glow plug current (amps)

[Vic8]

Following an encouraging response to my posting regarding power panels for a flight box I have decided to have a dedicated power panel for glow plug voltage only. In the event of needing a starter to crank over the engine, that again will be a dedicated power supply. My power panel has a 12v supply. In order to determine a suitable capacity of 12v SLA battery that would be required I measured the current that was needed to fire up the glow plug. I connected up my test leads to the various bits and measured 1.22v AC at the plug and a current of 0.8A flowing from the battery.
The AC voltage is not an issue however I was surprised at how much current was passing through the glow plug. Is 0.8A a ball park figure for the average standard glow plug, I do not expect the power panel`s internal resistance to have much influence.
Thanks

[Don Fry]

I have just measured the resistance of a new OS type F plug. It has a resistance of between 0.3 and 0.4 ohms. Applying ohms law this would then be drawing 3ish amps with a 1.2 volt (nominal) nimh battery. This feels about right to me, about 5 watts

With regard to your setup I thought power panels supply 12 volts to the plug, but pulse the supply so that the plug gets the correct wattage. The voltage/current you quote does not seem high enough. Are you sure that your measuring kit is giving a good reading when I think it is dealing with a rapidly pulsing supply.

 

 

Edited By Donald Fry on 04/03/2015 13:04:10

[Simon Chaddock]

I am surprised you only saw 0.8A, I thought anywhere between 2A & 3A through the plug was a more usual value..

[Vic8]

Posted by Donald Fry on 04/03/2015 13:01:01:

I have just measured the resistance of a new OS type F plug. It has a resistance of between 0.3 and 0.4 ohms. Applying ohms law this would then be drawing 3ish amps with a 1.2 volt (nominal) nimh battery. This feels about right to me, about 5 watts

With regard to your setup I thought power panels supply 12 volts to the plug, but pulse the supply so that the plug gets the correct wattage. The voltage/current you quote does not seem high enough. Are you sure that your measuring kit is giving a good reading when I think it is dealing with a rapidly pulsing supply.

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Yes both you and Simon Chaddock are correct hence my posting. Any measurement taken is only as good as the equipment used, however the readings are as seen by lining up the needle with the mirror underneath. Long gone are the days of using kit with an indate calibration certificate, no matter, even with a 20% error rate there is 1A in the circuit from the battery to the plug. Perhaps the topic will produce a definative answer as to the current drawn by a standard plug.

I have just finished making further enquiries and have been told that my readings may be skewed because of the way the power panel creates the glow plug voltage (pulses). However, in the experience of the gentleman I was speaking to he said that due to the voltage wave (my words) he thought that 0.8A was a good possibility. Furthermore, he has the use of a producing a set voltage from a lead acid cell and finds that at 1.5v a plug will take 1 - 1.5A.

 

 

 

 

Edited By Vic8 on 04/03/2015 13:46:21

[Phil Green]

Posted by Donald Fry on 04/03/2015 13:01:01:

I have just measured the resistance of a new OS type F plug. It has a resistance of between 0.3 and 0.4 ohms.

Thats the cold resistance, the resistance when heated will be much higher.
One thing to watch, most power panels use PWM to control the current. A cheap digital multimeter doesnt like this at all and can mis-read alarmingly. An averaging dmm or an analogue meter is better for reading modulated current.

Cheers
Phil

[Vic8]

Posted by Phil Green on 04/03/2015 13:48:04:

Posted by Donald Fry on 04/03/2015 13:01:01:

I have just measured the resistance of a new OS type F plug. It has a resistance of between 0.3 and 0.4 ohms.

Thats the cold resistance, the resistance when heated will be much higher.
One thing to watch, most power panels use PWM to control the current. A cheap digital multimeter doesnt like this at all and can mis-read alarmingly. An averaging dmm or an analogue meter is better for reading modulated current.

Cheers
Phil

A valid point and one I should have considered in light of the replies above, who checks the checkers. I have just carried out the same test using 3 stand alone analogue ammeters, ranging from the cheap and cheerful to what I consider to be an accurate instrument. Each ammeter was connected in series between the positive battery terminal and the positive input to the power panel. The readings are; 1A, just less then 1A and almost 0.7A. With my initial findings of 0.8A the average of the results make it 0.85A battery drain. I wonder if I have really got to the bottom of the matter.
Thanks to you all.

