I'd suggest a pragmatic approach. Remove C5 and check if the circuit is behaving according to your needs.
If you look at LM317 load transient response your 2 ms rate is the
dominant system characteristic. The 317 is more than capable of
handling that.
View attachment 163156
Regards, Dana.
Hi,
Now we know you have an adjustable regulated power supply.
So why still the LM317? I vote for a simple resistor plus a mosfet for PWM.
In simples case two parts.
Where do you see a drawback?
Klaus
Substituting ordinary diodes for LEDs doesn't give an accurate simulation. What you are doing is trying to control the brightness under constant current conditions by 'chopping' the output from the LM317 between constant current mode and constant voltage mode. The constant voltage being a little over 1.25V and hence insufficient to turn the LEDs on.
If I could make a suggestion: wire the LM317 as a simple current regulator (load connected to Vadj and series load resistor in Vo) and move the transistor to the bottom of the LED chain. Functionally it will work the same but it avoids all the problems of regulator recovery time and the current will still go to zero when the transistor is turned off. For even better results, use a MOSFET instead of bipolar transistor.
Brian.
So for this project, I have split the responsibility of the circuitry in different boards. This specific board which powers the leds receives 30V and 5v PWM.
If I am to drive 2 opto couplers (per board) with 5v pwm then I am not bound to search for mosfets with logic level. I could have the opto couplers drive the mosfets with 30V if needed on the gate correct?
Also I learnt that the more voltage on the gate (within limits obviously) the less resistance between the source and drain and the less heat it generates. So considering that:
each led drops between 3.2 and 3.4v, and that I have 7 leds in series, they will drop at least 22.4V and and most 23.8V and the LM317 will drop a further 1.25+3v leaving the mosfet potentially with only 1.95V @ a max of around 600mA ie: around 1.17W ? Is that correct? What about the voltage will it be able to function properly? Im not sure how to choose an N channel mosfet.
have you done the power calculations for the LM317.. ?
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