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Post  Tom Espo Tue Aug 10, 2021 4:48 pm

I've been using a power panel to start my models and its powered by a 12 volt motorcycle battery. The glow plug outlets are rheostadt controlled but the voltage through all settings is always 12 volts. I have another older power panel with no rheostadt knob and the voltage for the glow connections is also 12. Why haven't my plugs burned out? I thought plugs needed only 1.5 volts.
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Post  Ken Cook Tue Aug 10, 2021 4:59 pm

Tom, your correct in that glow plug voltage is 1.5 volts. Your older panel probably has a ceramic resistor vs a rheostat wired into it which steps down the voltage.
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Post  Tom Espo Tue Aug 10, 2021 5:13 pm

But I checked the voltage coming out of both power panels, its 12 volts.
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Post  balogh Tue Aug 10, 2021 5:29 pm

The voltage is 12V but it is fed intermittently through a tiristor or similar circuit such that the current is on and off thus the time-average is near 3 amps which is the right current for the glow plug.
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Post  Levent Suberk Tue Aug 10, 2021 5:45 pm

@Tom Espo wrote:But I checked the voltage coming out of both power panels, its 12 volts.

You read 12 Volts with a voltmeter but output is PWM (pulse width modulation), working in intervals.
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Post  rsv1cox Tue Aug 10, 2021 6:16 pm

@Tom Espo wrote:I've been using a power panel to start my models and its powered by a 12 volt motorcycle battery. The glow plug outlets are rheostadt controlled but the voltage through all settings is always 12 volts. I have another older power panel with no rheostadt knob and the voltage for the glow connections is also 12. Why haven't my plugs burned out? I thought plugs needed only 1.5 volts.

You need to understand Ohm's law:

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship

Glow plugs are designed to operate at 1.5 VDC.  Your power panel has a device to drop 12VDC down to 1.5VDC.  Voltage is just electrical pressure, it's current that does the job.  i = e/r or more conventionally i=v/r. i being current (measured in amperes), v being voltage, r being the resistance of the glow plugs element.  With your rheostadt you are varying the current through the glow plugs element not the voltage causing the plug to glow more or less bright.  You can still burn your glow plug out by exceeding i.  

With a 1 1/2 volt DC battery you have no control, only the charge left in the battery.
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Post  Tom Espo Wed Aug 11, 2021 9:06 am

Great review of basic electricity, I remember now that eagles fly over Indians and rabbits: e=ir. Thanks very much everyone!
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Post  Yabby Wed Aug 11, 2021 4:01 pm

If you want to drop/control voltage and for it to be variable you need a potentiometer, a rheostat limits current as they are different circuits.

A rheostat is basically an adjustable series resistor as one end of the rheostat is basically dangling free, so the adjustable connection is in series with the load. A potentiometer is a variable resistor with 3 connections. the variable resistor is basically connected across +ve and ground, or which ever theory (hole theory, electron flow) you wish to subscribe to :-) The variable connection on the resistor part goes to the load. This now forms a voltage divider and will control voltage.

You can use pretty simple and cheap solid state LMxyz Voltage regulators with a couple of resistors, maybe a cap or two and that will limit voltage for you. needs a heat sync as you are burning off the excess voltage. You can also put a resistor in series with the load to limit current if you so wish, but the VReg will do that to some extent. Read the spec sheet.

A buck converter is a DC to DC down converter with reasonable efficiency, so if you have a nice Xvolts source battery the converter isnt wasting all of the energy in the down conversion. here is a hobby level simple one.

https://core-electronics.com.au/20w-adjustable-dc-dc-buck-converter-with-digital-display.html

Probably the best to use is a "switch mode" supply which is far more complex electronically but if you buy a pre made ready to go unit, it SHOULD be the most energy efficient. :-) This link explains them fairly well, as there are switch mode supplies and then hybrid buck switch mode supplies for DC to DC switch mode, and regs added in etc.

https://www.electronics-tutorials.ws/power/switch-mode-power-supply.html

The thing is just buy one pre-made from somewhere. Electronics places sell them, usually pretty cheap and they are small and light. Put them in a nice case if they dont have one and connect it up to a good 6V or 12V battery of whatever type you like and away you go. If you look for this stuff via electronics hobby type sites and stores I think you will do better than through model plane stores.

Me, I still use two D cell batteries lol! cheers Clapping Cos Im too bloody lazy to read the spec sheets and make one or even order one and assemble it all. Maybe I can get one of my grads at work to build it for me!!! sunny Clapping

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Post  rsv1cox Wed Aug 11, 2021 5:02 pm

I took the easy way out Yabby and bought one already made similar to this.

Glow plug voltage Glow_p11

Tower I think, years old but blessed every time I use it.  I wired an on/off switch in series with the battery to ensure off is off and give me peace of mind.  Two D cells wrapped with electrical tape was my go-to also for years until I got sick of buying them. Savings paid for the panel.
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Post  Admin Wed Aug 11, 2021 5:05 pm

Without knowing more about your panel, it is possibly something like this below.

Glow plug voltage Real_c10
Glow head powered. Blue probe is what is displayed on your panel. Green probe is what you'd measure across the head. Values are arbitrary (I don't actually recall how much current a typical glow head draws). Glow heads have a positive temperature coefficient which means the resistance increases as it heats up, so measuring the resistance of a cold head would be much lower than if you measured one glowing.

Glow plug voltage Real_c11
Glow head disconnected.


Not having a commercially made power panel, and looking up the one Bob posted, it could be an IC controlled unit that changes the duty cycle of the pulses driving a mosfet. With no load/open circuit, you could still measure 12 volts (similar to the diagrams above).

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Post  Kim Wed Aug 11, 2021 6:11 pm

@rsv1cox wrote:I took the easy way out Yabby and bought one already made similar to this.

Glow plug voltage Glow_p11

Tower I think, years old but blessed every time I use it.  I wired an on/off switch in series with the battery to ensure off is off and give me peace of mind.  Two D cells wrapped with electrical tape was my go-to also for years until I got sick of buying them.  Savings paid for the panel.  

If the Swap Meets ever come back, there are BUNCHES of power panels---often with complete field boxes that can be had for incredibly cheap prices.  The exodus flyers from glow power can offer quite a bargain for fledglings who are wanting to sling some castor.
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