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LED Driver Selection


Hi.  Newbie here, to DesignSpark, to this forum, and to LED lighting design.

Sorry for my overlong post, but brevity was never my strong suit.  I hope some kind and much wiser person can assist me with their knowhow, please.


I am upgrading 3x ceiling light fittings from G4 halogen bulbs to G4 LED bulbs.

For this project, I need G4 LED bulbs that are 10mm diameter or less, but giving maximum light output.  I have selected G4, 12vDC, 3W, 300LM, 6500K, non-dimmable, LED bulbs, “BTF” brand, (made by Foshanshi Bitefomaoyiyouxiangongsi, and sold in UK by BTFEU on Amazon).

Before upgrading, each light fitting includes 6x G4, 12v, 10W, halogen bulbs, wired in parallel, and a simple step-down transformer with input 240vAC and output 11.5vAC, 60W, 5A maximum load.  To power 6x LED bulbs wired in parallel, I know that I need to use an appropriate LED Driver instead of the existing AC-AC step-down transformer.

The type of G4, 12vDC, 3W, 300LM, LED bulb I am using contains 16x type 2835 LEDs.  The specification for a typical type 2835 LED is 3.1vDC, 60mA, 0.2W, 20LM per LED.  So I have assumed that to make this type of G4, 12vDC, 3W, 300LM, LED bulb using 16x type 2835 LEDs, the simplest internal PCB would have 4x parallel strings of 4x type 2835 LEDs in series?  Then the whole G4 LED bulb will draw 4x 60mA = 240mA current at 12vDC, and 6 of these G4 LED bulbs wired in parallel will draw 1440mA current at 12vDC.  But maybe my assumptions are not correct?

To locate the LED Driver within the light fitting, I have to keep the same circular form factor as the existing transformer, 80mm diameter x 25mm high.  After reading the RS guide to LED lighting, it seemed a constant current driver would be best, and I selected RS PRO part no 176-6764.  This is a Constant Current LED Driver, “Snappy” brand, model SP30-1400IL(UE)-1, with input 240vAC, output 1400mA constant current, 10.5-21vDC, 30W max load (made by Ningbo Snappy Optoelectronics Co, and sold in UK under the RS PRO brand).

This 1400mA constant current LED driver would seem to be an almost exact match to the expected current of 1440mA drawn by the 6x G4 LED bulbs wired in parallel.  The LED Driver is dimmable (either 0-10vDC or PWM) but because I do not want my light fitting to be dimmable, just switch on / off, the dimming wires remain open circuit (I think this is the normal procedure to obtain constant 100% output).  This LED driver also incorporates an auto-restart feature after a temporary fault condition.

I installed the LED Driver to the light fitting with the 6x G4 LED bulbs.  When I switch on the 240vAC supply, all 6 LEDs flash on briefly for 0.1 second, then light up fully for 1 second, then go off for 1 second, then this same “quick flash – slow flash – off” cycle repeats until I switch off the 240vAC power (I disconnected the power after just a few cycles to hopefully avoid damaging either the LEDs or the LED Driver).

The LED bulbs all seemed to function perfectly for the short time they were on.  They showed the correct brightness (300LM) and the correct colour temperature (6500K).  The LED bulbs appear to be a high-quality product, and having a substantial ceramic base obviously helps to keep the bulb cooler so that it should last longer.

I think the flashing cycle must be caused by the LED driver experiencing a temporary fault condition, which makes it cut out and then automatically restart, repeatedly? This suggests there must be some mismatch between the selected type of LED Driver and the selected type of 6x G4 LED bulbs wired in parallel?  But I cannot work out what could be causing this mismatch?

Please would some wiser person than me, advise where I have made a mistake in my design assumptions?

And please also suggest the most appropriate specification for a different type of LED driver (it must be available in the same circular form factor 80mm diameter x 25mm high) which I can use to power these 6x G4 LED bulbs wired in parallel, and operating at 12vDC, to avoid the mismatch problem that is occurring now?

Should I perhaps be using a 12vDC Constant Voltage driver instead of a Constant Current driver?

Any help will be greatly appreciated, because I cannot work out myself what is the problem here.


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