# Limiting the Current of a Constant Current LED Driver which is Driving a Halogen Bulb

I am powering a 12V, 35W halogen bulb with a MeanWell HLG-100H-20B constant current power supply. The "B" version of this power supply has a really neat dimming control system which lets one control the output current via a potentiometer, a variable voltage source, or PWM. This MeanWell unit can pump out 4.8A at its maximum setting, which is obviously too much for my 35W, 12V bulb. To reach full brightness on my bulb, I only need roughly 3A (~60% current from the MeanWell unit). Unfortunately, the "B" version of this unit does not have an adjustable maximum output current like the "AB" version does, and the "AB" is not in-stock anywhere.

So, regardless of how I control the output current, I can't let it get past 3A. This is pretty easy to do (I can just limit my PWM pulse duration or pot resistance, etc.), but my fear is that a failure or a goof-up in the control circuit could make the output current higher than 3A. I do have DC fuses to prevent the lights from blowing, but I'd like to avoid replacing fuses as much as possible.

Is there any sort of module method for limiting the current at a specified value? If my PWM source goofs up and the MeanWell unit starts outputting 4A, for example, I'd want something to cap the current going to the lights or fuses at 3A. Perhaps some sort of current divider circuit with a transistor or two to open an additional circuit path?

Thanks in advance for any advice. By the way, I'm a mechanical design engineer with limited electronics experience (if you couldn't tell) so please let me know if I'm spewing nonsense here.

EDIT: I think I figured it out, like 10 minutes after posting this question. See my crappy paint drawing below. Basically, I'll put 6.87 ohms of resistance in parallel with the bulb+fuse. The current division means that when the system is at max power (4.8A) the bulb's path will only experience ~3A. This will scale linearly with output current.

Fuses won't have the well defined cut-off at 3.001 A that you might be hoping for. It's more likely that the lamp will blow and protect the fuse. I don't think that any great complexity is worth the trouble to protect a relatively cheap lamp.

You can add parallel resistance to divert 1 A away from the bulb but this is a potential failure and a waste of energy (12 W).

Figure 1. The typical Mean Well PSU design. Source: Dimmable mains PSU control.

Instead I would be inclined to either wire a resistor securely in parallel with the dimmer pot to bring the maximum control voltage down to 7.5 V.

simulate this circuit – Schematic created using CircuitLab

Figure 2. The simplest current limiter for resistance control.

The constant current source, as best as I can make out, is a 0.1 mA so if the dimmer resistance control isn't allowed go above 75 kΩ you will have the current limit control required. (100k || 330k = 77k.)

simulate this circuit

Figure 3. The simplest current limiter for PWM control.

With Figure 3 we have limited the control voltage to 7.5 V max with R2. Applying PWM with Q1 will reduce the voltage which will be filtered by Figure 1's R1 / C1 low-pass filter. Note that with PWM at 0% the control voltage will be at the maximum (7.5 V = 75% of specified current) and with PWM at 100% the control voltage will be at minimum (0 V = 0% of specified current). i.e. You need to invert your PWM strategy.

• @Toucher: Sorry, for some reason I worked this out for a 4 A PSU. Yours is 4.8 A but you should be able to calculate the resistor values to get the control you want. Mar 27, 2020 at 21:21
• I didn't realize just how wasteful the parallel resistance idea was initially. What if, instead of a parallel resistor, I add a motor with a little fan? Just a crazy thought. I'd be afraid of the light flickering, but maybe there's a solution to that? Mar 27, 2020 at 21:42
• Bad idea. The motor is not a constant "resistance". It will be very low if the motor is stalled. It will increase as the motor speeds up. (The motor draws high current when stalled and low current when running close to rated speed.) Motors are usually run at constant voltage. Mar 27, 2020 at 22:01