| Electronics > Projects, Designs, and Technical Stuff |
| 9VDC + 5VDC SMPS for driving LEDS |
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| Blade2Raiden:
--- Quote from: blueskull on February 10, 2020, 05:23:39 am --- --- Quote from: Blade2Raiden on February 09, 2020, 08:21:36 pm ---design is quite simple, I like it, but I'm going from main source so 120VAC, so I'd need something to step-down first and a transformer might be too heavy and probably won't output enough power for my application, I'm wondering if there's IC's powerful enough for that (well I think so after LED light bulbs work this way), 3 amps is my minimum though and I must be able to add a 5VDC output, that's where I'm quite lost --- End quote --- If you are asking questions like in the OP, you shouldn't touch 120V at all. Get a factory made SMPS module (12V 2.5A seems good), step it down to whatever voltage you want, and use it. Messing with mains without knowing what you are doing is one of the quickest ways to win a Darwin award. Also, stay away from GaN switches unless you know what are you doing. --- End quote --- I'm an electrician. I don't "fear" 120V, I know how to work with it. 12V 2,5A would be enough, you're right and it would simplify parts needed but how do I current drive 1,5A? That's a little harder to achieve, a TO220 transistor maybe? If you know a good one tell me. |
| tooki:
--- Quote from: TerminalJack505 on February 10, 2020, 01:28:02 am ---You can drive the circuit I posted with PWM from a MCU. The duty cycle would have to be inverted if you use it as shown. That is drive it with a 70% duty cycle to turn the LEDs on 30% of the time. If you are making just one of these and you use a 30V supply then another thing you can do is string 3 LEDs in series along with a hand picked resistor (nominal 27 \$\Omega\$, 1/2 Watt) and then tie all these strings to either a logic-level N-channel mosfet or an NPN BJT (of sufficient power rating.) You can then drive the MOSFET/BJT with PWM from the MCU. Also, if you need the MCU only for PWM then you could instead use a 555 timer for the PWM. Dave had a video where he does pretty much what you are wanting to do. (Although it looks like he switches the LEDs from the high-side.) --- End quote --- FWIW, I have had MUCH better results with this LM393-based PWM dimmer than with any 555 dimmer I’ve tried: https://www.electro-tech-online.com/articles/simple-pwm-circuit-for-lamp-dimming-etc-improves-on-555-pwm-circuits.797/ I built the circuit without the CD4050, configured for ~20KHz switching, with the PWM output feeding directly into an IRL540N MOSFET. It handles the 12V/1A load I have without even beginning to get warm, and testing at 5A with a tiny heatsink still produces absolutely negligible warming. What I like about this circuit is that it allows FAR better dimming down to low levels. |
| Blade2Raiden:
For those who haven't read all the answers these LED's are current driven so any FET transistor won't do, It'll have to be Bipolar I have access to MMBT3904 and MMBT3906 at work but I don't know their specs yet I'll need to look that up back at home |
| T3sl4co1l:
PWM control is fine here. You could also consider controlling the supply itself, but then you'd want an independent 5V supply so the MCU isn't browning out at light loads or anything (also could be something of a chicken-egg problem, say if the MCU commands the 9V supply to enable in the first place :) ). Definitely get a COTS module. It's not just about safety, it's about performance, reliability, RF emissions -- they've already checked out and solved all those issues for you, the value is very good. Strictly speaking, you should spend a good $10k+ putting your own supply design through testing; now, also strictly speaking, that only matters if you intend to sell it, so if you're running a one-off, who cares, right? But keep in mind, anyone whose radio spectrum you're stomping on, takes legal priority, and it is their right to prompt the FCC to send C&Ds or fines to you. It's also simply polite to not throw out noise. :) And to use something UL-approved, so no one loses insurance in case the thing starts a fire (whether this is your own house, an apartment shared with others, or a place of work). Tim |
| TerminalJack505:
Tooki, nice looking project. Yeah, that would be a good option for generating the PWM as well. Dave swapped pins 3 and 7 (discharge and output) on his circuit and got a duty cycle range of about 1% to 99%. Dave's circuit was running at about 400Hz, which seems like a reasonable frequency for lighting. At 20kHz, I'm surprised the MOSFET in your circuit doesn't get hot without using the 4050 to drive the gate. |
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