[Simon Chaddock]

Simple. Measure the glow plug current using a 1.5V DC source. i.e. A battery.

[Don Fry]

This is getting complicated for getting a plug to the oft quoted cherry red. In answer to you original quest, how big a 12v battery is needed for your panel, if only drain on it is a glow plug, any 12v battery will do the job just fine. But it still seems complicated to go to the bother of paying for 12v, when you only want a volt and a bit.

[jrman]

Use a Cyclon cell, an ammeter and a glow plug lead. Job done.

[Don Fry]

With a 10 watt potentiometer, between 2 and 5 ohm, to regulate the current.

[jrman]

Seriously the minimum you need is a 2.5 mAh Cyclon cell and a plug lead. The ammeter is useful to check whether the plug has blown. I have never needed to regulate the current or in fact, seen the need. I have also never blown a plug using the 2v Cyclon direct through a normal plug lead. Keep it simple.

[GrahamWh]

2.5 Ah rather than mAh?

[onetenor]

If wanting an independent supply why start at 12v. glow plugs are either 1.5v( or 1.2v)+ 2 volt. Theefore why not start at 2 v . A sealed lead acid or gel type single cell. ample power for dozens of charges from even a small one. we used to use Ex -GPO single cell 2 volt accumulator as they were known Couple of bob from one or other of our local surplus stores Gerry and her dad and later her husband in Whitechapel Liverpool were the best bet .Sadly no longer there for a long time. These should be available on the net somewhere This is the real KISS.

[John Olsen 1]

Glow plugs will vary in the current they draw, depending on design, eg hot or cold. So a simple resistance to drop the voltage may cause problems if you change to a different plug. But I suspect many power panels do just use a resistance. Note that the resistance of the meter you are using will be affecting the reading you get. The current will quite likely be higher without the meter.

If I was using a 12V battery supply for the plug, I would be looking for a glow driver that can accept the 12 Volt input. Most glow drivers are based on a switch mode power supply so will not waste power. Then later if you want a starter as well, the same battery can also be used for that. I have been using a separate glow stick myself, but I use a small sealed lead acid battery for the starter which is quite convenient. Ideally this would be a deep cycle one. This could easily run a glow driver as well as the starter. Such a battery also has enough energy stored to recharge a few smallish (eg 3S 2200mAh or so) Lipos if you run a mixture of power types.

John

[Phil Green]

The reason most power panels are 12v is that they provide for a pump and a starter, both of which are generally 12v.
Since the 12v battery is also capable of driving the plug via PWM, a dedicated 2v battery is unnecessary.
Cheers
Phil

oops - John made the same point at the same time!  Jinx!

Edited By Phil Green on 04/03/2015 21:06:59

[onetenor]

Why start at 12 v why not start at 2 v single wet or gel cell with a foot length of stiff wire as a sliding resisitor. Fixed to one terminal of cell and a croc clip on the end of one of the wires. Moved inwards til the plug glows brightly What more do we need. No need to know voltages and wattages amperages. Old hat but very effective. We used Ex=GPO cells which we got from Gerry ang her dad ,L:ater her husband. , at their shop of wonders in Whitechapel Liverpool. KISS John

[Shaunie]

With switch mode power supplies you may find it easier to visualise things in terms of Watts. As you measured around 3 A 1.2 V giving 4 Watts or thereabouts at the plug and 12V 0.7A at the input to the power panel which is 8.4 Watts. This gives a conversion efficiency of about 50%, not brilliant but quite hard to get a good efficiency at such a low output voltage. You have to remember that it is not just a dropper but a converter it doesn't just reduce the voltage but also increases the current. If the converter was perfect for 1.2V 3A output it would take .3A and 12V input.

I can get a bit more technical if anyone wishes.

I built my own panel many years ago and achieved about 80% efficiency. I now have a JP one and it's rubbish as you can only set current which is the best way to destroy plugs. What you need is a nice stable voltage then if the plug is cooled by excess fuel it will draw more current helping to evaporate the fuel off. I found an Enya number 3 at 1.2V takes 3A dry and 4A wet. When I get chance I intend to reinstate my old regulator into the JP panel.

Shaunie.

[Steve Hargreaves - Moderator]

As others have said I use the Cylon cell with an ammeter in series (to prove the plug is glowing!!) & generally see between 2 & 3A depending on the plug used.....