Instant-on 12v power supply for switched 120vac outlet? (LED homemade fixtures)

eggsalad

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Hi! First time poster in this subforum, but I've been on CPF for quite a while. Hope you folks can help.

I've built some architectural interior lighting using both 12v LED tape and "sign modules". They're all plugged in to 120vac outlets controlled by wall switches.

The problem I'm having is that I've tried all manner of 12vdc power supplies in my (copious) bins of "stuff" and none of them will illuminate the LEDs instantly. Some take as much as 3-4 seconds (!) before the fixtures light up.

Can someone please point me to a PSU that will illuminate the fixtures instantaneously (or darn near) when I flip the wall switch?

Thanks!

edit: I need units in the 3-5 amp range
 

Dave_H

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I checked a number of older 120vac wall-plug and "brick" type 12v adapters kicking around, up to 3A output, and most came on "instantaneously" i.e. within 1 second, albeit at light load. One took 1.5-2 seconds.

These are older "freebies" so brand names and models are likely not useful.

Properly-designed supply will have some delay to limit inrush current. One that came on very fast caused a "snap" at the ac plug indicating poor or no inrush limiting.

Specifically what type of supplies have you tried? What is your upper limit of delay (1 second)?

Dave
 

eggsalad

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1 second would be a lot. I have plenty of off-the-shelf LED fixtures on wall switches that illuminate in well under 1 second, as do E26 LED "bulbs".

I've tried probably about the same types you have. Wall warts are better, "bricks" are worse. But there has to be something faster.
 

Dave_H

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Supplies which come on very fast may have a lot of inrush current which is not easy on switch or relay contacts. Could you possibly switch the dc outputs, if supply can be left on?

Purpose-built LED drivers may well come on quickly but a dc supply meant for another purpose e.g. charger, with no requirement to turn on quickly, seems normal.

Some LED E26 bulbs I've opened use linear current regulation (LEDs stacked to appropriate total Vf) with no apparent large input capacitor which means they can come up very fast.

Dave
 

turbodog

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Dave_H

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I checked my supply. Put meter probes on output, flipped switch on, plugged in rapidly. Meter read 13.8 before I could push the plug all the way in. Probably 1/4 second

Nice but a bit pricey and overkill (for 3-5A mentioned), perhaps OP can mention price range.

You can get 12v from PC power supplies, not sure how well those work but may be a bit bulky.

Dave
 

fulee9999

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I imagine the "sign modules" cause the delay between the power supply being connected and the leds coming on...?
If you try it with a power block like MEAN WELL LRS-50-12 or a PC power supply does it lag as well?
 

eggsalad

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Purpose-built LED drivers may well come on quickly but a dc supply meant for another purpose e.g. charger, with no requirement to turn on quickly, seems normal.

Great! Where do I find a purpose-built LED driver?
 

jtr1962

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Can someone please point me to a PSU that will illuminate the fixtures instantaneously (or darn near) when I flip the wall switch?

Thanks!

edit: I need units in the 3-5 amp range
Probably the only type where you're highly likely to get "instant-on" would be old-school supplies using 60 Hz transformers and linear regulators. A 12V, 5A supply like that is going to weigh probably north of 5 pounds, and it'll get at least very warm in use, if not hot. There's a good reason they started making high-frequency switching supplies. Greater efficiency, much smaller transformers, less waste heat.

The delays you're experiencing are a product of design. A lot of these supplies slow start to limit the inrush currents into the filter capacitors and rectifier diodes. Another reason might be power factor correction.

I've mostly designed/built drivers for automotive uses, with a few other similar applications thrown in. In all cases these were DC/DC converters where I had a raw source of DC power available. Yes, they come on instantly, probably within milliseconds, but they wouldn't be much help to you here since I assume you want a 120VAC to 12VDC supply. I never designed or made those, although I understand the principles involved. The reason why was the market is already saturated with such supplies selling for far lower cost than I could make them for, and there are a bunch of liability and product certification issues for anything using 120VAC.

There are two ways I'd move forward on this if it were my project. Obviously, one way is to keep trying supplies until you hit on one that works. That Competition Electronics supply sounds like it fits the bill but is overkill for what you want.

