12VAC G4 led lamp lights up bright on 12VDC.

I'm still seeing spots.

These are what I got. 12VAC 3W led 30W halogen bulb replacements.

I plugged a 12V wall wart in and stuck one pin inside and the other to the outer barrel and it's real bright even through the rectifier. They'd make really good flashlight lamps on 12VDC.

The link says they are 12v DC lamps? Why have you AC in the title?

It is impossible to know if they are identical to similar looking lamps that I have been using for some time.

Recently I got some 12v GU5.3 / MR16 Ac/Dc lamps in Sainsbury's for £4 each. I only noticed now that they are suitable for AC as well as DC.

They have a plastic lens on the front that can easily be sawn off with a small hacksaw. Then they are significantly brighter than any other LED lamps I have tried. They have 4 LEDs and claim to provide 350 lumens. However I don't think I could tell a lumen from a lux even on a dark night.

It is easy to take out the parts and fit them into other lampholders and they also have the big benefit that the lamp is separate from the electronics so the electronics do not get fried - which happened with two other very bright small-format lamps that were much more expensive.

...R

There was discussion before but I lost that thread.

These have 12 leds, use 3W and supposed to be 600 lumens used as G4 lamp replacements.
I bought 10, got $1.29 ea. (checked the receipt) Just looked on site, price went up!

Turns out that G4 is supposed to be 12VAC and these disks have rectifiers. Most G4 bulbs are halogen bulbs, car headlight type for spotlights.

There was some concern that 12VAC peaks > 12V and 12VDC might give poor light. It might do better with another volt or 2 but it throws a bright light on 12DC.

It'd make a good flashlight or bike headlight.

For the price, I bought a few.

Based on my experience if they are cramming all those watts onto a thin PCB there will be a great risk of premature smoke :slight_smile:

...R

3 whole Watts at 12V through 12 leds as 4 chains of 3 leds with a resistor, getting about 20mA each all on a 30mm disk didn't get warm in the short periods I lit it up.

In your experience, do 5mm white leds get hot running 20mA? There's 12 on a 30mm disk, they're not in a packed grid though 4 pairs do share 1 side between 2. This is to replace a same-size 30W halogen.

If all 3W went through 1 special big led, I might wonder, but not 12 leds around 20mA.

These can strobe and PWM. They don't have to be spotlights.

GoForSmoke:
3 whole Watts at 12V through 12 leds as 4 chains of 3 leds with a resistor, getting about 20mA each all on a 30mm disk didn't get warm in the short periods I lit it up.

Interesting.

I am typing this in the light from an MR11 led lamp with 12 LEDs and they are hot enough that it would be uncomfortable to leave my finger touching them for any length of time.

The very much brighter Sainsbury lamp (with 4 LEDs) is hotter - but in that case the lamp is seaparate from the electronics.

And now I can't see properly for blue dots in front of my eyes :slight_smile:

...R

Looking at Amazon I see a few MR11 12-led bulbs rated at 6W…

Running a disk steady on 12V and to a touch it's not burning hot but it gets hot. It gets hotter if I keep my finger pressed on 2 or 3 leds, might get a 1st degree burn if I hold it long.

It sheds enough heat that insulating parts, even the back, quickly raises the temperature. They need to be mounted with good air flow and a heat sink of some kind wouldn't hurt but isn't necessary and could serve as a bracket.

GoForSmoke:
It sheds enough heat that insulating parts, even the back, quickly raises the temperature. They need to be mounted with good air flow and a heat sink of some kind wouldn't hurt but isn't necessary and could serve as a bracket.

The ones I had that failed were in lamp units that had been designed for halogen bulbs so there was no airflow. While they lasted quite some time they died long before the energy saving had offset the higher initial cost. I think I paid £6 or £8 each for them a couple of years ago.

The Sainsbury lamps have a big aluminium heat sink / reflector.

...R

They seem to get brighter when they get heated. Perhaps Vf drops a bit.

A constant current source might be good.

GoForSmoke:
A constant current source might be good.

Isn’t the electronics on the device intended to ensure they get the correct current?

I have no idea how to drive those ultra-bright LEDs even after Googling for some help when one of them failed (thinking I might be able to fix it).

…R

These have a rectifier dropping some of the voltage, feeding the rest to 4 strings of 3 leds through 4 51 ohm resistors.

Each led gets a lot more than 20mA though. It gets 250/4 mA, not 250/12. Leds (components) in series all use the same current (Kirchoff) and 4 paths for 3W at 12V leaves 250/4 mA. I think that that explains why they get hot where at 20mA, 5mm leds don't.

They're leds, they can PWM to run cooler. You could send data with one for all that.

I don't need the full brightness of any one, run at 90% means 10% not only cooling down but not making heat either, it should be more than 10% cooler.

I've seen DIY touch control lamps that use cap sense, I might try one to replace my desk light.

GoForSmoke:
They're leds, they can PWM to run cooler.

From something I read somewhere I think it is possible to grossly over power them for very short pulses to give brighter light without causing damage but I have never been able to find how that works - in the sense that I imagine the longer intervals between the bright flashes would offset the value of the extra brightness of the flashes.

I have been assuming (perhaps incorrectly) that the circuitry on the LED lamp was intended to take advantage of that overload ability.

You can probably figure that this is a subject that only passes across my consciousness occasionally.

...R

If you get too much current flow it will damage the junction. Limit current and you can put 5V through a red led, even 12V (they make em with built-in resistors) right?

Ohm's Law in action E = I/R where R is const and if I is current controlled, volts must obey.

E/volts here is VCC - VFtotal, note that if the temperature causes VF to drop, E/volts won't change.

With the cheap resistor method I = E/R where R is const and volts increases with temperature.
if the total VF of the led(s) leaves a small value E, a small change in VF can cause a big increase in current and led damage at some rate.

GoForSmoke:
If you get too much current flow it will damage the junction.

I’m aware of that. But there is a big difference between the max constant current and the max short-term current and I thought I read somewhere that you could get more light by pulsing the things close to the max short-term current.

However from a couple of datasheets I looked up just now that does not seem to be the case - they don’t quote brightness for the peak current. My memory may be faulty (very likely) or the way they are made and used may have changed.

…R