Glow Plugs Voltage

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Jan 17, 2009
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I am done machining my Edward 5 and started the construction of the display base.
The base includes the fuel and gas tanks, a drawer for all the special keys, extractor a whatnot that goes with the engine and the powering and monitor system for the 5 glow plugs.

I plan to heat the plugs with AC. A 24VA transformer rewound with 5 secondaries for about 1.5V each.
I want to sense each plug because if one goes bad I have no indication other than possibly rough running of failure to start. I have to disconnect them all and use my ohm-meter to find the bad plug.

I could use only one secondary but it will be too too thick a wire to bend sharply on the bobbin and would require a 5 floating shunts on the plugs tips side.
With 5 secondaries all the shunt can be grounded and the secondaries are smaller gauges.

But all this is not the reason of this post, just background for the real question.

First a bit of history
those of us old enough will remember that Glow Plugs use to be 2V, back then the only battery available was Lead-Acid. A single cell charges at around 2.3V, as soon as disconnected from the charger they fall to 2V and stay there until discharged to 1.8V. End of story.
Then came those wonderful NiCd, the high discharge rate made it possible to use a small capacity cell that could be coupled to the plug connector and the plug igniter stick was born.
I suppose, but have no direct knowledge that the old 2V plug would work enough on a freshly charged NiCd cell.
Everybody called the NiCd cell a 1.5V cell and the 1.5 Glow plug was born.


A NiCd cell is charged until it reaches 1.5V. When connected to the load it quickly drops to 1.4V
Depending on the discharge rate I/Capacity and temperature it can be discharged down to 1-1.1 V
During a slow discharge the cell voltage is averaging 1.35V on a brisk discharge we may say the average voltage is about 1.2V.

Calling NiCd cell a 1.5V cell is just marketing talk, technically speaking convey zero information.

Now to the question: Has any one ACTUALLY measured the voltage and current on a properly working, capable to start the engine, plug powered by a middle of the charge stick or single cell NiCd?

All the other data is just guessing.
I have been messing around with glow-plug engines since 1955 and they were always powered with 1.5V. The standard flying field power source was the classic # 6 carbon-zinc cell ( no longer made). I have never seen or heard of a 2V plug.

Re your question, apparently a single ni-cad cell works well enough so that the market place supports several different brands of plug sticks. What is the concern?

According to this Wiki article on model glow plugs,, a typical glow plug will pull about 3 amps at 1.25-2 volts. A "cold" plug needs 1.25-1.5 volts, and a "hot" plug needs 2 volts. From Ohm's laws we know that Watts=Volts x Amps so:
For a hot plug W=2 x 3 or 6W.
For a cold plug W=3.75 - 4.5W.

You are planning on powering the glow plugs with single phase AC. Which, if this was anything but the purely resistive load that it is, would change the equation by adding the power factor into the mix. With a purely resistive load the power factor is 1.0 so DC equations are equivalent.

Within reason, you can use whatever voltage you want to supply the glow plug, as long as you limit the current so that you are not trying to dissipate more wattage than the glow plug can handle. Whatever voltage you wind up using in your transformer, you need to limit the power the glow plug is trying to dissipate to the above values of 6W for a hot plug and 4.5W for a cold plug. Otherwise your glow plug will work REALLY, REALLY well. For a short time. Once.

. I have never seen or heard of a 2V plug.

Fox made a 2V plug and I remember that one had to specify 1.5V or 2V when buying.
I was flying U control back in the '60 in Italy. The only source available there was a D cell Carbon Zinc or a 7Ah Single Cell Lead Acid very expensive and short lived. We usually begged for a bad car battery at the local garage and dug trough the tar cover to access one of the usually still good elements.

Anyway thank for the electronic lesson but I am winding the transformer so I can set the voltage to the correct value without current limiting.
As Don has noted amps required to each plug will be determined by the type of pug you use, I believe it would be wise to feed the secondary voltage via a rectifier so you are supplying DC to the plugs. There are glo-plugs made especially for 4stroke engines, several manufacturers, OS being most used in the UK.
When you have decided which plug spec suits your needs and you have them it's a simple job to measure the current at various voltages, so preferable to have a voltage adjuster in your circuit, this could be on the primary but best on the secondary as you may want to retain a glo on the lower cylinders, especially at tick-over or low revs.
I believe it would be wise to feed the secondary voltage via a rectifier so you are supplying DC to the plugs.
Not necessary.
The plug is a resistive heater. The heating effect is proportional to I squared.
A full wave rectified sinusoidal current has exactly the same RMS as a DC current, assuming you scale it properly.
One may worry that the temperature may fluctuate during the cycle.
At 60 Hz the heating frequency is 120Hz that is the same firing Period of a 4 strokes engines running at 14,400 RPM. If the catalytic heating effect is good enough to keep the plug hot at 2,000 RPM, 60 Hz current should be plenty fast.

I do plan to add a primary control to change the output from about 1.5 to 1V.
Within limits changing the plug current is the same as changing the advance.

