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LED driver circuit.

I have a number of G9 non-dimmable led lamps (230V, 3W, 300lm). One just failed with very low light output, so I thought I'd have a look inside. Other than the led itself, there were only three components - an MB10F bridge rectifier, an FC2082G constant current led driver (TO-252 pack), and a 27 ohm resistor to set the led current.

The circuit appeared at first look to be the "Typical application" on page 1 at . But what is odd is that there was no ground connection to the chip. Pin

2 had been cut off. The circuits shown in that data sheet, and another one I checked, all have the earth connected. One other minor oddity is the resistor value. If that led is rated at 3W, then at 230V that's 13mA. According to Diagram 1 on page 4, a 27 ohm rext should set an output current of 20 - 30mA. For 13mA, about 50 ohm is required.

A couple of questions. What difference does the lack of an earth connection to the chip make, and is the led being overrun?

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Jeff
Reply to
Jeff Layman
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I reckon 27R ought to be about 22mA ~= 0.6v/R

I don't understand. The current set resistor goes between two pins of the chip and one of them is the ground return to negative supply.

Are you really saying the pin 2 is not connected or does it rely on current flow through the case or heatsinking to return to ground?

The LED might be being overrun a bit but it would be a pathetic LED die that couldn't survive 20mA indefinitely. Even the most feeble indicator LEDs can cope with that sort of current without even getting warm.

60 of them in series might get the chill off though but I wouldn't expect them to be warm enough to be life shortening. 
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Regards, 
Martin Brown
Reply to
Martin Brown

Chip has a power pad underneath for pin 2... that has to be connected. Re: The current.. some of the voltage will be dropped by the current regulator.

George H.

Reply to
George Herold

As soon as I'd sent the OP I realised there might be a connection on the heatsink. The led driver chip is mounted directly beneath the diode bridge on the other side of the circuit board. When I levered off the driver chip I found a soldered connection from it to the -ve of the diode. Seems an odd way of doing it as there is a decent amount of room to mount the chips so that they do not overlap at all. I'd be surprised,

Agreed, but the "Typical application" on page 7 of the data sheet shows a 3W 220V circuit where R = 62 ohm! I'll try to remember to dig out an unused lamp and connect it up externally with an AC ammeter in circuit. It will be interesting to see what current it shows.

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Jeff
Reply to
Jeff Layman

Your parts list didn't include a large capacitor after the bridge rectifier. This implies the LED and current limiter are operating off pulsating DC so there will be periods of time when no current flows. That could be the reason for the larger than expected peak operating current - once time averaged it does equate to the 3W rating?

piglet

Reply to
piglet

Good point, as there was no capacitor in the led lamp base.

I tried connecting a digital multimeter in series with the lamp but the lamp does not light at all on the 200mA range. I connected a 10 ohm resistor in series with the lamp instead, and the lamp lights with that. The voltage drop across that resistor was 120mV, so the current seems to be 12mA, which is about right.

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Jeff
Reply to
Jeff Layman

The lack of any capacitor, even a small value one, makes the device extremely vulnerable to high voltage transients. Switching an inductive load on the same circuit could destroy the driver chip.

piglet

Reply to
piglet

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