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Author Topic: Using BC327 instead of BC337 but not working as expected?  (Read 5534 times)
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The page you linked to indicates that those LEDs are designed to work from a constant 12v supply and take not more than 3A. Presumably, this means around 1A per colour. So to dim them, all you need to do is to switch 12V to them using PWM. Start with a regulated high-current 12V supply, perhaps a standard ATX supply. If you want to use TLC5940s, then I would use P-channel mosfets to do the switching. They don't need to be logic level ones, something like IRF9530 will do. Connect source to +12V, drain to the +ve side of the LED strip, and gate to TLC output. Also connect a pullup resistor of around 1K ohms between the gate and +12v. Connect the negative side of the LED strip to ground. [EDIT: set the TLC current to around 20mA, i.e. more than the 12mA that will be sourced through the 1K resistors, to ensure that the TLC outputs saturate.]

A slightly cheaper option would be to use PNP darlington transistors in place of the mosfets, with base series resistors instead of the pullup resistors. However, you would then need to increase the supply voltage to around 14V to get full brightness.
« Last Edit: January 15, 2013, 12:23:16 pm by dc42 » Logged

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Thanks David that sounds pretty simple smiley I think I will try the first option, that way I can use the same 12v power source for the LED strips and the MR16 LED bulbs smiley

I will get some IRF9530's ordered to try out.

Thanks again

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Many thanks, it seems to work perfectly with the IRF9530's smiley
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Hello...  I was putting together 5 * 3wRGB(350ma per color) to create single RGB wash fixture and wanted to use the TLC5940, seen you guys went thru some trial/errors in the post.  I'm techy but not with electronics so much, does anyone have the exact circuit I could see or the BOM and parts.   Knowing the TLC can't reach 350ma and there's a solution around it, as was discussed in this thread early but can anyone summarize this recent build?  It was a bit hard for me to follow at 100%.... Thanks!
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The page you linked to indicates that those LEDs are designed to work from a constant 12v supply and take not more than 3A. Presumably, this means around 1A per colour. So to dim them, all you need to do is to switch 12V to them using PWM. Start with a regulated high-current 12V supply, perhaps a standard ATX supply. If you want to use TLC5940s, then I would use P-channel mosfets to do the switching. They don't need to be logic level ones, something like IRF9530 will do. Connect source to +12V, drain to the +ve side of the LED strip, and gate to TLC output. Also connect a pullup resistor of around 1K ohms between the gate and +12v. Connect the negative side of the LED strip to ground. [EDIT: set the TLC current to around 20mA, i.e. more than the 12mA that will be sourced through the 1K resistors, to ensure that the TLC outputs saturate.]

A slightly cheaper option would be to use PNP darlington transistors in place of the mosfets, with base series resistors instead of the pullup resistors. However, you would then need to increase the supply voltage to around 14V to get full brightness.

This is working nicely, but I have just realized the LED strips I have are common anode smiley-sad Could I use the IRF9530's to run them somehow?
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There is no easy way of using IRF9630s to drive common anode LED strips. Here are two suggestions.

The left-hand one uses PNP darlingtons, which will drop around 2V. So you will need a 14V power supply and small heatsinks for the darlingtons. You can probably omit the 1K resistor, because the darlingtons have 8K resistors built in to them.

The right-hand one uses an octal inverter chip to feed logic level mosfets. There are lots of variations on this (for example, using transistors instead of the 74HC240), however the main thing is that you need to invert the signal polarity between the TLC outputs and the mosfets gates, otherwise the PWN will work in reverse and you won't be able to turn the LED strips complete off.


* Scan 139.JPG (84.36 KB, 1653x1165 - viewed 35 times.)
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Hi David. Thanks for the help as always!

I circuit on the left seems more straight forward, but like you say it will be dropping some voltage so will produce some heat. I might get some TIP127's to try out.

The circuit on the right looks quite complicated, and the 74HC240 looks like it might require a fair amount of circuitry to run it?

Thanks
« Last Edit: February 10, 2013, 07:22:05 pm by dtokez » Logged

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The only additional circuitry the 74HC240 needs is to connect the Vcc and ground pins, put a 0.1uF decoupling cap between them, and also connect the 2 enable inputs to ground. It's an octal buffer, so 2 of them will handle all 16 outputs from the TLC5940.
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Thanks they don't sound to complicated after all, I think I will get a few to try out. There seems to be a few manufacturers making them, in general with a IC like this, will they all be the same?

Thanks again

edit.
which of the logic level mosfets do you think would be the cheapest? I can only make out IRL540 from you circuit diagram, are the others likely to be any cheaper than those?

Thanks!
« Last Edit: February 11, 2013, 07:15:21 pm by dtokez » Logged

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Thanks they don't sound to complicated after all, I think I will get a few to try out. There seems to be a few manufacturers making them, in general with a IC like this, will they all be the same?

Yes, they will all be the same. Be careful, there is a 74HCT240 as well as a 74HC240. The 'T' version is less suitable for this application.

which of the logic level mosfets do you think would be the cheapest? I can only make out IRL540 from you circuit diagram, are the others likely to be any cheaper than those?

From my supplier (Farnell UK), in order of increasing price they are STU85N3LH5 and then NTD4815-35G. I assumed you only wanted through-hole mosfets; there are much cheaper SMD ones.
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Brill, thanks David!

I'm searching eBay at the moment, I have used Farnell and RS but I rarely have orders large enough to qualify for the free shipping smiley-sad

Would the SN74HC240N or the 74HC240A be the same part?

The IRL540's seem to be the cheapest and most readily available on eBay so I think I will go with a few of them. Probably will stick tho through hole this time because my PCB layout skills are lacking somewhat and usually find it easier to route through hole hole 2.54 pitch stuff.

Thanks again

« Last Edit: February 12, 2013, 07:53:06 pm by dtokez » Logged

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Would the SN74HC240N or the 74HC240A be the same part?

SN74HC240N is the right part, from Texas Instruments. 74HC240A doesn't sounds like a complete part number, however MC74HC240AN is also the right part, from Motorola.
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Probably will stick tho through hole this time because my PCB layout skills are lacking somewhat and usually find it easier to route through hole hole 2.54 pitch stuff.

All the parts I listed are through hole. The first 2 have 0.09" lead spacing, but I have used them with 0.1" stripboard. If you are prepared to go SMD and use SOT23 packages, then there are much cheaper mosfets available than the ones I listed.
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Finally got the parts and some time to test out the circuit. I got STU85N3LH5's in the end. After a couple of hours of hiccups I can glady say its working smiley first I had a strange ground problem that was the cause of some very weird behavior and then I hooked up the 74HC240 not realizing that the pin out is such that the inputs are staggered from side to side of the device  smiley-red

Thanks David as always! That should be the hardware aspect sorted so I can start building it and using all the channels I will need smiley

« Last Edit: February 23, 2013, 09:07:24 pm by dtokez » Logged

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