Checking voltage on LED power supply ?

[JoeH]

Not RV related per se, but figured someone here would know. I recently purchased a set of LED lights to put some path lights to avoid tripping. Hooked up the lights and everything works fine.
Next, I decided to put a strip of 5050 led's on some stairs. The strip LED's have + and - markings and are 12v . The output from the power supply for the path lights is also 12v so I figured I'd hook into that. I got out the meter and I get nothing when I check the voltage, yet the lights work. Check the meter with a different power supply I have for some other lights and it reads 12v as it should. So, why would the new power supply indicate no voltage and yet at the same time, successfully light up the 12v LED lights?

[JoeH]

Gdetrailer wrote:

JoeH wrote:
All great info and much appreciated. I have a Harbor Freight free analog meter out in the storage trailer. I'll see what that does just for fun .
I was also reading that hooking the 5050 LED strip lights to AC would work but reduce their life span. Recommendation was to get a 12v AC to 12v DC rectifier/converter.

Generally, running LEDs on AC is not a problem.

LEDs only "conduct" in the forward direction.

5050s require 3.0V per diode in the forward direction and are capable of handling up to 5V per diode reverse voltage.

SEE HERE FOR 5050 SPECS

Typically multiple LEDs are connected in "series" (pos of one connected to negative of the next one) same idea as a flashlight that takes multiple batteries.

Many "strip" light assemblies for 12V will connect 3-6 LEDs in series to run off of 12V depending on the diode forward voltage..

Which is why when you buy flexible and cutable strips they have a place marked for cutting.. cutting anywhere else and you break the series circuit..

Good example is pix below.


Click For Full-Size Image.

The issue I have with that setup there is no "current regulation". Slight variations in voltage can cause the LED current to swing past max values with only .1V change..

Better LED assemblies will use a current limiting resistor which will give a longer life.

Best LED assemblies will use a current regulator power supply which will give you max life and tolerate larger voltage swings.

If it makes you feel better, just s simple "12V" DC power supply is all you need.. Of course that power supply would bypass your current landscape transformer that may have timer and or dusk to dawn sensor just for your new added LED lights..

But I suspect leaving your new LEDs attached to the landscape supply will not do much harm if any to the LEDs life..

Thanks for the info once again... I'm going to move the strips to DC just incase the AC reduces life.... too much effort went into placing them to have them **** out early ! that's tomorrows project

[Gdetrailer]

JoeH wrote:
All great info and much appreciated. I have a Harbor Freight free analog meter out in the storage trailer. I'll see what that does just for fun .
I was also reading that hooking the 5050 LED strip lights to AC would work but reduce their life span. Recommendation was to get a 12v AC to 12v DC rectifier/converter.

Generally, running LEDs on AC is not a problem.

LEDs only "conduct" in the forward direction.

5050s require 3.0V per diode in the forward direction and are capable of handling up to 5V per diode reverse voltage.

SEE HERE FOR 5050 SPECS

Typically multiple LEDs are connected in "series" (pos of one connected to negative of the next one) same idea as a flashlight that takes multiple batteries.

Many "strip" light assemblies for 12V will connect 3-6 LEDs in series to run off of 12V depending on the diode forward voltage..

Which is why when you buy flexible and cutable strips they have a place marked for cutting.. cutting anywhere else and you break the series circuit..

Good example is pix below.


Click For Full-Size Image.

The issue I have with that setup there is no "current regulation". Slight variations in voltage can cause the LED current to swing past max values with only .1V change..

Better LED assemblies will use a current limiting resistor which will give a longer life.

Best LED assemblies will use a current regulator power supply which will give you max life and tolerate larger voltage swings.

If it makes you feel better, just s simple "12V" DC power supply is all you need.. Of course that power supply would bypass your current landscape transformer that may have timer and or dusk to dawn sensor just for your new added LED lights..

But I suspect leaving your new LEDs attached to the landscape supply will not do much harm if any to the LEDs life..