The other way is a hack which frankly I think would be more trouble than it's worth. I'm thinking something along the lines of a small 12V battery which charges from the power supply. A small lead-acid battery would do fine on a 13.8VDC supply. From the battery you have a 120VAC relay to connect to the LED strip. The relay coil is connected to the same 120VAC as the power supply. When you turn the switch on, the relay goes on almost instantly, connects the strip to the battery, and the strip goes on instantly. You don't need a huge capacity battery since it's only carrying the load for a few seconds, until the power supply kicks on. Once the supply is on, it powers the strip, and float charges the battery. You would need to put a diode between the supply and battery so that the supply doesn't see the battery voltage when it's off. The diode would need to be rated for at least as many amps as the LED strip.

If supercaps didn't self-discharge so fast, this would be a perfect application for them. To carry 5A for maybe 5 seconds with a 1V drop, you would need a supercap of at least 25F, and rated for at least maybe 15V. But due to the self-discharge, this isn't a good application for a supercap.

EDIT: Not exactly what you're looking for but here's a 24V, 72W supply:


Now the reason I linked to this was I converted a circline fluorescent fixture in the bathroom to LEDs using the same brand driver, but in my case I used their constant current (350 mA) driver. It comes on within maybe 1/2 second maximum. I figure the one I linked to might function similarly since it's the same brand.

They do have a 12V, 4A version according to the datasheet. If it turns on quickly, it might be just what you're looking for. I couldn't find the 12V version on either Amazon or Future Electronics but someone must sell it.
 
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eggsalad

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Probably the only type where you're highly likely to get "instant-on" would be old-school supplies using 60 Hz transformers and linear regulators. A 12V, 5A supply like that is going to weigh probably north of 5 pounds, and it'll get at least very warm in use, if not hot. There's a good reason they started making high-frequency switching supplies. Greater efficiency, much smaller transformers, less waste heat.

You're absolutely on to something here. The PSU from my collection that works the best is a *very* old Radio Shack (that's old!) 12 volt regulated power supply.

The other way is a hack which frankly I think would be more trouble than it's worth. I'm thinking something along the lines of a small 12V battery which charges from the power supply. A small lead-acid battery would do fine on a 13.8VDC supply. From the battery you have a 120VAC relay to connect to the LED strip. The relay coil is connected to the same 120VAC as the power supply. When you turn the switch on, the relay goes on almost instantly, connects the strip to the battery, and the strip goes on instantly. You don't need a huge capacity battery since it's only carrying the load for a few seconds, until the power supply kicks on. Once the supply is on, it powers the strip, and float charges the battery. You would need to put a diode between the supply and battery so that the supply doesn't see the battery voltage when it's off. The diode would need to be rated for at least as many amps as the LED strip.

You say it's a hack, and maybe you're right. But I have almost all of the parts in place! The original design intent was to run this fixture from a 12V battery bank charged by solar, complete (as you described) with a 120VAC relay. Turned out I simply don't have enough solar capacity, but this means I have everything I need for your "hack" aside from the diode.

I would be really grateful if you could name and/or link the *exact* diode I'd need, and a sketch of the schematic. All problems would be solved.

Thank you.
 

jtr1962

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You're absolutely on to something here. The PSU from my collection that works the best is a *very* old Radio Shack (that's old!) 12 volt regulated power supply.
I have one of those. It's a 13.8V, 3A supply. Big and heavy compared to AC adaptors which perform the same function.
You say it's a hack, and maybe you're right. But I have almost all of the parts in place! The original design intent was to run this fixture from a 12V battery bank charged by solar, complete (as you described) with a 120VAC relay. Turned out I simply don't have enough solar capacity, but this means I have everything I need for your "hack" aside from the diode.

I would be really grateful if you could name and/or link the *exact* diode I'd need, and a sketch of the schematic. All problems would be solved.

Thank you.
The diode isn't critical. Any silicon diode rated for at least, say, 5 amps will do. If you have common diodes like the 1N4001 just put maybe 5 of them in parallel. Or you could get these on eBay. Maybe put 3 of these 3A diodes in parallel so you have a margin of safety. The power supply may well already have circuitry to prevent it seeing any battery voltage when it's off, so the diode might not be needed.

Here's a quick and dirty schematic:
LED Strip Schematic.jpg

Use the normally open contacts on the relay to connect the LED strip to the battery.
 

turbodog

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Given the couple of watts an always on supply would be... just leave it on 24/7, then switch the output (like the picture). You just need a separate a/c supply for the circuit. And use a cap instead of a battery... less to replace over time.
 

eggsalad

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Given the couple of watts an always on supply would be... just leave it on 24/7, then switch the output (like the picture). You just need a separate a/c supply for the circuit. And use a cap instead of a battery... less to replace over time.
Another fine idea! I appreciate your insight!
 
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