Lightbulb trick on filament circuit?

[Rundmaus]

Hi there,

has anyone tried to apply the "lightbulb in series" trick for powering up equipment to filament circuits?

While the filaments are still cold, the voltage drop should be mainly over the (sensibly rated) series lightbulb which should light up and limit the inrush current. Later the hot filaments take most of the voltage leaving only a small drop over the bulb.

Do you think it is possible to find a bulb with usable ratings?

Greetings,
Rundmaus

[M Gregg]

Originally Posted by Rundmaus

Hi there,

has anyone tried to apply the "lightbulb in series" trick for powering up equipment to filament circuits?

While the filaments are still cold, the voltage drop should be mainly over the (sensibly rated) series lightbulb which should light up and limit the inrush current. Later the hot filaments take most of the voltage leaving only a small drop over the bulb.

Do you think it is possible to find a bulb with usable ratings?

Greetings,
Rundmaus

There was such a lamp made for the job. Dont know if you can get them now!

Barretters and Ballasts

Remember you need to take resistance readings from the hot and cold lamp.

I still use the inrush current suppressor + relay.

Regards
M. Gregg

[tomchr]

I use soft-start on my filament regulators. Works well.

~Tom

[aardvarkash10]

its a helluvalot easier to wire in a constant current device...

[Miles Prower]

Originally Posted by Rundmaus

has anyone tried to apply the "lightbulb in series" trick for powering up equipment to filament circuits?

Light bulb in series with what?

For many dogs' ages it has been a quick 'n' dirty technique to soft start power supplies that include huge reservoir capacitors, high voltage and low voltage alike. Connect a light bulb socket in series with the power xfmr primary, use ~100W light bulb, when the bright glow dims down, close a switch to short out the bulb. Of course, it only works if you don't forget to open the switch across the bulb socket when you power down.

"Idiot proofing" requires more involved automatic inrush limiting that depends as little as possible on user intervention.

[Rundmaus]

Originally Posted by Miles Prower

Light bulb in series with what?

My thought was to put a (sensibly dimensioned) light bulb in series to the (many paralleled) heater filaments to reduce inrush current.

Anyone tried?

Greetings,
Andreas

[barrymagrec]

Most filament lamps have exactly the wrong characteristic for what you want to do, i.e. they are lower resistance cold so the effect would be to increase the switch on surge.
Thermistors are available to do this job but for parallel heaters why bother?

Barry

[DF96]

For this trick to work you need a bulb with a long thin filament, which will heat up much more quickly than a valve heater. This means that it works best with series heater strings, not parallel. Some AC/DC radios do something similar.

The initial surge is not reduced in magnitude (it is slightly increased) but it is reduced in time, as the bulb quickly heats up and reduces the current. The heaters slowly catch up, and the bulb then gets dimmer. I'm not sure it is worth doing, unless you have some spare voltage to drop and need a bulb anyway (e.g. to iluminate a radio dial).

[kevinkr]

There are NTC thermistors that are much more suited to this sort of application. Take a look here for ideas: Power Thermistor - Inrush Current Limiter | Ametherm

Constant current heating is not a good idea with parallel filament strings, but for single tubes it obviously eliminates the inrush completely. You do need to allow for the additional warm up time however..

[M Gregg]

Originally Posted by Rundmaus

My thought was to put a (sensibly dimensioned) light bulb in series to the (many paralleled) heater filaments to reduce inrush current.

Anyone tried?

Greetings,
Andreas

I think you will find that the light bulb filament is low resistance when cold and it goes higher as it warms up! So where is the resistance at power on to soft start the heaters? I would think that you need high resistance at start on the lamp that goes low when hot!


If you want to play then:

I guess it will come on quite quickly and provide the resistance after the initial surge! then go dim as the heaters warm up. Volts * amps gives watts, so from wattage of lamp; calculate the current at power on for resistance compare to heater resistance when hot, work out volt drop across each (filament and heater) using Kirchhoff’s law. Then apply the correct voltage and current for the heaters. Sounds the same as two heaters in series, only the lamp will warm up quicker.

Or put a 6V lamp in series with a tube heater and apply 6.3V then see what current / volt drop you get and adjust the applied voltage to suite!
You will then get over voltage on the tube heater until the lamp warms up at start!

The 6.3v will stop the tube heater getting over voltage at start when testing!

Regards
M. Gregg