[JoeH]

All great info and much appreciated. I have a Harbor Freight free analog meter out in the storage trailer. I'll see what that does just for fun .
I was also reading that hooking the 5050 LED strip lights to AC would work but reduce their life span. Recommendation was to get a 12v AC to 12v DC rectifier/converter.

[MEXICOWANDERER]

Many Chinese meters have problems reading high frequency A.C.

[Gdetrailer]

For those that doubt that a switching power supply can present a AC output, I am posting some pix..

This is a power supply that came with 12V undercounter halogen puck lights..


Click For Full-Size Image.

It is a twin 60W supply (total of 120W),length is about 5.25", width about 1.5", thickness maybe 3/4".

From the dimensions alone it would be impossible to be a 60hz transformer..

Not to mention it is designed to be "dimmable" using a standard triac based dimmer..


Click For Full-Size Image.


Click For Full-Size Image.

You will notice on the close ups it lists the output CLEARLY as 11.5V AC, not DC.. Doesn't say if what the output frequency is but I believe it will be at the switching frequency.

I checked output with several different brand DMM meters, no load neither DMM registered any voltage.

Adding one incadescent bulb one meter registered 8V, the other 6V..

So, depending on the meter, it IS possible that the meter may not be able to register AC frequencies above or below typical 50hz/60hz correctly or at all..

This is where a analog meter might be the way to go, I have one good analog multimeter but it is packed away so I not gonna bother testing this any further..

[Gdetrailer]

JoeH wrote:

Gdetrailer wrote:

JoeH wrote:

Tom_M wrote:
Switch your meter to AC and check. Most likely one supply is AC and the other is DC.

Nope-- meter stays on "0" regardless of being on AC or DC.... but the LED lights out in the yard work. Strange

Post a pix of the power supply with the model number..

I suspect it is a high frequency switching supply and has no or little filtering on the output.. Most meters will not read at all or properly 15Khz-30khz frequencies which most switching supplies operate at which could explain why you can't get a reading but the lights work.

Quit raining and I went out to get a pic-- it is AC output but no reading on meter.

Coming up blanks on the brand and model numbers but with the pix, it is most definitely 12V AC output.

Doesn't look very big so I suspect it is a switching power supply and the output is at the switching frequency which may be the reason a meter doesn't read it.. Old school low voltage 60hz transformer systems were a fairly good sized box and heavy for the size.

If you had an analog meter it may show some voltage or if you had a oscilloscope you would be able to see the high frequency waveform..

LEDs are nothing more than a diode that happens to make light when a forward voltage is present, since AC alternates, the diode conducts 1/2 of the waveform making a LED workable on DC or AC..

However, some LEDs have a built in switching regulator that controls the voltage and current going through the LED.. This type of LED may or may not work every time with a high frequency AC power source.

In your case, everything has lined up and your additional LEDs are working fine with the yard light power supply you have.

[Gdetrailer]

jkwilson wrote:

Gdetrailer wrote:

JoeH wrote:

Tom_M wrote:
Switch your meter to AC and check. Most likely one supply is AC and the other is DC.

Nope-- meter stays on "0" regardless of being on AC or DC.... but the LED lights out in the yard work. Strange

Post a pix of the power supply with the model number..

I suspect it is a high frequency switching supply and has no or little filtering on the output.. Most meters will not read at all or properly 15Khz-30khz frequencies which most switching supplies operate at which could explain why you can't get a reading but the lights work.

Even with poor filtration, the output of a switch-mode supply is still DC and could be measured with any DC volt meter.

No, not always the case, switched mode power supplies by their very design CAN present a high frequency AC output.

Switching supplies convert 120V AC 60hz to 120V DC via a bridge rectifier and brute force filtering via 160V low microfarad cap (470 or so).

That unregulated and unfiltered 120V DC then goes into a high frequency chopper..

Chopper output goes into a small high frequency transformer..

