Hi
I'm having a bit of trouble regulating voltage and current from my power supply its a S-150-5 (5v 30A) but i want to reduce it to 3.3v 150ma to drive TLC5940's, i have tried using a LM317 to get the voltage down to about 3.3v but every time i connect one of the chips it blows. All i can think of is the current is still to high. if anyone has any ideas it would be really helpful
thanks
What voltage are you reading on Vout of your regulator?
What is the "it" that blows - the reg or the TLC5940?
Can you post your circuit?
its the tlc5940's that keep blowing the voltage coming out of the regulator is 3.374 i noticed when i turn the power supply on it spikes at about 7v could that be whats blowing them ? how can i stop that spike ?
how can i stop that spike ?
You can check your regulator circuit - it shouldn't be spiking!
I'm using the following diagram
And I'm Using a 130ohm for R1 and a 220ohm for R2
this has me puzzled
What about a switch after the resistors? Power it up THEN flick the switch to on.
Yeah that would work, even maybe some type of delayed relay ? but i would still like to know why this circuit is not working.
I would have expected some caps for transient suppression - have you got any fitted?
What does the datasheet suggest?
I agree with the capacitors suggestion.
Also, how did you map pin numbers to Vin, Vout, and ADJ? Any chance you got that wrong (1=ADJ, 2=VOUT, 3=VIN for the TO-220 package).
MUST have output caps with the LM317.
Their voltage fluctuates wildly without them.
Recently we blew up the .1uF output cap we had on ours and decided to just desolder it and forget it.
Toasted 2 sensors in a row before scoping the PS and finding the spikes. Solder in a new .1 and all has been well since.
We also have a 2200uF on the input side of the 317 to ensure it gets a nice steady voltage in, coupled with another .1uF
I followed the advice with .1uF capacitor on the output of the LM317, except that I had managed to put the capacitor between ADJ and ground instead of VOUT and ground. For some reason this worked fine though.
When I discovered my mistake I put the capacitor between VOUT and ground, and now I have blown 3 capacitors.
Looking at the datasheet they first of all say that it is an option to use a output capacitor, but also they indicate a value of 1uF.
Could this be the reason why the caps are blowing up, that they are too small?
hc
and now I have blown 3 capacitors.
I find this amazing, you just don't blow up capacitors unless you over voltage them or connect electrolytics the wrong way round or exceed the ripple current. I don't see how on earth you could be doing either of these things. :o
What input voltage are you applying?
Glad to amaze you.
Vinput is +9v, I have a 1.8 ohm resistor between ADJ and Vout, and the schematic is like in this picture (from an instructable). I first placed the .1uF between 1(ADJ) and gnd, then moved it to 2(VOUT) and gnd.
in my setup the pwm pin is an output from the tlc5940 (with a 10k pullupresistor).
As I mentioned before, it worked flawlessly when the capacitors were between ADJ and gnd.
hc
it worked flawlessly when the capacitors were between ADJ and gnd.
Now that don't surprise me. What does surprise me is that any idiot would think what you posted could possibly work.
How do you know the capacitors have 'blown', is there physical damage?
I'm just glancing at this... but... HC
To me... that's the CURRENT regulator configuration... which means you are NOT controlling the voltage... I realize that you are PWM'ing the ADJ... but that's not a good voltage regulator circuit.
I didn´t invent this circuit, and it seems to be a quite common circuit for controlling powerleds.
Here is from the instructable I used as startingpoint:
"The LM317 regulator gives out a constant voltage of 1,25 volts between ADJ and Vout, so by adding a resistor between these two outputs, you'll get a constant current.
Ohm's law says that U/I=R, which means that Voltage divided by Ampere makes resistance.
so if you want to connect one or more luxeon 1W LEDs, which has a power consumption of 350mA, the calculation should look like this: 1,25 (the constant reference voltage of the LM317) divided by 0,350 (the LEDs power consumption) makes 3,57. So if the resistor is 3,57, constant current will be 350mA. The closest E12 value is 3,9 ohms, it will give you a constant current of 321mA. However you can't see any difference in the light output.
If you use 3W LEDs, which has a current consumption of 700mA, the calculation should be: 1,25 divided by 0,7 makes 1,78. The closest E12 value is 1,8 ohms, the output will be 694mA
the resistor must be at least 1W in both calculations."
Somewhere in the comments the author of this instructable adds the diagram I linked to in the previous post.
Anyway, if you have a suggestion for a better way of using the LM317 with a TLC5940 running a 3w led on each output channel, I would love to see it, since I am no expert.
hc
1.8 ohms between VOUT and ADJ??? That's 1.25/1.8 = 694mA!
I don't think this circuit is what you want at all.
Ok we seem to be drifting from the original question. I get the feeling you are not telling the full tale, or at least it is coming out in dribs and drabs.
You started out by saying you were regulating 9V down to power the TLC chip that kept blowing up. We have now abandoned that and are looking at a constant current drive of an LED with PWM.
I would consider that this is a highly unstable circuit and I am not surprised you can't get it to work. You have no decoupling on the input nor the output of the LM317, in other words nothing to damp the instability. You are switching rapidly large amounts of current.
What is the current you are trying to drive through the LED?
Is it 150mA? What is the forward voltage drop across the LED?
I would imagine you have at least 1W of power to dissipate here.
The instructables did not have a PWM switcher in the -ve lead of the LED although it said it was compatible with PWM it did not say how. Unfortunately just because it is on instructables doesn't mean it is any good. Adding the PWM makes an unstable circuit even more unstable.
From the instrutables he says:-
I couldn't get my schematic drawing program to work, so here is a hand drawn.
So he can't get a drawing package to work but he can design a circuit?
if you have a suggestion for a better way of using the LM317 with a TLC5940 running a 3w led on each output channel, I would love to see it
Well if you paste me into all those conditions then no I don't have a better circuit but as your circuit doesn't work I don't see where you win.
RuggedCircuits: Yes, I want 700mA, that´s what the 3w led needs.
GrumpyMike:Yes, I use decoupling capacitors on the powersupply, and tried doing it on each lm317 as well (as you mentioned in another thread).
I haven´t blown up any tlc5940s, I have blown up 3 .1uF capacitors, and
I did not power the tlc5940 with 9v, please reread the post before giving me more aggressive feedback.
the instructable does mention the pwm circuit, but in the comments, as I wrote.
The circuit was not unstable before I moved the .1uF capacitors, I have had it running for hours with 11 3w leds.
hc
I did not power the tlc5940 with 9v, please reread the post before giving me more aggressive feedback.
Yes sorry I didn't realise you had hijacked the thread from the original poster.