Meanwell LDH-45 DIM- connection

In a new product we will have microprocessor controlled lighting, and use LDH-45 (given the Power in and LED requirements).

Issue: Datasheet says “DIM- Don’t connect to Vout- or Vin-“
I understand the Vout- note, but not connecting to Vin- is a problem.

Our design is typical DC circuit with same Gnd signal for microprocessor, LDH-45, and all Power.

  • Microprocessor supplied Analog DIM signal, so DIM- = Microprocessor Gnd = Power Gnd = Vin-
  • That violates the note in LDH-45 datasheet since Vin-=DIM-

What is the proper connection or circuit for this kind of application?


Hello PaulPort and welcome to Digi-Key’s TechForum. Baised on my review of the 3rd Page of the LDH-45B-700 Datasheet, the unit uses just a potentiometer between pins 3 and 4. No external power, reference or signal is provided in the example.

Taken from the MEAN WELL USA inc. datasheet:

In this diagram the LDH-45 monitors the voltage drop from Analog DIM to DIM-, keeping Analog DIM- hot. If the unit detects an open circuit (ie. you are not using dimming) the brightness goes to max. If the unit detects a short on the pins, it shuts the LEDs off.

The datasheet does note that the adjustment range is just between .25V~1.3V before operating in the saturation on either side.

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Hi PaulPort,

If you are doing PWM type dimming, then you may need to use something like an optocoupler, which isolates the MCU from the LDH-45, but which would effectively open and short (well, have a very low Vce voltage) PWM DIM to DIM-.

Something like this:

EDIT: I found out that this LED driver does NOT have an internal voltage available for the circuit above, so it will not work as drawn up.

There is another possibility for making it work and still passing EMI testing: that would be to use an isolated DC-DC converter in addition to the optocoupler shown in the circuit above. The following isolated DC-DC converters can be used to generate an isolated 5V supply which can be used in conjunction with an optocoupler to pass a PWM signal from the MCU to the PWM DIM pin of the LDH-45 and still pass EMI testing.

The appropriate choice depends on whether one needs surface mount or through-hole, and the voltage of the non-isolated supply which is available to power the rest of the circuit. To pass EMI/EMC standard EN55032 class A or B, one would need to add a few capacitors and possibly an inductor around the DC-DC converter, as described in their respective datasheets.


FYI Reply by a Meanwell Engineer:
“The LDH-45 specifies -Vin and -Dim cannot be connected because there is a choke in between for EMI debug. For operational, you can connect -Vin and -Dim and won’t harm the unit but EMI might not pass.”