How can I add soft on/off functionality in simple lamp

[Nathatt]

I have a small LED lamp and I would like it to slowly ramp up when turned on and slowly fade out (the reverse) when turned off.

The input power will be removed at power off
The LEDs take 6.6VDC, 50mA at full on
The input voltage is on a vehicle so it is 9-16 VDC
The solution must be very "cost effective"

I have explored thermistors and different voltage regulators, LED drivers, etc but have not found a suitable solution to the design problem.

 

[IRstuff]

Well, you need a current source that's programmable/controllable. Beyond that, your desirements will dictate the cost.

TTFN

FAQ731-376

 

[VEBill]

Capacitor.

LEDs typically have a series resistor. Add a capacitor across the LEDs after the resistor. It might have to be a large capacitance value, but large value supercaps are getting cheaper. It's certainly the simplest solution, but it might not be the cheapest in large quantities. But for a one-off it's a no brainer.

PS - resistor power rating might have to be increased with some combinations of circumstances. Worth double checking.

 

[VEBill]

With 9 to 16 volts input, you wouldn't be using a simple series resistor.

But a capacitor will still give you a ramp as you have described.

But if this is for production, a swItcher would be better. But you have to think how it knows the power has been cut while it still has power to ramp down on power off.

 

[Nathatt]

Thanks for the replies.

The capacitor for the slow off is perfect and is in fact in
my current design, but then I am stuck on the ramp on.

I am afraid a switcher is both $$ and an EMI/EMC concern.

 

[VEBill]

Does your present design have the capacitor *directly in parallel* with the LED string as I described?

If the 'slow off' is perfect, and the 'ramp on' isn't (too fast?) then I suspect you're talking about a normal power supply capacitor somewhere else in the circuit.

 

[Nathatt]

It is in parallel with the LED and a current set resistor. I'll take another look and see if I am missing something.

I also ran across this (another helpful person sent it my way). I would welcome any comments if you see potential issues.

http://www.zen22142.zen.co.uk/Circuits/Power/softstart.htm

Thanks again.

 

[OperaHouse]

I would use a LM339 op amp. These are a quad amp with open collector outputs that can be wired in parallel for quite a bit of current. Put the LED in the collector circuit and operate it in unity gain. use a resistor like 1M to charge a cap on the op amp input. The other side of the resistor goes to either plus or minus rail to turn on/off. A further advancement would be to offset the voltage so it ends just below LED cutoff. More complex, but smaller than a large cap!

 

[MacGyverS2000]

To determine "your" definition of "cost effective", how many lamps are we talking about here? One/lifetime? One million/year?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Nathatt]

We expect a few thousand/yr but we'll see how it goes.

I am just getting back to the design this morning.

 

[MacGyverS2000]

For that level, I would consider Opera's suggestion...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[OperaHouse]

"The input power will be removed at power off"

How fixed is this requirement? Decaying the light with no external power is possible but considerable added expense if always on power is not available. How long a decay for the lamp?

 

[VEBill]

Which brings us back to: "Capacitor". ;-)

 

[TurbineGen]

What kind of ramp time are we talking about? This sounds like an automotive dashboard light system of some sort. VE1BLL's suggestion should work fine. The ramp up time and ramp down would be different since the charge circuit has the resistor and LED to control ramp up and the discharge only has the LED.

OperaHouse's suggestion certainly limits the size of the capacitor and controls the ramp up. However, there's always the LM393, which is a dual output rather than a quad. Here's the datasheet:

http://www.national.com/ds/LM/LM393.pdf

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[PHovnanian]

LEDs have a very nonlinear voltage vs current (and output) characteristic. So a simple RC ramp circuit may still make the LED appear to 'snap' on. The op-amp solution (as a current source) can solve this. But you'll have to supply some energy storage to the circuit (a big cap) to keep the op-amp and LED powered during the ramp down.

If this device has a microprocessor in it with a spare output pin, a bit of s/w to PWM the LED might be the simplest solution.