Glow plug for ejection system

[Neway]

recently is in vogue the use of glow plug n°8 to ignite ejection charges. See Aerotech electronic forward closure or NO MATCH
Both use a CR123 3V lithium battery, but glow plug must be supplied with lower voltage (1,5V is written in Electronic forward closure manual).

In this manual you can see the photo of both sides of PCB. It is clear that one side of the glow is directly connected to + of battery via ejection canister. The second side of the glow (the one with the wire soldered) reach the PCB, pass through the diode D3 and reach the (I think) power mosfet.


The one million dollars questions are: How the voltage is reduced? With a PWM control of the mosfet?
What is the function of the diode D3?

 

[TheAviator]

Adriano,

I think it is MUCH simpler than that. All semiconductor circuit elements have a voltage change across them. Silicon diodes operating in full conduction are in the neighborhood of .7V. MOSFETs are rated by "R_on," or on resistance. Take the current passing through and multiply by R_on to get the voltage drop across the FET. A battery also has an internal resistance, which is generally quite large for the tiny, high capacity batteries. (A quick Google search suggests 1-3 ohms for the CR123.) All of these factors drop the voltage that the glow plug sees.

 

[Zack Lau]

A series diode is a very reliable way of insuring reverse polarity protection.

 

[bobkrech]

Have you read the instructions for use? https://www.aerotech-rocketry.com/c...tions/EFC_Instructions/efc-1_instructions.pdf This will tell you how the unit works.

To answer your questions.

How the voltage is reduced? There is no reason to reduce the voltage. It's current that heats the glow plug. The more current you put into the glow plug the shorter time it requires to get hot. A CR123 battery can supply > 10 A for < 1/2 second. https://www.batteryspace.com/prod-specs/cr123A_1300.pdf This will get the glow plug hot quickly.

With a PWM control of the MOSFET? PWM control of the MOSFET is not required. The resistance of the glow plug increases as it gets hot, so to first order the current gets limited by the glow plug. To prevent long term burnout, there is a microprocessor controlled 1 second electrical pulse width sent to the glow plug to activate it and then to shut it off.

What is the function of the diode D3? The diode is a polarity control for safety. If the battery was put into the holder backward, the MOSFET would immediately fire setting of the BP and probably electrically damaging other components.

Bob

 

[Neway]

You probably are both right, but I have this guess because in the user manual is written. I have a n° 8 glow plug. today I make some try and let you know

.........
Glow plug power duration...........................................1.0 seconds
Firing current.......................................................................3 amps
Firing voltage.....................................................................1.5 volts
Estimated firings per battery.....................................................200
Continuity check threshold............................................2000 ohms
Typical glow plug resistance..........................................<0.5 ohms

Thanks
Adriano

 

[Neway]

I made a try with a CR2 (similar to CR123 but a bit smaller).

With 2 big diodes --> nothing happened
With 1 big diode --> nothing happened
Without diode --> red filament. I measured 3,8 A peak with a multimeter

So using directly the battery is fine.

 

[sandmantoy]

Is there a simple board that can be made for my perfectflite altimeter to light a glow plug for 1- 1.5 seconds. The only thing stopping me from doing this is the latch time and my limited knowledge of electronics. I always have room in my bays for more electronics, I just don't know how to go about making what I want. I have used regulator boards for changing voltages on some of my experiments that you can buy already made. I hate to buy a CR2 just to scavenge it and it not do what I want anyway.

 

[bobkrech]

I made a try with a CR2 (similar to CR123 but a bit smaller).

With 2 big diodes --> nothing happened
With 1 big diode --> nothing happened
Without diode --> red filament. I measured 3,8 A peak with a multimeter

So using directly the battery is fine.

Diodes allow current in one direction only. Are you certain you had the diodes connected properly? Try reversing the direction.

The diode drop is only 0.7 volts. Without the diode in the circuit, the glow plug will activate immediately if the battery is put in backwards, so they are an important safey feature that should not be left out.

Thr CR2 battery does not have as much electrode area as the CR123 so its impedence is higher, and will not source as much current as the CR123. The CR123 will source over 10 amps.

Bob

 

[Neway]

Diodes allow current in one direction only. Are you certain you had the diodes connected properly? Try reversing the direction.

Yes, infact the were a bit warm after 1 second of closed circuit.