Powering 12V 35W halogen bulbs with a constant current supply

Question

I'm revisiting an old project, and I want to see if there is anything I could have done differently, or if I missed any important details or considerations.

Basically, I used a constant current driver (designed for LEDs) to power and dim a halogen bulb. The driver is a MeanWell HLG-100H-20AB, and the bulb is 35W, 12V halogen. This constant current driver can be controlled via variable resistance (10kOhm at min. brightness and 100kOhm at max. brightness), and its max current output can be adjusted as low as 3A. I calculated that the bulb needs 2.917A to shine at its maximum, so I set the max current on the MeanWell to ~3A and arranged my potentiometer and resistors such that the max resistance delivered would be ~97.5kOhm, meaning, theoretically, that the Meanwell unit should not output more than 2.917A.

Here are my questions:

1. This worked well from what I saw. The light dimmed as I expected, and the bulb didn't burst. But, I'm wondering if there's anything crucial that I could have missed. I don't generally see people dimming halogen or incandescent bulbs with constant current devices, and there must be a reason for this. Any ideas?
2. I'm assuming that if I had used the MeanWell to output, lets say 4A, the bulb would have failed, since my understanding is that the MeanWell unit is capable of "force feeding" current. Is this actually true? Or, does the unit just modulate voltage to indirectly control current?
3. I noticed that the spec sheet has a "constant current region" listed between 10V and 20V. I don't understand what this means, or how it would affect the halogen bulb. For instance, when running only 1.5A to the bulb, for example, its a safe bet that the voltage across the bulb's termninals is less than 10V. Yet, the MeanWell unit still supplied steady current. What gives?
4. Am I correct in thinking that this setup negates voltage drop considerations for the bulb over a long cable?

Thanks!

Answer 1

I don't generally see people dimming halogen or incandescent bulbs with constant current devices, and there must be a reason for this. Any ideas?

Price. It is much easier to power a bulb with a battery, mains, transformer.

Or, does the unit just modulate voltage to indirectly control current?

Correctly, almost all current sources do it that way. Practically, there is no such a device like intrinsic current source, just theoretical. But it could be true somewhere in the universe, so we're ready to use our equations.

Am I correct in thinking that this setup negates voltage drop considerations for the bulb over a long cable?

Yes.

Answer 2

I'm assuming that if I had used the MeanWell to output, lets say 4A, the bulb would have failed,

Most likely.

I noticed that the spec sheet has a "constant current region" listed between 10V and 20V. I don't understand what this means, or how it would affect the halogen bulb. For instance, when running only 1.5A to the bulb, for example, its a safe bet that the voltage across the bulb's termninals is less than 10V. Yet, the MeanWell unit still supplied steady current. What gives?

The unit is designed to provide constant current if the voltages do not need to go outside of that range to do that. If you tried to drive a 1M\$\Omega\$ resistance with your unit set to 3A, the voltage would probably rise to 20V, or somewhat above and give less than 3A current. If you short circuited the outputs of your unit, the voltage would probably drop somewhere below 10V, and the output current would be something we cannot determine from that spec.

Am I correct in thinking that this setup negates voltage drop considerations for the bulb over a long cable?

Yes.

Answer 3

What you did seems fine, but your fear is well-founded.

Tungsten (and other wire) filaments have a positive temperature coefficient of resistance. Running them at constant current may not quickly achieve a stable final temperature, power or brightness.

Instead, the filament temperature and lamp brightness may continue to rise for a long time in a positive feedback loop. To be safe, it is better to use a voltage supply rather than a current supply. This will insure that the system rapidly converges on the final steady state level. The final filament temperature is relatively insensitive to voltage, and relatively hyper-reactive to current.

It is probably safe to use a current source as long as the current is low enough (as you did) OR if the supply also has a safe voltage limit so that it can never go over the rated voltage.

Answer 4

Nothing wrong to power incandescent bulb from that power supply. Just set max voltage to 12V. In CC mode setting current as in normal condition may cause only slow start. Cold filament has low resistance, it rises when heats. It even improve life span of bulb. Dimming circuit with PWM is OK. Resistance of wires should be taken in consideration, but if it is known value, you can rise output voltage to compensate wires drop voltage. For fire safety current should not exceed max current for wire gage.