I do not know the difference between a 2 stroke plug and a 4 stroke plug.
The biggest problem I had when I was a kid trying to start engines with glow plugs was the carbon zinc cells with their high internal resistance. Nicads and NiMh are so much lower internal resistance so the plug still glows in the presence of a cooling incoming air/fuel charge. If I was to design a glow plug driver it would be a constant voltage affair with very short leads and the absolute lowest output impedance I could make. Those single cell sticks that you place on top of the glow plug work so well though I've never bothered.

How big is the transformer and also the primary and secondary winding gauges? If it's too small then you'll have to deal with drooping voltage just when you need it to be rock solid.
Having DC to drive the plugs opens up many possibilities for converter circuits which will provide a definite 1.5v and ease of monitoring.
Transformer will be all over the shop load/no load and limited monitoring possibilities.
I think Enya as well used to make the 2v plugs at least it's what I remember from the days of control line flying back in the mid 60's.
Just about every different plug brand, style, model will have different requirements if you want to get into the nitty gritty, BUT an off-the-shelf C size nicad usually covers all without any problems.
Current and voltage monitoring can be easily done on DC or AC via a few ACS712 or if using DC supply, INA219 modules and naturally DC for Arduino and lcd display or whatever.
Actually there are 3 AC voltages that are used in equations.
For 60Hz:
Peak-to-Peak, RMS, and DC equivalent(rectified AC).
P-P= 1:1, RMS = 0.727 of Peak, and DCE = 0.707 of Peak, these values are used when calculating watts.
1.5v DCE = 1.5 x 1.414 = 2.122 Peak or 1.5 x 1.376 = 2.063 RMS

24VA = 24 volts RMS output at 1 amp or 24 watts, don't forget now that the more current that is pulled through the transformer the more the voltage droop will be.
If you want 1.5 volts DCE (rectified AC) you need to know the resistance of the glow plug. Each plug will draw between 2 & 3 amps so your looking at 10 - 15 amps total or 15 - 30 watts. Your 24VA transformer isn't going to work for to long before it overheats and dies. But there is a way around this, I'll explain later.

The reason we RC people use 1.5 volts is from the old carbon pile batteries we used back in the day. But, this has become more because in order to classify a plug as hot or cold or 4 stroke we needed a standard that had 1 fixed value (voltage) and variables (resistance & current), Ohms law stuff and voltage is the easiest to measure. I use a 12V 7ah SLA battery with 2 voltage regulators (6v to 3v), I usually set it to 1.5 volts in summer and 1.8-2.0 volts in winter depending on the temperature, as low as -20C/0F.

My advice to you if you want to use your 24VA transformer (110 down to 24 VAC) is with a full wave bridge rectifier, a filtering dielectric cap, and 6 - 10 voltage regulators if you want to monitor each cylinder. To go from 24 volts down to say 2 volts is a lot to ask from a single linear voltage regulator and it would need a huge heatsink and cooling fan. You could go with 1 really big linear down to 5v and then 5 regs (1 for each cyl.) down to 1.5v with a really big heatsink and cooling fan.

My best advice would be to either get a filament transformer 45 watt used for vacuum tubes 1.6v or get a very old computer or it's power supply (AT not ATX) and use the 5v output and then 5 linear regs with a very big heatsink for the 1.5v, a resistor and LED in shunt to indicate failure of a plug. There is no free lunch here.

One way to get around the high current draw is by using what some glow plug power supply manufacturers are doing and that is using pulsed DC. Since the glow plug has a heat up and cool down cycle we can use something like a 555 timer to pulse the DC. This would not heat the regulators up as much or as fast and you could use a smaller heatsink. So long as you adjust the cycle time high enough to start the engine you could later turn it down to keep it running clean and the PS cooler. The pulsed DC would look like AC on a meter. I hope to make a 20 cylinder 2 stroke glow plug motor one day and this pulsed DC will be the way I will be going.

If you need further help just ask.

If you rectify the 24V, you'll need a switch mode power supply to drop to 1.5V. Using a linear for the big drop will mean 2-3 amps at 24V which won't last long.
Why use 24v when you only need 1.5v.?
If a standard dc supply was used, say a 6v or 12V battery for convenience of supply and cost, pwm the supply as suggested in #11.
Mosfet output would mean very little generated heat in output elements and if logic level, can be easily driven via 5v logic as in microcontroller.
Thank all of you for the lesson in electronics but as a designer of power supply for the last 50 year I think I can manage.

My question was about the Plug data, all the clever powering solution means nothing if the "load" is poorly defined and understood.

A 24 VA transformer is perfectly adequate to power 5 plugs for the 10 seconds necessary to start the engine. 1.5V x 4A x 5 Plugs = 30 VA Transformers are highly overloadable for a short time, anybody that has boiled water knows that the pot temperature rises slowly.

I never said I would use 24V, I did mention the use of a 24 VA (VoltAmpers is the correct power rating unit for all AC machines). I bought a 120/12V transformer removed the 12 V secondary and rewound with 5 separate 1.5V secondaries.
5 secondaries for 2 reasons: a) Smaller wire bends easy on a small bobbing with less "belly". b) I want to sense the current. Just sense fro plug failure not for measurement. One winding need the shunt floating requiring differential amplifiers. 5 wingdings allow the sense shunt to be grounded a simple comparator can distinguish between a sound plug and an open plug.

