Rudeboy

This was your original post... it says right there you wanted to test how much voltage a plug could take... not that you wanted to find out about "other means" to light a plug.

About that power supply, you are right: a 3A power supply is not enough to kill a plug. You need something bigger.
Everything that forms a closed electrical circuit, hooked up to a power source, is a conductor. The moment current flows, wether it is a lot or very little, electricity is being conducted.

A 12V/7Ah battery to start a car...? Must be an awful small engine and a VERY good battery...

You are also right in stating that you can't blow a plug with voltage. It is not the voltage that kills the plug, it's the current. BUT, voltage and current are linked together. The resistance of the plug is a fixed value (actually it varies slightly with the temp of the coil, but that is of no importance here). If you increase the voltage over the plug, the current will follow... up to the point where the plug wire can no longer carry the current and melts.

RC-Captain

I know enough about electonics and power supplies to say some power supplies have a fuse , some don't .If using one that has a POT to adjust the voltage , I would beleive it would have a fuse. Only because one could regulate the voltage, and if connected with the wrong polarity, something will have to give out.

Your method can be used but like I said a power supply has limits VS batteries don't becasue they only drain and not provide constant flow of voltage.

Rudeboy

Batteries have limits too... that's what we've been trying to tell you all along! You can only draw so much current from a certain type of battery. How much depends on the internal resistance of the cell type... the lower the internal resistance, the more current the cell can deliver.

There are power supplies with TWO pots (like the one in the link fgonzale provided): one to limit voltage, one to limit current.

RC-Captain

Exactly , it melts

True but for what other reason would I need to find out the amount of voltage the plug can handle ?
I'll answer: so I can use other means of getting the plug to glow. The two reasons go hand and hand.

Rudeboy

Well then you could have just read what it says on the package...

downunder

How about one of you guys who knows more about electricity than me (I changed the batteries in a torch and it threw out a beam of dark ) try finding the resistance of a cold plug and then check the resistance with the engine running? And if you've got a twin plug engine, do it on the plug that doesn't get used to start the engine too. Now that's something I'd really like to know

blikseme300

My Feedback: (118)

Sorry to burst some bubbles here - you all seem to misunderstand what makes glow ignition work. The glow plug is a dual function device. The glowing coil will get the engine running, but the platinum coating is the catalyst that keeps it going once the battery (cell) is removed. Excessive current (heat) will cause the platinum to burn off and then you just have a nice heater coil.

Common symptom of this kind of plug damage is that the engine quits after removing the glow driver. The coil will still glow when current flows, but it is damaged.

Safe Flying!

Rudeboy

Downunder,

That would be an interesting experiment: just hook up a digital Ohm meter to the plug while the engine is running and one should see the resistance wander when going from idle to full throttle and back.

fgonzale

For your simplicity, use a AA battery (NiMH recommended @2000mAh for the glow plug starter, smaller, lighter and cheaper than a 9V battery, and won't do any harm to a glow plug. If you plan to run an electrical experiment with a glow plug, then make sure you understand the theory well. A current limiting power supply is the best way to go, a 3A one might not be enough i have one that runs up to 15A. And yes there are much more advanced methods of protection than fuses on electronic equipment, like current limiting, as RudeBoy says, you have two pots there and you can play with both limiting current and varying voltage while at the same time you get meter readings to know the exact moment when the plug blows.

Rgds,

Felipe

fgonzale

For Rudeboy, don't try that, as when the engine is running you can't measure the resistance that way, when the engine is running the glow plug acts as a coil that actually generates electricity because its magnetic varying conditions as the head moves. so if you plug a digital or analog ohmeter it will damage it. One thing that can be done is to measure the voltage out of the glow plug and take a look at it on an oscilloscope as it is not a plain DC voltage. I have not done this, but you should be able to see a signal on the oscilloscope with a frequency that should be proportional to the RPM of the engine, looks interesting and worht to try it. Indeed when the engine is running you need to calculate the impedance, which is different that the resistance as it contains another component caused by the nature of the glow plug that is a coil + resistance. Anyway don't recomment plugging in an ohmeter directly as will receive voltage that will kill the meter likely.

