12v DC LED Bulb Polarity

[SMG]

I am replacing my MR16 halogens with LED`s. I have a new led driver rated at 102watts, & 8 bulbs rated at 94 watts total. In my experience an AC bulb will fit any way around, but a DC bulb is DC, so, which pin is Pos, & which pin is Neg. Or, does the bulb fit either way??

Thanks

 

[Mursal]

Details between image 5 and 6 on this link

http://mr-ives.blogspot.ie/2011/03/replacing-mr16-halogen-bulbs-with-led.html

 

[riveralt]

I am replacing my MR16 halogens with LED`s. I have a new led driver rated at 102watts, & 8 bulbs rated at 94 watts total. In my experience an AC bulb will fit any way around, but a DC bulb is DC, so, which pin is Pos, & which pin is Neg. Or, does the bulb fit either way??

Thanks

As long as you wire the led driver correctly and unless the led lights state specifically, I don't think it matters which way the pins go in.

BTW some AC bulbs (Edison type) do have specific polarity - live goes to the centre point and neutral to the screw part of the casing.

 

[SMG]

Thanks for those reply's, I plan to fit the driver unit in the loft, where there is plenty of ventilation. Each bulb has its own connection, & all 8 connect to a terminal block, which will then connected to the 12v DC driver. The maximum wattage is less than the drivers maximum output. It just didn't make sense to me connecting the bulb to any polarity.

I`m not the brightest bulb in the pack!

Cheers.

 

[ericmark]

Details between image 5 and 6 on this link

http://mr-ives.blogspot.ie/2011/03/replacing-mr16-halogen-bulbs-with-led.html[/QUOTE]
Rather surprised to find DC power supply all mine are AC and I would have expected the MR 16 lamp to have a rectifier inside the housing together with some control components as LED's use around 1.2 to 3 volt not 12 volt.

Running lamps designed for AC on DC could overload components within the lamp so I pulled out a new lamp to look at if AC or DC and there is nothing on the packet saying which it is.

With a LED the long lead is positive but with MR 16 both same length. I would not expect there to be a problem swapping the polarity and with a standard LED wrong way around it just does not work and it does not cause any damage.

I would not expect the rectifier to be damaged feeding DC rather than AC but it could in theory overload the rectifier one diode being unused and the other taking twice the power.

RS Components do seem to give more information than other suppliers and their bulb was listed as "Voltage Rating 9 → 15 V ac/V dc" so clearly could be AC or DC supplied.

The surprise was "for indoor use only" and must be used in a well ventilated area not totally enclosed.

 

[SMG]

Yes m8, there are "External" drivers & internal only drivers, the latter has a case which allows full cooling,the case is ful of holes, that's the type I am getting. Aparently, the DC bulbs are "Dimmable" but you can only use DC, (I wont be dimming them), the bulb I suspect must have some sort of polarity swap (Diodes) depending on the flow. I have buggered LED bulbs in the past using a standard transformer, so for this project, I want to get it right.

 

[seasickstevie]

BTW some AC bulbs (Edison type) do have specific polarity - live goes to the centre point and neutral to the screw part of the casing.

AC doesn't have a 'polarity'. Well it does, but it changes it's mind fifty times a second!


I think you're confusing with the regs for the old style ES lampholders that used to say live should be on the centre pin, so as to reduce the chances of touching live (as would be more likely if connected to the outer contact).

 

[JohnW2]

AC doesn't have a 'polarity'. Well it does, but it changes it's mind fifty times a second!

100 times per second, actually

Kind Regards, John

 

[JohnW2]

RS Components do seem to give more information than other suppliers and their bulb was listed as "Voltage Rating 9 → 15 V ac/V dc" so clearly could be AC or DC supplied.

I have no idea how these things are normally arranged but, just just thinking aloud, a bridge rectifier (plus appropriate current-controlling components) would enable an LED lamp to work off AC or either polarity of DC.

Kind Regards, John

 

[EFLImpudence]

I think you're confusing with the regs for the old style ES lampholders that used to say live should be on the centre pin, so as to reduce the chances of touching live (as would be more likely if connected to the outer contact).

Still do, don't they when the thread is metal?

 

[ericmark]

RS Components do seem to give more information than other suppliers and their bulb was listed as "Voltage Rating 9 → 15 V ac/V dc" so clearly could be AC or DC supplied.

I have no idea how these things are normally arranged but, just just thinking aloud, a bridge rectifier (plus appropriate current-controlling components) would enable an LED lamp to work off AC or either polarity of DC.

Kind Regards, John

Yes but with DC you will get twice the watts through one pair of diodes and if half wave or using the LED its self then half the power to LED.

As to external and internal it was not the drivers it was the bulbs which were internal only.

If one looks at a LED controller able to dim the LED these do seem to be DC with an output of 7.2v to 12v. These looking at data sheet are consent current but when you look at the typical I-V curve not quite that simple.

However from what I can see DC is used to dim LED lights but where dimming is not required you can use AC or DC.

The DC units are switch mode power supplies but the AC units are normally Toroidal Transformers. There are however some switch mode which can also go from 0 watts output.

looking at this list you will note "LT YT50 Premium Electronic Lighting Transformer 0w - 50w ( Special Offer )" clearly able to go 0 - 50W so can work LED lamps, but most have a 20w - 60w or similar which means they would not work with LED as the LED would not draw the minimum of 20W.

 

[JohnW2]

I have no idea how these things are normally arranged but, just just thinking aloud, a bridge rectifier (plus appropriate current-controlling components) would enable an LED lamp to work off AC or either polarity of DC.

Yes but with DC you will get twice the watts through one pair of diodes and if half wave or using the LED its self then half the power to LED.

Not with a bridge (i.e. full-wave) rectifier. In any event, one would not just be connecting a 'passive load' to the output of the bridge - the current throughg the LED will be controlled by circuitry (at its crudest, just a resistor) following the bridge.

Kind Regards, John

 

[SMG]

I "sort of" understand where you guys are coming from regarding rectifiers, apparently there is a small circuit board inside the 12v led bulb, I would assume the electronics would take care of any volt drop & polarity. I didn't want to connect all the bulbs then switch on just to see 8 flashbulbs.

 

[SMG]

Just a footnote to this, I bought a DC LED Driver, & DC bulbs. connect the bulbs either way round & they work fine. In the words of old Harry Worth, "I don't know why but there it is".

 

[JohnW2]

Just a footnote to this, I bought a DC LED Driver, & DC bulbs. connect the bulbs either way round & they work fine. In the words of old Harry Worth, "I don't know why but there it is".

Interesting. That being the case, as I said before, my guess would be that there is a 'bridge rectifier' within the bulb. With that, the LEDs themselves would always get electricity of correct polarity, regardless of 'which way round' the DC was supplied to the bulb.

Kind Regards, John