Reading spec sheets (meanwell LDD-700LW)

Scott M · 2017-12-03T15:55:15+00:00

I'm trying to design an interface to a meanwell LDD-700LW, a constant curre nt regulator for LEDs. I'm having trouble understanding the spec sheet. (I' m no guru and spec sheets often confuse me a little). It has a PWM input to provide dimming. If left floating, some internal pull up causes the device to go full on. If pulled to ground, it turns the light off. Cycled at some rate, it provides dimming. Simple enough. The device gets fed (in my application) 20v or 24v, depending on where I ho ok it up. Well within range. But the PWM input is marked differently: Leave open if not use Power ON with dimming: DIM ~ -Vin >3.5 ~ 8VDC or open circuit Power OFF : DIM ~ -Vin < 0.5VDC or short I can't figure out if that's trying to say the safe range for turning the L ED on is 3.5-8v (and bad things happen above 8v)... or if it's saying that the voltage needed to turn it on is, eh, maybe 3.5v will do it, you might n eed as much as 8v... Leaving me confused. If I want reliable operation, do I feed it a little ov er 8v, or strictly less? Clarity appreciated. For the curious, here's why I care. On/Of/Dim is to be controlled by a micr oprocessor, via a ULN2003A (originally, the ULN2003A was going to drive the LED directly). So I was planning to use the ULN2003A to pull the LDD's inp ut to ground when I want the light off, and do PWM through it as needed. Th e pullup voltage would be the LDD's problem and all I had to do was pull do wn. Simple, right? Except... If the microprocessor fails or reboots, the ULN will stop conducting and th e PWM input will go high, turning on the LED. NOT what I want. The uncontro lled state has to keep the LED off. Maybe I just bought the wrong part, but since I have it I'd like to make it work. (If someone knows of an equivale nt that is active low, mention it!) So my thought was to use a 2N222A as a logic inverter. Pull up the base so it conducts by default. Use the ULN to pull the base back to ground as need ed. Use the transistor's collector...

Dec 03, 2017 40 Replies

Rate this thread:

rickman 8 years ago

More common in lighting controls is a range of 0-10 volts DC. Interesting in that there are still two flavors of that spec. One is where the lighting module puts out a current and a passive pot will give 0-10 volts over the resistance range. The other requires the control to source a voltage in that range and the lighting module has a passive input. One way the two standards can be mixed, the other way not. lol

Rick C Viewed the eclipse at Wintercrest Farms, on the centerline of totality since 1998

Vote