Zweefkees93 t1_ixml8o1 wrote
Reply to comment by irreligiosity in How can I trigger a string of PWM RGB LEDs using a solid state relay without introducing resistance and significantly dimming the LEDs ? by irreligiosity
Ah eehhh yea that's a bit weird. I'm at work at the moment. But I'll draw a quick schematic when I have time later today.
Am I correct in assuming:
- The mains> 24v is the led powesupply/controller. This needs to stay powered to prevent it from taking forever to boot?
- The 12v PSU is your controllsignal that switches rapidly to flash the leds?
It might not be that simple by the way. Yes you can control the leds by mosfet easily. But the pwm controller probably is controlling the current to. So I'm not 100% sure what will happen if you turn off all the leds behind it. (Probably won't blow up, but might go into error state and needs to reboot). I have made these pwm controllers myself. So I don't have experience with the storebought controllers.
Either way. I'll draw a schematic based on these assumptions, if incorrect I can change it ;).
irreligiosity OP t1_ixmw5ln wrote
The 12v PSU is just a variable control DC power supply that I used to trigger the relay. Mains -> 24v is the LED power brick & controller. The mosfet and relay are between the 24v and the LED strip.
Energizing the relay does cause the 24v to drain to the LEDs, but they are dimmer than if I just energize the LEDs without the relay/mosfet between them.
Zweefkees93 t1_ixnvbjg wrote
Ok, I'm guessing it's a RGB strip? Do you know if it's common anode or common cathode? It might be in the datasheet. Basically, are all the plusses form the leds connected together and the minuses go to the controller per color. Or is it the other way around? Most likely the connector has 4 pins. One of them is connected to all the leds, the other three are connected per color. You need to switch the common one. (Or all three of the other ones, but then you need 3 MOSFETs instead of one). The only downside is, if the common is the plus (which it most likely is) you need a p channel mosfet, they are a bit more expensive and less efficient. But should still produce a drop in power that shouldn't be visible.
Sorry, I had to work until 23.00 today. So i didn't have time for the schematic. But I looked at your schematic a bit better. You used a N-channel mosfet. There is a bit of nuance there. But it boils down to: The mosfet needs to be in the negative lead. So 24v+>led>mosfet>24v-. Using N-channel is the positive rail is possible, but not without extra drivers/boostcircuitry. But using your schematic you van just remove the MOSFET all together. A MOSFET, solid-state relay, spdt (or any non solid-state relay), bjt, and a bunch of other devices are basically switches. Sometimes you need to use a MOSFET to switch a relay, but the other way around is almost never the case. And that is what your doing in that schematich. Just remove the MOSFET and switch the positive rail with the SSR only. That should work just fine. I'm guessing the dimming effect is caused by the wrong usage of the N-channel MOSFET. But as some said before. SSR's, mosfets and a whole host of alternatieves range in their "on resistance" (in other words, the amount of voltage that drops over the switching device, resulting in the dimming of the leds) form 1 miliohm to over an ohm. On 240v that doesn't matter to much. Because 1v drop is still 239v. Or not even .5%. but 24v becomes 23v. Which is over 4%.
If I get a second tomorrow I'll draw the schematic! (I'm in industrial automation. So work tends to get a bit crazy when machines won't work as they are supposed to do :P)
[deleted] t1_ixnvz5x wrote
[removed]
Zweefkees93 t1_ixpuf1d wrote
Here is the schematic.
The mechanical relay and SSR are basicly the same thing. You just need to be sure that it is capable of switching DC. Some SSR's require an AC signal because they switch in the zeropoint. If it can switch AC it doesnt matter where the LED's are. (between plus and relay, or between relay and minus)
Mosfets can switch waaayyyyy faster. And dont wear out if you switch them a thousend times a second for 20 years. (yes, thechnicaly there is some wear by flowing current thrue the substrate. But in thes application its completely irelevant. This becomse relevent when the conductors with is counted in atoms instead of mm^(2). ). With mosfets it does matter where the LED's are!
Be carefull when selecting the mosfet you use. Like I said, low R^(on) means more efficient and less power (brightness) loss. The voltage on the gate should be below the max gate-drain voltage. But 12v is usualy ok. And P-channel mosefts tend to be more expensive and have a bit higher R^(on) value.
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