Interfacing microprocessor and buckpuck LED driver

[npomeroy]

I am trying to use two output pins on a picaxe microcontroller to control two sets of high power LEDs. The LEDs are Luxeon Rebels 700mA and are driven by the buckpuck (http://www.luxdrive.com/content/3021-BuckPuck.pdf for the datasheet). I've got the 6-wire version of the driver which will control an LED bank, and using the circuits in figs 15, 16 in the datasheet can effectively get the strobe pattern I want (using PNP transistor so output high = ON). But I want two different flash patters on two LED banks.

The problem is that I can not yet use two ouputs, with different flash patterns: When I try (i.e. with the outputs controlling two separate buckpucks), both LED banks shine continuously. EVEN when I set up a second microchip for the second driver, sharing the 12v supply, when I connect both LED banks they both shine continuously. The only way round it I have found is to use completely separate power supplies (i.e. a second 9v battery for one of the driver-microprocessor-LED bank set).

 

[npomeroy]

Here are the circuits

I'll try to post images of the circuits here:

Excuse the rough hand drawing.

 

[Laplace]

So given your schematic I surmise the BuckPucks are wired in parallel with the Vin+, Vin-, LED-, & REF pins connected together. If that is the case, then the wisdom of connecting the REF pins together is suspect. For troubleshooting try removing the micro-controller and getting it to work with 2 pots/switches (Figure 12) instead while the Vin and LED- pins are connected in parallel. Just guessing here.

 

[(*steve*)]

Just as an aside, the symbol for a PNP transistor is this:

not this:

Which is what you appear to have drawn.

 

[npomeroy]

I was simply copying the symbol on the buckpuck documentation.
I must admit I get confused over the orientation of transistors. When I first tried the circuit the LEDs did not light to full brightness via the transistor so I reversed the transistor and they worked much better. Is it possible for the circuit in Fig 15 to work with a reversed transistor? These are PN200 (recommnded repalcement for BC557).

Just as an aside, the symbol for a PNP transistor is this:

not this:

Which is what you appear to have drawn.

 

[npomeroy]

So given your schematic I surmise the BuckPucks are wired in parallel with the Vin+, Vin-, LED-, & REF pins connected together. If that is the case, then the wisdom of connecting the REF pins together is suspect. For troubleshooting try removing the micro-controller and getting it to work with 2 pots/switches (Figure 12) instead while the Vin and LED- pins are connected in parallel. Just guessing here.

The buckpucks are in parallel but only w.r.t. the Vin+ amd Vin-. The CTRL and REF wires go from the respective buckpucks to their own microchip only. But a continuity test shows that Vin- and LED- are connected. So both LED banks have the ground line in common.

 

[npomeroy]

Ah silly me - I see now that black triangle in fig 15 is an arrow head. I have done quite a bit of practical circuit building including using transistors (am a high school physics teacher) but never had any formal electronics training and am not good on transistor theory. Now recognising the polarity of the transistor in fig 15 I realise mine have collector-emitter polarity reversed. The other way round they worked but the LEDs did not give full brightness. I wondered if the transistor was not saturated and a different resistor value was required, but reversing them gave full brightness and the circuit behaved correctly, i.e. +5v applied to the base turned on the LED. With a single Picaxe circuit and one driver my circuit works perfectly. The mystery to me is why sharing the power supplies (and probably significantly the LED - lines) forces the LEDs ON.

Unfortunately at this stage I have soldered it up (with separate power supplies) and am reluctant to pull it to bits for additional diagnostic work - am hoping someone may intuitively see what the problem is. Sorry about the rough circuit diagram - I can provide neater and additional details if it would help.

By the way this is to drive two separately patterned flashing lights on a radio controlled helicopter. I am using 700mA luxeon LEDs so they show well in sunlight. The whole thing weighs nearly 4kg so a bit of extra circuitry isn't too bad, although I was not intending to need a second battery.

Just as an aside, the symbol for a PNP transistor is this:
...
not this:

...

Which is what you appear to have drawn.

 

[davenn]

I was simply copying the symbol on the buckpuck documentation.
I must admit I get confused over the orientation of transistors.

hi there npomeroy

there's an old memory jogger for NPN and PNP transistor identification

PNP = Point n Plate ---- the arrow points towards the plate
so if it doesnt do that it has to be a NPN

Dave

 

[npomeroy]

Judging by my previous observations, I suspected that connecting the negative power input wires of the two otherwise completely separate circuits would create the same problem but it has no effect.

So in the meantime I think I will put up with having 2 independent systems (and use little button lithiums on one of them to save weight. The other is powered by the helicopter's 24v Lithium polymer via a low dropout 12v regulator).

But I am concerned if I have wired the transistors wrongly (even though the system flashes as intended) so have included a digram below. For clarity in case you have not digested the Buckpuck details: REF supplies +5v. Short circuit between REF and CTRL switches LEDs off. Open circuit between REF and CTRL (or 5K resistance) switches LEDs on. Intermediate Rs provide dimming.
... In my application, if the transistors die or for any reason REF-CTRL goes open circuit the LEDs will stay on continuously and one of them has only a small heat sink (it fits inside the vertical fin for red flasher on top) and could damage the airframe if it overheated.

 

[Laplace]

So if I understand this correctly the problem only surfaces when the two BuckPuck Vin power terminals are connected in parallel to the same power supply. Also, note Figure 18 of the datasheet which shows a filter capacitor across the BuckPuck power terminals. This suggests that the SMPS places high frequency noise on the power input. Is it possible the LED driver is interfering with each other's operation via this high frequency noise? Would it help to use a noise cancelling choke and capacitor on each BuckPuck power input to eliminate noise cross-coupling? There must be some good reason why the datasheet recommends that capacitor on the power input.