave them off for constant current drive up to 100ma per channel. Then you need to choose a resistor to match the current limit you need and place it here. The correct value for the current needed can be located in the datasheet for the TLC5940NT chip.

Common currents are:

20 ma - Use 1.91k ohm such as Mouser pn# 660-MF1/4DCT52R1911F.

100 ma - Use 380 ohm such as Mouser pn# 660-MF1/4LCT52R391J.


Then install the 120 ohm resistor at the top of the board’s center where it is marked."

I've left out the 4.7k ohm resistor, I want to light basic MR16 led 12v 1w spots or basic single colour led strings.  So I'm assuming I'll be leaving off the moffets ? and need to jumper, I know the leds are 20ma, but not sure on the spots, if I go for 100ma can I still use the connections for 20ma strings? my instinct tells me yes, but going to need to put a resistor somewhere in the string to make up the difference.  (Electronics is totally new to me, so I've got my trainer wheels on)

Jumper the moffets where the oval marks are?  I see three oval marks around the capacitors above the chips, how do I jumper these?



Ian

[n1ist]

The MR16 can be used in two modes - it can either directly drive low current LEDS (up to ~100mA per channel) from the TLC5940, or it can use MOSFETs to boost the current to drive lamps like the LED MR16 bulbs.  Neither of these choices have anything to do with the input voltage.

Since you are powering MR16 lights, you will be using the FETs.
- Install the 4k7 resistor above the TLC5940
- Install the 16 3k resistors and 16 100nF caps in the row above the FETs
- Install the FETs

If you are powering simple series strings of discrete LEDs
- Install a 1k91 (for 20mA) or 380R (for 100mA) resistor above the TLC5940.  In this mode, the output current setting MUST match the rating of your LED strings
- Leave off the 16 resistors and 16 caps above the FETs
- Your version of the PCB doesn't have the ovals on the silkscreen for the FETs.  For each of the 16 FETs, jumper the upper and middle terminal

/mike

[Pyrotech]

Oh thanks Mike, the ovals threw me a bit.  So basically I think my best bet as a starter is a board dedicated to LED's and one dedicated to MR16's.   

[D56VillageNut]

Ian,

Check out this thread.  It's where RJ describes the changes to the version 2 of the MR16.  This may help clarify things a bit for you.

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Alan T

[Pyrotech]

Ha Alan I see the ovals----I'm getting there.  though on reflection still don't understand why I cannot run a 12v 1w MR16 LED off of the basic stripped down board?  or rather 16 singles.  one per channel  aside from needing a larger power supply.

I thought working out lift times and delays for fireworks was bad enough  

oh well mega tress and matrices this time next year..............

[rm357]

Low power mode is "seriously" low power. While an individual channel can sink 100ma, the total power rating of the TLC5940 integrated circuit (IC) at room temp is about 2.5 watts. If you don't want to overheat and burn up the IC, you need to keep it below that.

If running 12v, 1 watt on a channel will probably be OK (~83ma), but if you turn on 3 channels of those at the same time, the IC will be getting pretty hot.

By adding the mosfets, the limiting factor for current is the board itself - RJ recommends keeping it below 35 amps for the board and below 2.5 amps on a single channel.

As a side note:
Power (watts) = Voltage (Volts) x Current (Amps)

Also, RJ is working on fixing the firmware for the low power configuration. The mosfets invert the control signal, making 255 = off and 0 = full on.
RM

[Pyrotech]

RM

Okay, 1 light on 1 channel, on okay poss 3 lights, 16 lights on 16 channels on at once, bad for chip. got you.

With the moffets switching quirk as well, best to build  full boards from beginning.

I'd somehow or the other actually got the 83mA per MR16 myself,,,, which means that the max I could bare run is 2.6 lamps.  I think I was working on the 35amps and 2.5 amp figures for a bare board.

Oh well now to get my head around common grounds and anodes etc. 

I think I'll be back........

DIY must have another meaning.

[rm357]

Actually, the MR16 is a common anode device (look at the back of the board - all of the "pos" channel lugs are tied together with the input "pos" lug)... The switching is actually done by controlling the grounding through the mosfets...

It's a minor distinction if you are using both the "pos" and "gnd" lugs on the MR16. However, there is a cool side effect in that you can actually use several different power supplies with different voltages at the same time for your loads. Just hook the grounds of all the power supplies and the "Gnd" input lug on the MR16 together, tie the positive from the power supply to the appropriate loads (lights, motors, selenoids, whatever) and the other lead from the load to the appropriate channel "Gnd" on the MR16. Of course, the MR16 needs at least 7v tied to it's "pos" lug to power the board itself.

RM

[RJ]

Becareful with the :

motors, selenoids, whatever  as inductive loads such as motors and solenoids have a way to pop mosfets and other items unless they have protection diodes on them.

RJ

[soakes94]

Here is a thought, would you be able to use smaller MOSFETs? As these ones can handle more power than actually required for my setup, I would be looking for maybe 600/800ma? Would it be possible to use something like 863-MPS2222AG? As these would be able to handle my power requirements and cost a fraction of the MOSFETS.


Thanks Steve