The main reason I use AC is that I hate batteries. If I were flying my engine I would use a battery but next to and AC outlet I have other options.
As a professional in the field I can say this with authority that 95% of all battery powered consumer products have undersized capacity and poorly designed charging circuit as we know of e-cigarettes burning in pockets, laptop igniting in airplanes, drills letting the user damage the battery by deep discharge and cell phones having various problems of their own.
Now to the question: Has any one ACTUALLY measured the voltage and current on a properly working, capable to start the engine, plug powered by a middle of the charge stick or single cell NiCd?

Thank all of you for the lesson in electronics but as a designer of power supply for the last 50 year I think I can manage.

Well it seems you are not so much of an expert that you profess.
Many people here offer their suggestions and many are correct in their statements.
You have already been informed that every plug from every manufacturer has differing characteristics and that your approach would require a "suck it and see approach".
OR you could go with some of the more " current suggestions" in the way of electronic control.
People here offer their help but it seems you might be more interested in simply blowing your own trumpet and insult everyones opinion into the bargain.
For me at least, good luck next time you have need of " uneducated assistance".

BTW, when you do your transformer VA calculations, remember to include losses and inefficiency.
Bluejet, if you have any comprehension of the written word you would have realized I did not ask for suggestion on how to power the plug. Did not involve the battery types, their chemistry or their specification. Did not raise the issue of wire resistance, adjustable voltage or any subject regarding the power source.
I simply asked for a fact, if you know the answer you receive a thank for sharing.
If you think that is appropriate to digress of topics just because you think you know stuff you get politely rebuffed. Obviously you do not have the maturity to engage in technical discourse.

By the way, darling, I designed and got paid for, more than 500 transformers (who is counting) and never forgot to calculate the losses when that was appropriate.

As a courtesy I refrained to point out the many errors in your replies, therefore I do not need you to question my professional credentials.

I suppose all aircraft manufacturer use the engineering "suck it" approach rather than find out their components specifications, as variable as they may be.

All considered all other respondent bits of information collate into a useful specification for a typical plug. I got my answer from those truly interested in helping, for which I thank them. Thank to their help the 5 plug and the transformer are very happy.
So, basically just blowing your own trumpet then.
Good to remember.
I imagine you missed the bit where you were told the characteristics of plugs.
There are almost too many links on 2S vs 4S plugs & I find a lot of it is chatter repeating what someone else has heard. I figure if OS develops a dedicated plug for 4S engines, there must be a reason. Having said that, I have run 2S plugs in 4S engines. Its mostly all about the element but also influenced by nitro% in fuel and what you want out of the engine. Some will run, but not last as long before element distorts. Some have indistinguishable high end but not as good an idle. Some 2S plugs have acceptable element but the plug bodies are a bit short for 4S heads. These commercial engines may have slightly different compression ratio than what we build which is a big factor for glow plugs, so there are a lot of variables.

Most RC guys are not running constant power to the plugs while running. Some folks will have a simple on board battery or 'system' that powers the plug only during idle. Its hard to know if this is for perceived insurance for a dead stick landing or it actually works. Because I would have to estimate 98% (in the thousands) of 4S engines just rely on just powered glow at starting & including inverted engines. I see it on-board glow more in scale planes, but there are probably more high $ models of other classes that rely on reliable glow.

This link has a bit more elaboration of running effects & different plug brands/ratings that might be available to you depending on where you live.

I have taken apart some of the simple igniters & from what I can see the only circuitry is dedicated to feeding the little current reader if it has one, otherwise they look just like the non-metered igniters.
I think by & large they are depending on the nominal battery voltage. The cell size (sufficient maH capacity) is important for not only multiple starts between charging, but voltage suppression under load (C-rating). These in-line Ni* battery igniters generally do the job, but there are better electronic starter boxes out there that regulate power more reliably and efficiently. Most use Li* chemistry cells at different nominal voltage so require circuitry to step down the voltage & are probably more efficient by micro plulsing... but all this is above my pay grade.

Our multi-cylinders require some kind of permanent wiring harness in any event just out of practicality. So I guess we have the choice as to when to ignite or not and/or by how much. And more cylinders means more power, This V-12 glow has what appears to be (12) dedicated NiMh cells. But hard to see from the pic if there is anything else in the circuit.


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The OP asked just the question:

"Now to the question: Has any one ACTUALLY measured the voltage and current on a properly working, capable to start the engine, plug powered by a middle of the charge stick or single cell NiCd?"

I see nothing wrong in asking that, on the basis that no question is stupid if you don't know the answer, and what are these forums for if not to ask questions on model engineering from like minded folk who might just know the answer?

However, it seems like most answers here deviated into answering and commenting on everything BUT the original question, like has anyone ACTUALLY measured the voltage and current..... etc etc. I might have missed it, but if I did it's no wonder!

I'm not sure Googling this question would have produce an exact answer to the original question either. Asking the question here surely is the better option?

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