Remote Glow Plug Ignitor

Discussion in 'General Electronics Discussion' started by deanenp, Jan 25, 2013.

1. deanenp

5
0
Jan 23, 2013
Hi All

I'm entirely new to electronics and the last time I covered any theory on electricity was more than 10yrs ago, so please excuse my lack of knowledge.

My Problem:

I fly RC Helicopters and would like to create a remote glow plug ignition system. To elaborate, a glow plug is simply a heating element that, with the aid of a starter motor, starts the helis engine. Glow plug can usually be heated using a 1.25 -1.5V 1700mAh battery that is attached to the engine and then removed once started. I would prefer to have an on board ignition system that can be activated by a tactile switch.

The system would make use of the 555 timer IC on a 10 sec timer i.e. the glow plug should be heated for 10 secs and then switches of until the switch is pressed again. From my research (taken from the nomograph monostable operation section of the datasheet) a 10M resistor and 1.2uF capacitor should give me a 10 sec high output. The entire system would be powered by a 7.4V 2200mAh lipo via a voltage regulator set to 5.2V (10A const, 20A peak). The datasheet also indicates that the trigger voltage must be lower than 1/3 of the input voltage.

I plan to design the circuit as illustrated in the monostable section of the http://english.cxem.net/beginner/beginner1.php website.

Questions:

1. Have I got the correct resistor and capacitor values for the 10 sec timing? - don't worry about this question the design website provides the values for the resistor and capacity for 10 sec (100K and 100uF).
2. How do I get the trigger voltage to be less that 1/3 the input, or would the timer resistor do this anyway?
3. Is the layout in the pic incorrect for my problem and if so how can I modify it for my use?
4. What can I use to boost the output current (power transistor - maybe?) and drop the voltage to 1.25 - 1.5V (resistor/regulator)?

I'd appreciate any help.

Thanks,

Deanen

Last edited: Jan 25, 2013
2. duke37

5,364
772
Jan 9, 2011
1. The resistor could go up to 1M and the capacitor down to 10uF. This would be smaller and lighter.
2. The trigger voltage is inherent in the 555. you could add a transistor amplifier too make the inpit more sesitive but I do not understand why you should want this. A resistor from pin 5 to ground may do the job.
3. What pic?
4. You do not want a resistor or series regulator, these waste power. A switch mode buck convertor would do the job but I would make a push-pull oscillator and a 9:1 trnasformer to get the right output voltage. I would run the glow plug on AC so that lossy rectifiers are not used.
4a. You do not say what current the glow plug uses. 10 sec at that current out of the lipo may not be significant.

3. deanenp

5
0
Jan 23, 2013
Thanks.

My understanding was that the trigger voltage had to be less than 1/3 the input for the timer to start (I may has misinterpreted this). So in this case, my input from the voltage regulator is 5.2V and the trigger would have to be ~1.6V.

Sorry, just figured out how to attach the pic to the post.

Sorry, i left out some important info.

For the glow plug (plug resistance = ~0.5ohm) to heat up enough, the current should range from 2-4A, so the current output of the 555 wont be adequate at 250mA. I've read that a transistor connected to the output could increase the current or act as a switch i.e. activating the glow via the 5.2V input. If I chose the transistor option, I would have to drop the voltage to 1.25 - 1.5V to prevent the plug burning out. The Lipo is rated for 20-25C which means I can draw upto 55A (peak) for ~10sec.

I will need to do a bit more research to understand buck converters and the push pull oscillator, but thank you very much for the suggestions.

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4. duke37

5,364
772
Jan 9, 2011
I have met a 555 but I am not on speaking terms!

The trigger needs to go to less than one third of the supply voltage. Your switch will take it all the way down to zero so there will be no problem.

The 555 can run on 7.4V and will not need a stabiliser, it could be driven directly from the battery.

To get 2A into the glowplug from 7.4V, a 3ohm resistor would do.

Switching 3A is intermediate between low and high current. I would use a power FET with its gate connected directly to the 555.

5. deanenp

5
0
Jan 23, 2013
Thanks for that duke.

I've bread bordered the timer and that works pretty well. The timing is slightly off, but I suspect that's due to the capacitor tolerance.

I might have a few questions on FETs in a couple of days. Been reading the "easy" guides on how to chose them and been having some difficulty. I'm gonna have another read through them and hopefully it'll sink in.

Thanks

6. CDRIVEHauling 10' pipe on a Trek Shift3

4,960
652
May 8, 2012
This topic surprises me because I've observed the radio controllers near me. The battery packs that they use to heat the glow plugs are quite large and heavy, so I never questioned why they've never been integral to the aircraft. It's a hobby that considers every gram of weight. Admittedly, I'm talking about fixed wing aircraft. I imagine helicopter motors are started by spinning the rotor just like the fixed wing fliers spin the prop. Is this project an attempt to minimize your exposure to the rotor?

Chris

7. deanenp

5
0
Jan 23, 2013
Hi Chris

Helis are slightly different in that they make use of a clutch so when starting the engine, there minimal risk of being struck by the rotors unless you accidently start at full throttle.

With planes, a similar process is used to start the engine, though planes don't have a clutch, instead the starter motor is attached directly to either the propeller or propeller nut/cone.

Glow plugs only need a 1.25V battery to start the engine. So I think that the large batteries you've seen are more likely used to power the starter motor.

To be entire honest, this project is my attempt at being tight with my cash. There's a general trend in RC hobbies were accessories cost a fortune. I don't believe a voltage reg, 555 timer, FET, couple of resistors and capacitors should cost upwards of £15, especially when when I can buy all the components for less than £4 and have the knowledge of how to repair the device when/if it breaks.

I think I've just become a grump old fart, even though I'm 28.

8. CDRIVEHauling 10' pipe on a Trek Shift3

4,960
652
May 8, 2012
Hey, it's always a good to be ahead of the curve!

9. duke37

5,364
772
Jan 9, 2011
You say the glow plug is just a resistance and that the battery is capable of very high current output.
Power in a resistance is proportional to voltage^2 so if you supply 7.4V to the plug, it will need to be connected for 1/(7.4^2/1.25^2) of time, I think. This is about 3% of time.

You will need a 555 running at high frequency, driving a transistor to switch the current to the plug. The frequency will need to be high enough so that the temperature only rises a small amount on each pulse.
There will be very little power lost.
A second 555 can do the overall time as you have already tried.
Two 555 can be obtained in one chip (556).

10. CDRIVEHauling 10' pipe on a Trek Shift3

4,960
652
May 8, 2012
Since a hi current Lipo is going to take up a bit of real estate anyway have you considered a two battery system? A small 9V battery can power the 555 while a 1.5V cell can drive the glow plug. Cheap too!

Chris