How can I trigger a string of PWM RGB LEDs using a solid state relay without introducing resistance and significantly dimming the LEDs ?

Submitted by irreligiosity t3_z27xi1 in DIY

I'm doing a Christmas lights show this year with addressable pixels and want to include some of my old RGB LED lights, but the inline PWM controller for them prevents me from rapidly switching them on and off because it takes a second to boot up. I figured as a solution I could just keep the controller powered and use a relay to switch the power to the LED strand, but this results in the LEDs being dimmer than normal.

I tried using both a BJT and Mosfet in conjunction with the solid state relay, but it didn't make any difference - the LEDs were still dimmer than they should be. How can I pass the PWM signal from the controller to the LEDs without adding a significant amount of resistance and dimming the lights?

The light strand is a 24v, 300mA rgb LED with a built in PWM controller.

The solid state relay is SSR-25DA 25A 250V

The Mosfet I tried was a 2N7000G

The BJT I tried was a MJE172

Thanks for the help.

​

**Resolved - going to use SPDT relays. Thanks for the help.

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ntyperteasy t1_ixf63yr wrote

This is a bit complicated for a DIY thread, but here, hold my beer...

I found one data sheet for that relay and it says it has a 1.6 V "voltage drop" which is an indication of the loss in the relay when on. This is a poor man's way of specifying the "on state resistance". This is why you are getting such a dim output. Since you are controlling the low level signal after the PWM controller, the voltage drop is too much. If you insist on doing it this way, then you need a way to increase the voltage again, but that seems a bit ridiculous...

If you buy the SSR from a place like Digikey that lets you sort by specifications and has detailed datasheets, you can sort them by the "on state resistance". They have models that range from 0.001 ohm to 10 ohms. I will guess you have one on the higher end of this range...

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irreligiosity OP t1_ixf8oxo wrote

Replying to ntyperteasy (#623,612)

FWIW, I also asked in the electrical engineering sub-reddit, but they have less traffic. I figured it is still technically a DIY question as I have no EE experience outside of DIY.

I was under the impression (maybe mistakenly) that I was able to get around the relay voltage drop by using the Mosfet. I had hooked up the gate pin to the output of the relay, the source pin to the PWM source and the drain pin to the LEDs. I guess I thought once the FET was open, the voltage would flow through it as if the relay wasn't even there.

I'm mostly just trying to solve this problem with things I have on hand because I've spent too much this year switching over to addressable pixel LEDs. I know I can technically buy a dmx receiver that outputs PWM for LEDs, but I'd unfortunately need several for different voltage strands.

Thanks for the help though.

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Reverse-Backward t1_ixfaylz wrote

Shouldn’t you be able to do this using an interposing relay?

Typically used where low voltage triggers high voltage or vice-versa.

An interposing relay should just have dry contacts that close (or open) when you want to command the control coil to operate. Two different power levels can exist on an interposing relay with no influence on each other. Therefore no dim LED’s once you close in the contacts.

https://control.com/textbook/relay-control-systems/interposing-relays/ Ignore the overly complicated explanation in this linked article, but focus on how they show the DC relay on the PLC side (controlling device) connected to the relay coil which then closes the other circuit to the motor starter (your LED lights)

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ntyperteasy t1_ixfcnbh wrote

Replying to irreligiosity (#623,870)

Do you need to switch fast? Why not just a cheap mechanical relay? The voltage drop should be vanishingly small. Should be good for a year or two... And assuming your PWM is around 100 Hz, most AC rated relays should be ok with that. It's not RF....

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Zweefkees93 t1_ixizflw wrote

100hz on spdt relays might be possible. But why not use mosfets. A good (let's say 50% oversized) mosfet has a on resistance of about 0.01something ohm. At 300mA this means a voltage drop over the mosfet of at most 10mV which is basically nothing. And those are able to do 1khz easely. Even 100khz without to much trouble... 100hz on relays wont be reliable to actually switch on every time. And the relays will burn out quickly. What was the problem with the mosfet? Do you have a schematic how you connected everything? I have an electrical background, I'd be happy to help!

Ps: the mosfet you used is only for 200mA... Not 300. Or did you use the mosfet to switch the solid state relay? If you only use the mosfet it'll be wayyyy easier

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Zweefkees93 t1_ixml8o1 wrote

Replying to irreligiosity (#651,236)

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:

  1. The mains> 24v is the led powesupply/controller. This needs to stay powered to prevent it from taking forever to boot?
  2. 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 ;).

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irreligiosity OP t1_ixmw5ln wrote

Replying to Zweefkees93 (#651,574)

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.

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Zweefkees93 t1_ixnvbjg wrote

Replying to irreligiosity (#652,867)

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)

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Zweefkees93 t1_ixpuf1d wrote

Replying to irreligiosity (#652,867)

Here is the schematic.

https://imgur.com/a/1g77Lo8

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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