Cortezz
Junior Member level 3
Hi!
I designed a dimmer for 230VAC 50Hz for a few GU10 LED bulb with the classic 555 PWM circuit.
It basically works but I have some major issues could you help me to improve it?
The basic concept:
First problems were:
Trials for problem #1:
Trials for problem #2:
Remaining issues:
Questions:
Thanks in advance!
I designed a dimmer for 230VAC 50Hz for a few GU10 LED bulb with the classic 555 PWM circuit.
It basically works but I have some major issues could you help me to improve it?
The basic concept:
- 555 based PWM tuned around 500Hz
- rectified AC for the LED bulb (the 10k on the circuit)
- a high voltage BJT (BUX 87) for switching
- a (darlington) opto for separating the two
- 15V zener to protect the opto from high voltage
- at the very first design R9, C7, C8 werent there at all
- R10 is just there for LT Spice
First problems were:
- at zero pot level the bulbs were glowing already
thought: even at zero level the 555 produces some spikes and with the darlington opto it simply turns the BJT on - the dimming was working partially but there was some shimmering in the light level
thought: as the PWM has some different frequency and phase as the mains 50Hz they were interfering
Trials for problem #1:
- adding R9 to make the BJT less sensitive to low currents
result: worked partly: but wans't stable the min and max level was moving continuesly with temperature - adding C7 to speend up the BJT switching
result: not much
Trials for problem #2:
- adding C8 to smoothing the rectified mains voltage to avoid the 50Hz vs PWM phase interfering (attachment)
result: works
Remaining issues:
- I can't achieve a stable zero, intermediate and max state with the dimming
- at max level the dimmer starts to blink at ~2Hz the bulbs for some reason (BJT temperature oscillation?!)
- at higher level the BJT gets very hot
Questions:
- Is it possible somehow to achieve a perfect zero PWM level using this 555 layout?
- How should I change the working to a more digital kind of operating?
I mean now it's more like an analog type as the BJT starts to turn on even a low current.
And the LED bulbs are very sensitive so I guess that's why they glow already on the min pot level.
There should be a hysteresis maybe. Or should I use some other digital opto rather then this darlington type? - Should I use rather a MOSFET (IRF730) for switching? (faster switching, higher turn-on-voltage-threshold, etc?)
- Do you have any other suggestions to improve this circuit?
Thanks in advance!