12VDC vs 12VAC for GU5.3 LED Spots

[Seb101]

Hi,

I'm doing a lighting project and I'm mixing some LED strip lighting with some LED spots (GU5.3).

When I've like-for-like replaced other 12V halogen spots for LED spots in the house I've just swapped the bulbs directly keeping the existing AC transformers. Presumably these 'bulbs' have a built-in rectifier that allows them to run properly from an AC source?

Question is, for the new project, I have a 12VDC (100W) constant voltage driver which is driving the LED strip lighting and the lighting controller, will the same GU5.3 spots run correctly (and safely) from the 12VDC source?

One more question - I also want to drive the switched-live line of a timed extractor fan from the same 12VDC circuit - can I put a 12V SPST coil relay on this line driven from the 12V output from the controller?

Anything to watch out for?

Thanks!

 

[Seb101]

Essentially this:

 

[bernardgreen]

Depending on what W- and V+ are that plan should work. Make sure the relay has at least 500 volt isolation between coil and contacts. Do NOT use a 12 volt relay intended for use in a vehicle.

 

[Seb101]

Depending on what W- and V+ are that plan should work. Make sure the relay has at least 500 volt isolation between coil and contacts. Do NOT use a 12 volt relay intended for use in a vehicle.

Well, honestly, that's where the experimentation is going to come in. According to the LED controller specifications, the controller has a single +12VDC (V+) terminal, then the R- G- B- negative terminals are PWM dimmed to allow colour mixing for the strip light. It then also has a switched -12VDC terminal designated for white lights (W-). Whether or not that is suitable to drive GU5.3 spots and a relay is the real question here.

I intend to rig this all up on a bench before installing anything. This is what the controller looks like for reference. The Power button should turn everything on/off (inc the extractor) the colour wheel works the RGB outputs and the 'W' button switches the white output. Fits on a standard dry-wall box.

 

[NuaghtyNigel]

LED's are diodes, so they will rectify an AC voltage without an additional rectifier. HOWEVER, LED's tend to have a very low reverse voltage limit, (Vr), so they could be destroyed by connecting to an AC supply unless specified for such use.

I would also expect some flicker at mains frequency (50 Hz), and an associated loss of brightness as there is no smoothing. (The LED's will only conduct for half of each mains cycle.)

All in all a proper 12 Volt LED driver would be a much better choice in my view.

 

[Seb101]

LED's are diodes, so they will rectify an AC voltage without an additional rectifier. HOWEVER, LED's tend to have a very low reverse voltage limit, (Vr), so they could be destroyed by connecting to an AC supply unless specified for such use.

I would also expect some flicker at mains frequency (50 Hz), and an associated loss of brightness as there is no smoothing. (The LED's will only conduct for half of each mains cycle.)

All in all a proper 12 Volt LED driver would be a much better choice in my view.

I think the commercial LED bulb replacement units from Philips etc must have some of this technology built in, as they seem to function flawlessly on an AC supply. That's why I wonder if they will still be compatible with a DC input - they have no polarity markings for example.

 

[JohnW2]

I think the commercial LED bulb replacement units from Philips etc must have some of this technology built in, as they seem to function flawlessly on an AC supply. That's why I wonder if they will still be compatible with a DC input - they have no polarity markings for example.

I can but presume that 'non-polarised' LED lamps designed to be usable with DC must have a built in bridge rectifier (4 diodes), which would guarantee the correct supply polarity to the LED element, regardless of the polarity of the supply. If such a lamp was used (could be used) with an AC supply, the bridge rectifier would also prevent the LED elements being subjected to any reverse voltage. Whether that's the case with your particular lamps, I obviously don't know.

Kind Regards, John

 

[NuaghtyNigel]

LED's are diodes, so they will rectify an AC voltage without an additional rectifier. HOWEVER, LED's tend to have a very low reverse voltage limit, (Vr), so they could be destroyed by connecting to an AC supply unless specified for such use.

I would also expect some flicker at mains frequency (50 Hz), and an associated loss of brightness as there is no smoothing. (The LED's will only conduct for half of each mains cycle.)

All in all a proper 12 Volt LED driver would be a much better choice in my view.

I think the commercial LED bulb replacement units from Philips etc must have some of this technology built in, as they seem to function flawlessly on an AC supply. That's why I wonder if they will still be compatible with a DC input - they have no polarity markings for example.

You may well be right Seb.

If the lamps are intended to work on an AC supply (having a bridge rectifier built in) then they should would work perfectly well on a DC supply too. It is also possible that any such rectifier is only half wave, (i.e. a single diode) in which case the lamps would only work if the DC polarity was correct. There is no harm in trying.

Mains voltage devices have a switch-mode regulator built in, which rectifies the AC supply and provides a constant current (but not necessarily constant voltage) to the LED's. However, I'm not so sure about low voltage LED lamps. Wire-ended types probably don't have a rectifier built in owing to lack of space.

 

[Seb101]

You may well be right Seb.

If the lamps are intended to work on an AC supply (having a bridge rectifier built in) then they should would work perfectly well on a DC supply too. It is also possible that any such rectifier is only half wave, (i.e. a single diode) in which case the lamps would only work if the DC polarity was correct. There is no harm in trying.

Mains voltage devices have a switch-mode regulator built in, which rectifies the AC supply and provides a constant current (but not necessarily constant voltage) to the LED's. However, I'm not so sure about low voltage LED lamps. Wire-ended types probably don't have a rectifier built in owing to lack of space.

I will try it, and report back

 

[Seb101]

Depending on what W- and V+ are that plan should work. Make sure the relay has at least 500 volt isolation between coil and contacts. Do NOT use a 12 volt relay intended for use in a vehicle.

http://www.maplin.co.uk/p/round-base-10a12v-dc-dpdt-relay-jg58n

That seems to be acceptable?

 

[bernardgreen]

Insulation resistance: 100MΩ at 500Vdc

Yes that will be OK

You will beed to put a diode acreoos the coil to suppress any voltage spikes when the relay is de-energised. Otherwise the spikes could damage the controller

 

[Seb101]

Insulation resistance: 100MΩ at 500Vdc

Yes that will be OK

You will beed to put a diode acreoos the coil to suppress any voltage spikes when the relay is de-energised. Otherwise the spikes could damage the controller

A diode? Excuse my naivety but I'm not sure I've ever seen a diode used for this purpose, which orientation should it be in?

 

[JohnW2]

A diode? Excuse my naivety but I'm not sure I've ever seen a diode used for this purpose, which orientation should it be in?

'Reverse orientation' - i.e. so that it only conducts if/when the polarity of the potential difference across it is opposite to the polarity of the potential difference across the relay coil when it is energised.

Kind Regards, John

 

[Seb101]

Thanks John.

Like this?

What type of diode? Zenner, voltage suppression...

 

[bernardgreen]

This is how to fit the diode

Edit ...John was too quick