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EDIT: I have replaced the FL7732 circuit with the circuit I actually have ("D29B 6L15"). In addition I've added another question.

Please also note that I've accepted that using the 30W driver is not the way to go. I am asking, because I want to understand SMPSs and how they behave.

Where do I find reasonably priced LED-driver, which I can modify for variable currents?
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This is a follow up question to modify 30W led driver

Here is the schematic of the led driver: enter image description here

I have added a FET on the secondary side and PWM the LEDs. As expected the voltage of the capacitor rises and the brightness of the LEDs changes only very little until ~20% PWM at which point the LED driver apparently can't rise the voltage any longer.

The LEDs are rated twice the power I use them. Receiving a little bit more current while the PWM turns them on should IMO not hurt them.

However I don't know if this could hurt the driver itself.

So my questions:

  • is controlling the brightness using a FET and PWM safe for this circuit?
  • what could happen / what should I pay attention to?
  • is there a cheap / easy way to use existing LED drivers (flyback) to change the brightness of the LEDs.
  • The FL7322 circuit has a diode, resistor and capacitor across the primary winding. How important are those.
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closed as unclear what you're asking by Andy aka, Finbarr, RoyC, PeterJ, uint128_t Feb 25 '18 at 15:08

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Why are you not showing the schematic of the circuit you actually used? Who cares about a circuit that is similar? \$\endgroup\$ – Andy aka Feb 22 '18 at 9:59
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    \$\begingroup\$ Please post the actual schematic you're using, in clearly legible form, and specify the chip you're using. Otherwise you're just wasting people's time. \$\endgroup\$ – Finbarr Feb 22 '18 at 10:23
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    \$\begingroup\$ Seriously bad idea to PWM a current source. It will attempt to pass the designed current and generate whatever voltage it needs to do so. Nothing good will come of that. You'd be better off understanding how that current is generated (what CS (pin 1) is compared against (possibly the voltage on VS (pin 6) and controlling that. Of course, that means working at mains voltages and all the safety implications. Realistically, get a 30W isolated supply at the right voltage and start over. \$\endgroup\$ – Brian Drummond Feb 22 '18 at 12:59
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Secondary side PWM is no good .Bimpelrekkie has stated some reasons why.You could discard the cheap chip but it does have a low parts count even if EMC and inrush current issues are addressed.If you want to keep the chip you could experimentally change R sense to a larger value and note the lower LED string current giving a dimmer result.You should be able to get a reasonable control range .If the control range is enough for you then hook up a mosfet and an optocoupler to get things safely isolated from the mains .The mosfet will be cheap because the voltages and currents it sees are low .The opio can be a cheap 4N38 because speed is not important here.The mosfet does not waste any more power because it is acting as the current sense resistor .

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