Rgds,

fgonzale

Rudeboy

I'm no electrical engineer, just an amateur, but I fail to see how a glow plug coil in a running engine could be generating electricity... And what do you mean by "magnetic varying conditions as the head moves"?

I have a 5$ digital multimeter here I use at the field for quick and dirty measurements. Next time I run an engine that has a throttle I will hook it up and we'll see... if the meter dies in the process, I'm 5$ poorer...

William Robison

My Feedback: (3)

Brian:

The plug Resistance is inversely proportional to the coil temperature. When the combustion reaction is heating the coil, the electric current drops way off.

When I'm figuring glow current/voltage I use 4 amps at 1.2 volts as my base line, and this is at the high end of the range. Many will heat fine at 3 amps, depends on the individual plug.

with a single Ni Cad connected I've measured the current dropping below 300 ma at full speed on the engine, rising to roughly 700-800 ma at idle. When the engine quits the furrent immediately goes back to 3-4 amps.

This drop in current is what makes an electronic glow controller unnecessary, battery charge life is still very good without any control other than a switch, and an ordinary switch is a lot less expensive too.

Bill.

William Robison

My Feedback: (3)

RC-FIEND:

You got an arc when you connected your 60v set of transistor 9v batteries because at the instant of connection you did indeed have 60v applied. The moment the voltage was connected to the load (glow plug) the internal resistance of the batteries dropped the applied voltage way down. The transistor batteries have a maximum current in the range of 100-150 ma, the true applied voltage probably was less than one volt.

If you use a supply that will still deliver the 3-4 amps at higher voltages, it is a rare plug that will not turn into a puff of smoke before you reach 4 volts applied.

Bill.

blikseme300

My Feedback: (118)

Rudeboy is correct in questioning the possibility of current being generated by a moving piston relative to the glow plug. Can't happen, won't happen. Pistons are generally manufactured of aluminum alloy - aluminum has a very high reluctance and virtually no magnetism. Next time you have a magnet handy, try to pick up a piece of aluminum with it. Steel, on the other hand, has a low reluctance and is very magnetic capable.

Do a google on reluctance if interested.

You WILL see a change in resistance when the engine is running. The change in temperature of the plug will affect the resistance.

It is not the voltage that is applied to the glow plug that is important, it is the current passed thru it that matters. Do basic power calculations if you like. This is the same as when Ni-Cad cells are charged. You can apply 100vdc across a cell and it won't be damaged if you limit the current. (And remove the current source once fully charged, of course.)

Safe Flying!

William Robison

My Feedback: (3)

fgonzale:

Missed your posting earlier, sorry.

Electrical reluctance applies only when there is a change in current or voltage. It is the DC equivalent of impedance. A I said, it does not apply in a steady state condition.

As for any voltage generated by the piston movement? Never happen with an aluminum alloy piston, and even with a magnetized iron piston at most you would get nanovolts. The plug coil doesn't have enough turns, additionally it's magnetically shielded by the ferrous body of the plug. The only danger in connecting a DVM to the plug while the engine is running is catching your finger in the prop.

Bill.

meowy84

William, ah finally here you are. Glad you arrived to explain to these kids how things work.

May I also point out that doing this experiment with one plug is absolutely useless. whatever happened to repeatability and consistency in the scientific method? If I get one OS #8 plug to burn out after I apply a specific amount of "juice" that doesn't mean that the next OS #8 will burn out at that same precise value. The only true value is a average. Second, a 1.2V plug rating means about as much as the horsepower rating on a car. Unless you duplicate the exact atmospheric conditions and use the very equipment the factory used to arrive at the hp rating you'll never get duplicate it. Equipment varies. Put the same engine on a different dyno and you'll get different results. Also, materials accumulate stress. Let's say that the test plug was ok after applying x but burned out burned out after applying x+1 current and voltage. That means nothing because the plug could have been already severely damaged by x and x+1 simply put it over the edge. Also, the plug used in the experiment was an old used plug. If it was already well worn then chances are it didn't take much to put it over the edge and 'blow it'. As electrical conductors age their internal resistance increases. I could go on and on blowing holes in this plug experiment. The point is that this experiment proves nothing.

fgonzale

Good point on the voltage generated by the plug, haven't tried it. I forgot about the aluminum composition of the piston. Maybe not enough to generate a signal to kill a DVM, i will run the experiment with an oscilloscope to see if the gplug actually gives some sort of pulses even ata mV level everytime the piston comes up proportional to the RPM of the engine. I guess this is different topic now than the original experiment about burning a glow plug. Will let you know about it as soon as i get the equipment together.

