Tag: PWM

Dimmable mains PSU control

Mean Well HLG-80H-15A dimmable PSU.
Mean Well HLG-80H-15A dimmable PSU.

Many of the LED power supplies such as those by Mean Well, etc., offer three modes of control: output constant current level can be adjusted through the control input

  • by connecting a variable or fixed resistance to the control terminals,
  • using a 0 ~ 10 V DC control signal,
  • using a 10 V PWM signal between.

To do this the controller input is something similar to the circuit below.

Adjustable constant current PSU input circuit.
Adjustable constant current PSU input circuit.
  1. For DC voltage control we just apply the voltage, it gets to the controller with a slight lag depending on the R1/C1 delay and output power is set.
  2. For PWM a pulse train would be used as shown in Figure 2. This time R1 and C1 filter the PWM to obtain the average DC value in the 0 to 10 V range. Output power is set as before.

    A PWM control signal.
    PWM signal transitioning from high pulse width (75%) to low (25%) and back again. Note amplitude remains constant. This will result in a control input of 7.5 V and 2.5 V respectively giving 75% and 25% of rated current.
  3. To sense a resistance connected to the input the PSU  supplies a small current to the input terminals as shown by the constant current source. On the units I am familiar with 100 kΩ gives full brightness so that means the voltage drop across the 100 kΩ is 10 V and I = V/R = 10/100k = 0.1 mA.
    This theory is supported by the fact that if you use one pot to control multiple power-supplies that the required pot value is 100/n where n is the number of power-supplies. This makes sense as each PSU will drive 0.1 mA into the pot. So for five lamps in parallel on the one pot R = V/I = 10/0.5m = 2 kΩ.
  4. Finally, if nothing is connected to the control input the 0.1 mA will charge C1 to 10 V and give 100% brightness.

It’s simple and flexible and rather ingenious.

Link: Mean Well HLG-80H series. Note that these PSUs use a 1 – 10 V control signal rather than 0 – 10 V as discussed above. This will make little practical difference.

Datasheet extract

Mean Well HLG-80H dimmable PUS wiring and output vs control graphs.
Mean Well HLG-80H dimmable PUS wiring and output vs control graphs.

LED rated current

LEDs are specified to run in a certain current range and the datasheet gives the maximum rated current.

Extract from Cree 5mm round LED C503 datasheet.
Extract from Cree 5mm round LED C503 datasheet.

The extract from the Cree C503B datasheet gives us the following guidelines:

Cree C503B
Cree C503B.
  • Maximum continuous forward current is 50 mA (1). Take note that this is an absolute maximum rating and is quoted at 25°C. If running in a higher temperature environment the current will have to be reduced to prevent overheating. We should also read Note 1:

1. For long term performance the drive currents between 10mA and 30mA are recommended.

  • Peak forward current is quoted at 200 mA (2). To understand this we need to read Note 2:

2. Pulse width ≤0.1 msec, duty ≤1/10.

The C503B maximum duty cycle at maximum peak current.
The C503B maximum duty cycle at maximum peak current.

Some applications such as multiplexed displays the LEDs are blinked on and off fast enough that the eye can’t see. To keep the average light-levels high a very high current pulse is given. In the example above we can see that the pulsed current’s average value is 200 \( 200m \frac {100\mu}{100\mu + 900\mu} = 20 \mathrm mA\). It should end up comparable with the LED run at about 20 mA continuously.

Other applications for pulsed LEDs include strobes and camera flash units.

See Multiplexing – the basics for pulse application.