tlc5940 and MOSFET driver

Hi there to all.
I have started a project idea using a
Tlc5940 and MOSFET for switching a 24volt DC led string of lights. Load is between 150ma and 400ma per set. Am I right or wrong in thinking I need a pull up resistor on the TLC output if I'm using an nfet. The irlz14 primarily.
Also will I need a logic invertor. Sorry for my lack of knowledge as I'm very new to this and welcome any input.
Many thanks indeed

Matt

The TLC 'output' is a current sink. When it switches 'off' you don't see a high voltage, you see a high impedance.

So yes, you need a pullup for when the TLC switches off. This inverts the logic...

excellent thanks for your help and quick response. i may need to call on your expert knowlege if i hit anyfurther walls. i can post a schematic of my setup if it helps.

Hi there again.
I am in the process of putting together a breadboard test for the driving circuit. Going back to the pull up resistor. Will I need to wire a 24v feed via resistor to the gate of the fet. Or will 12volt via resistor be enough to fire the gate. Is the extra voltage required as the TLC on its own isn't enough to activate the gate via pwm.

Thanks for your help I appreciate it.

Matt

If you wire a pull up resistor to 24V you will fry the TLC5940.
However you only need 12V to turn on most FETs.
However you need to invert the logic, a pull up resistor will not do this for you.

Thanks for the quick response. So I need a logic invertor then. Otherwise otherwise when gate drive is the output is low. Have I got that right. Then I will need a logic invertor. Would the lm704sn work. Is there another way to invert the logic. What about a 2nd transistor in circuit somehow.

Many thanks

Matt

So I need a logic invertor then. Otherwise otherwise when gate drive is the output is low. Have I got that right.

I am not sure, the TLC powers the LED on when the output is pulled low. If you are using an external n-type FET to power your load you need a high to turn it on.

Would the lm704sn work.

Never come across that one, neither has google.
A 74LS04 would work. You get 6 inverters per IC.

Is there another way to invert the logic. What about a 2nd transistor in circuit somehow.

Yes you could do that.

Oops sorry that was exactly the one I meant to type in.. Bad type moment I think.
Is there a generic diagram for putting this together. I believe logically the fet will feed the input of the second transistor to create a loop Which will then sink the output back through the TLC pin. Thanks again for helping me with this.
Just another thought could I use bi polar pnp transistor.

Thanks matt

I believe logically the fet will feed the input of the second transistor to create a loop Which will then sink the output back through the TLC pin.

No you don't want to loop anything back through the TLC.

Attached is something I just knocked up.

TLC.pdf (20.8 KB)

Many thanks. I will look at the attached shortly. I can't seem to get attachments to work on my mobile. I'm sure I will have questions. Thanks again.
My plan is to ultimately have 3 TLC controlling a total of 48 channels. But its still early days.

hello mike

thanks for that attachment i have had a good look at it. is the triangle a diode by any chance and the resistors will be selected at 1k ohm each. lastly will i need to attach all neutrals together or should the control 5v side arduino tlc be seperate from the 24volt side.
just trying to get my head round it all.
thanks

matt

is the triangle a diode by any chance

No it is an inverter, one sixth of the 74LS04.

chance and the resistors will be selected at 1k ohm each.

It depends on:-

  1. If you use a transistor then it will depend on the current gain of the transistor and the current draw of your LED strip.
  2. If you use a FET then you should make it about 56R

will i need to attach all neutrals together

Yes but they are grounds. Neutral is only used for AC mains.

Thanks for that. Of course your right. I'm going to start experimenting in the next 10mins or so. Sorry for my lack of knowledge on the symbols. I'm an electrician by trade. But I have always wanted to get into electronics for years.
The circuit will still work without the logic invertor but backwards in terms of output if I'm right.
I really should probably measure the current draw of the strings with the controllers cut out 1st.
Thanks again
Matt

The circuit will still work without the logic invertor but backwards in terms of output if I'm right.

Yes but note that you will not be able to turn them off fully, they will always be dimly on.

well ive setup the circuit and am getting next to zero output from the lights. there is one of those 8 channel controllers in line with the leds though. should i remove it as suspect that maybe causing the problem. im using 56 ohm resistors. the crazy thing is when i disconnect the 5volt side the lights stay on a half brightness. any ideas.? i am a bit

sorry i hit the enter key

confused as to why its not working.

thanks

matt

there is one of those 8 channel controllers in line with the leds though.

What does that mean?
I am assuming your LED strips just need 24V to turn them on is that not the case?
Can you post a schematic of what you have wired.
What value have you got for the iRef resistor?
Can you light an LED if you put it between +5V and an output pin?

hi mike,

i have put a voltage check across the string and i seems to be 12 volt dc, the psu is 24volt ac. so there is a rectifier within the controller. I am going to need to rectify the string before i test again. in the mean time i have purchased some logic invertors from rs online and a 12volt rgb strip to test with next week.
could i simplify this design by using a p fet instead, i have some tip32ag.
thanks
matt

scematic1.bmp (750 KB)

the 8 funtion controller fades out the leds switches randomly etc,

This is the first we here of a controller, what sort of LED strip is it?
Are the LEDs individually controlled or is it the strip as a whole?
What are you actually trying to acheave?
In general you can't go giving PWM to an active device like a controller.

No that circuit will not work because you can only turn that transistor / FET off when you take the base / gate up to 24V and a pull up to 24V is too much for the TLC chip.