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Need Power Supply with AC Output


boom3

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Hi,

Not sure where this topic goes but...

I need a power supply that converts 110 VAC to 4.5 VAC.  I am trying to power strings of LEDs I got on clearance from World Market, and the  battery box (4 AA cells) has 9 functions. Short story, I hooked my trusty Fluke to the output of the box and it is putting out 4.5 VAC at 450 Hz ! My DC power supply is fine, but it wasn't lighting up the string; the battery box was, so I find the box is putting out AC.

 

I think a 60 Hz frequency would be fine, the oscillator in the box is putting out 450 Hz, I would think so as to use small components. I may just give up and use regular 12 V LED strings for this project (bookshelf lighting). Looked at Parts Express and Allied, no joy. 4.5 v is an oddball. Open to suggestions!

 

 

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Yep, it's AC. Confirmed with the frequency counter in my Fluke DMM, and also hooked the box up to a speaker and got some very interesting tones straight out British sci-fi sound effects. The 4 AAs feed a small circuit board that allows 8 effects and off. It is possible that what the DMM thinks is AC is pulsating DC. In certain modes, the speaker cone shows some deflections like it is receiving DC. Don't have a scope anymore so I can't see the waveforms.

 

After rummaging around in our pile of tech stuff, we found a reel of white LED strip and we also have some controllers made just for those, so that's the path I'm taking. The alternative would be to have the DC power supply feeding the battery terminals of each controller box and then each box controlling one string. 

 

Thanks Dave!

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Your battery box is a DC switching power supply. Your DMM is measuring the switching frequency. You'll find what you need if you look for a 4.5VDC supply.

 

Report back when you get the problem resolved. In the meantime, we'll be looking north for the mushroom cloud. 😁

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I wish it was that simple, Peter. I tried a 4.5 VDC supply, and the LED string will not light. The string will only light when connected to the box that came with it. As I said, the box has multiple modes, which are controlled by the circuit board which is outputting either pulsating DC or an AC waveform.

These strings are the "beads on wire" type, and using the "LEDs on tape" type will be easier to mount under the bookshelf lips. But my investment (very small) is not for naught; I can use these in costumes as I have in past.

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27 minutes ago, boom3 said:

I wish it was that simple, Peter. I tried a 4.5 VDC supply, and the LED string will not light. The string will only light when connected to the box that came with it. As I said, the box has multiple modes, which are controlled by the circuit board which is outputting either pulsating DC or an AC waveform...

I should have been more accurate: Look for a SWITCHING DC supply.

Any chance you can put the box on an oscilloscope and verify the waveform?

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5 hours ago, Peter P. said:

I should have been more accurate: Look for a SWITCHING DC supply.

Any chance you can put the box on an oscilloscope and verify the waveform?

would that I could. My Tektronix scope went back its owner years ago after an extended loan. I guess I should buy a scope plug in for my Mac or PC. I just haven't had a need to look at waveforms in a long time.

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On 4/30/2022 at 8:33 PM, Dave MacKay said:

Almost all LEDs require DC, not AC

If the LEDs don't have an extra circuit built in, they should work. You might see a flicker since they would only turn on for half the waveform, but 30 times a second is considered when you see and retain an image. 

 

 

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From the bicycle light hobby, I learned that LEDs don't need a steady power source like incandescent bulbs do. In order to save power, the LEDs, being a device that are instantly on when power is supplied to them, are pulsed at a frequency faster than the eye can detect that they're flashing. It would make sense that the OP's power supply is putting out pulsed DC, which can only be read on the AC scale on a DMM, or with an Oscope. Unless the said power supply has a problem, it would likely be simpler to get a 6VDC wall wart and use it to replace the 4 AA batteries, rather than trying to solve a problem that isn't there.

FWIW, cold startup current on an incandescent filament can be around ten times what the hot filament current is.

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