Hi sed!
I want to illuminate our hall with 7 white LEDs so we don't have to turn on the main light at night when we want to save energy or avoid too much light.
I started to build a SMPS with a TL494 PWM controller salvaged from an ATX SMPS [2]. Image/video gallery [0].
Before I hook it up to 230V AC (325V rectified) I'm testing it with 12V and something is wrong.
When I decrease the voltage at error amplifier 1 (EA1) with the 150K pot, the duty cycle increases which at first lets the current rise higher in the flyback primary and produces more LED current. But at some point the primary current drops sharply as if the transistor were shut off, but stops at 240 mA and then settles at 300 mA for the rest of the duty cycle, no matter how long that is [1]. The LED current drops and remains at about 0.6 mA. I want 20 mA.
At higher duty cycles the transistor gets hot.
Without the flyback connected the base current of the transistor (BULK128 or BULT118) is very clean [3]. With the flyback there are some bumps in the base current [4].
What's going on at these higher duty cycles? How can I get the primary current to keep increasing during the transistor on time?
I abused the TL494 quite a lot but without flyback it seems to work fine. Do you think it may be damaged?
Any other suggestion for improvement? I want this to be relatively efficient, so no linear PS.
Thanks, Bernhard
PS: The flyback primary is 139 mH. The secondary is self wound.
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