Manufacturer now has two ways to go, rectify and filter to DC or allow high frequency AC to go out..

For applications like a simple 12V light bulb, absolutely no reason to add in a few more parts like diodes and filtering to DC..

Old school 12V landscaping lights used a heavy 120V AC to 12V AC 60hz transformer and the output was 12V AC at 60 hz..

Newer landscape lighting most likely has replaced the heavy old school 60hz transformer and replaced with a switching supply..

And I suspect it most likely does not output DC, but AC at the switching frequency..

I HAVE seen this type of setup with indoor low voltage "puck" lights used under cabinets.. The switching power supplies used on many under cabinet halogen puck lights were designed to work with a dimmer and the output was 12V AC at the switching frequency..

Those power supplies typically do not work well with replacement LED modules which have a built in switching regulator (I have been there done that)..

I have a few of those Puck light power supplies laying around, I guess I need to dig one out and check with a meter but I believe that would explain why the OPs lights work but a meter doesn't..

[Fishbreath]

Your transformer indicates that the output is AC (Alternating Current), and I believe that the 5050's require DC (Direct Current).
I would guess that, since your landscape lights work with the transformer, that they are AC bulbs.
Bill

[JoeH]

Gdetrailer wrote:

JoeH wrote:

Tom_M wrote:
Switch your meter to AC and check. Most likely one supply is AC and the other is DC.

Nope-- meter stays on "0" regardless of being on AC or DC.... but the LED lights out in the yard work. Strange

Post a pix of the power supply with the model number..

I suspect it is a high frequency switching supply and has no or little filtering on the output.. Most meters will not read at all or properly 15Khz-30khz frequencies which most switching supplies operate at which could explain why you can't get a reading but the lights work.

Quit raining and I went out to get a pic-- it is AC output but no reading on meter.

[jkwilson]

Gdetrailer wrote:

JoeH wrote:

Tom_M wrote:
Switch your meter to AC and check. Most likely one supply is AC and the other is DC.

Nope-- meter stays on "0" regardless of being on AC or DC.... but the LED lights out in the yard work. Strange

Post a pix of the power supply with the model number..

I suspect it is a high frequency switching supply and has no or little filtering on the output.. Most meters will not read at all or properly 15Khz-30khz frequencies which most switching supplies operate at which could explain why you can't get a reading but the lights work.

Even with poor filtration, the output of a switch-mode supply is still DC and could be measured with any DC volt meter.

[JoeH]

Gdetrailer wrote:

JoeH wrote:

Tom_M wrote:
Switch your meter to AC and check. Most likely one supply is AC and the other is DC.

Nope-- meter stays on "0" regardless of being on AC or DC.... but the LED lights out in the yard work. Strange

Post a pix of the power supply with the model number..

I suspect it is a high frequency switching supply and has no or little filtering on the output.. Most meters will not read at all or properly 15Khz-30khz frequencies which most switching supplies operate at which could explain why you can't get a reading but the lights work.

I'll get a pic soon---- sounds like you might have solved it.

[Gdetrailer]

JoeH wrote:

Tom_M wrote:
Switch your meter to AC and check. Most likely one supply is AC and the other is DC.

Nope-- meter stays on "0" regardless of being on AC or DC.... but the LED lights out in the yard work. Strange

Post a pix of the power supply with the model number..

I suspect it is a high frequency switching supply and has no or little filtering on the output.. Most meters will not read at all or properly 15Khz-30khz frequencies which most switching supplies operate at which could explain why you can't get a reading but the lights work.

[JoeH]

Tom_M wrote:
Switch your meter to AC and check. Most likely one supply is AC and the other is DC.

Nope-- meter stays on "0" regardless of being on AC or DC.... but the LED lights out in the yard work. Strange

[wopachop]

JoeH wrote:
I got out the meter and I get nothing when I check the voltage, yet the lights work.

Conformal coating.

I didnt exactly understand your entire post but type that into Google.