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I have two 90w/12v infrared IR bulbs which is put into a r7s sockets. From thes socket goes in total four wires which is connected to our power supply. We have set two of the wires in serial so they pull 45 watts each, in total 90 watts.

The bulb is 90w / 12v

Our power supply is 12v / 8.3 amps

It should be able to power the bulbs since the power supply is 99.6 w and the bulbs are 90w in total.

But they don't, they cannot power the two bulbs.

However, if we put a bigger power supply which is 10a / 12v then it works.

If we then, after having the bigger power supply connected, switch to the smaller 99,6w power supply then it works as well.

It is only if we try to power the two bulbs directly from small one that it doesn't work.

My questions are

why can't the 99.6w power supply power the 90w IR bulbs? (They are in serial so they use 45w each)

is there a way we can lower the wattage of the bulbs? Maybe a resistance? Something else? I just need to lower them by 5 watts and I'm sure it'll work.

Thanks in advance, and thanks for reading my long post!

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  • \$\begingroup\$ Draw a circuit; don't use words to describe a circuit. We are EEs here and we describe the way things electrically connect by using erm... a circuit diagram. \$\endgroup\$ – Andy aka Dec 8 '18 at 10:14
  • \$\begingroup\$ Please draw a circuit and share the datasheets of Bulb and Power supply (or at least part numbers) \$\endgroup\$ – Satish Singupuram Dec 8 '18 at 10:32
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When the bulbs are cooler i.e. running at "45 W" they have a lower resistance than they would have if run at 90 W. Hence use more current and more power than you would expect when run under these conditions. This is probably exceeding the specification of your smaller power supply.

A power resistor will bring them back into the range for the smaller supply but this will need to dissipate a large amount of power. You are probably better off using the larger supply.

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  • \$\begingroup\$ You mean "they have a lower resistance and use more current, even though they use less power" \$\endgroup\$ – WhatRoughBeast Dec 8 '18 at 15:10
  • \$\begingroup\$ @WhatRoughBeast Edited to address the confusion. \$\endgroup\$ – RoyC Dec 8 '18 at 16:06
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Cold incandescent bulbs have a much lower resistance than when they are hot. They are positive temperature coefficient devices. That means that your power supply has to be able to provide enough current to start warming them up, so as to increase their resistance until their temperature reaches steady state. If it cannot handle this initial surge, it will simply shut down.

You have a couple of ways to address this problem and remain somewhat efficient.

  1. If your supply has a “soft start feature” set it to slowly ramp the voltage during 5secs or so. This way the bulbs will be warm enough when it reaches final voltage.
  2. Add an NTC device. A resistor with a negative temperature coefficient into the loop. Its high resistance will control the initial current and it will slowly decrease its dissipation until it reaches equilibrium. It will waste a bit of power during operation, but this can be minimized.
  3. Add a inrrush current limiting circuit at the output of the power supply. This can be as simple as a FET a couple of resistors a capacitor and perhaps a diode.

All of this does assume that the power supply can actually handle the current in steady state.

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