[8D]

RC-Captain

William, not only was it arcing , the plugs nipple was welding itself to the clip. I didn't measure the voltage when it was welding but......what difference would it make any way? (1) the plug was still glowing and was very very HOT . (2) who would use 60vdc to power a glow plug ?

meowy84, I don't have a clue as far as what you are trying to explain. I was always taught from grammar school on up , any experiment is only a theory , hypothesis, then a conclusion. If you want to test 1 million or every glow plug manufactured for tolerance , the world is yours . As far as I'm concerned ,melting a glow plug is not blowing the plug. There is not any type of explosion involved in the situation.


fgonzale, electric current as others has said doesn't exist after removing the glow starter. Now if you want to invent something , make a glow plug that can store electric current for the amount of time a normal RC pilot flies . I will give you one component needed , its called a capacitor.

The only thing you will get is a dead short whether idling or at full throttle. The meter will only read across the crankcase.

Crash90

My Feedback: (17)

It's pointless. He just doesn't get it. The glow plug never saw 60V. The glow plug never saw 60V. The glow plug never saw 60V. Place 3 automobile batteries in parallel and connect them to your glow plug. See what happens with only 12V.

RC-Captain

For a C hair of a second the plug saw 60v. The voltage had started at some level before dropping drastically

cwrr5

My Feedback: (32)

Sport_Pilot

As a mechanical engineer who has designed electronic HVAC control systems I can vouch for the fact that resistance goes up with temperature. For you see that is how an electronic control thermostat measures the temperature in a room. As the temperature rises the sensor (a precise heat sensitive resistor) has more resistance which reduces the voltage and current. The actuator has a pot and moves till its current match's the current flowing through the thermostat. The digital controls do the same but convert the analog signal to a digtal output which measures the room temperature.

downunder

Exactly Bill and this is why I suggested trying it on an engine with twin plugs to see if the second plug really does light up automatically. If it does then it's resistance should vary the same way as the plug that was used to start the engine. I can't think of any other way to check it other than blowing the coil right out of the first plug and you really don't want that floating around inside the engine

Sport_Pilot

RC FIEND,
You are ignoring a lot of people more educated than I. As someone posted V =I*R. Where V is voltage, I is current, and R is resistance. The battery's rated voltage is not necessarily what you get. A 1.2 volt ni-cad can deliver much higher voltage with load when freshly charged, and higher yet on a low load or a short. The R part of the equation includes the resistance of the battery, that is why ni-cads often work better than an alkaline battery, they have lower internal resistance. Don't forget that the arc also has resistance, in fact the arc is probably the largest resistance in the circuit. The heat from an arc is localized. On an electric arc welder the arc causes the steel to melt, but the copper wire, which melts at a much lower temperature than the steel, does not melt. So the glow plug never saw the heat nor the voltage of your 60V setup.

BTW melting the glow plug element is considered blowing the plug. Never heard of one exploding. It is just an exaggerated expression.

William Robison

My Feedback: (3)

Brian:

I've newver bothered to check resistance, but in the twin plug Saito engines using a "Cold" plug in the front hole gives a lower idle than a hot one. If you heat the front plug at idle the engine roughens, (you have advanced the timing) but if you heat it at speed there's no effect - the plug has "Come on."

Using two "F" plugs there's no difference when you heat the front plug at either idle or high speed.

My favorite set is an OS "F" plug in the rear which is heated for starting, and a K&B 1L plug in the front, heated only by combustion.

Bill.

PS: I WILL NOT discuss the ignition timing change here, do a search. I've explained it in detail elsewhere. wr.