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Variable resistors

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Sparks18/04/2019 19:20:41
269 forum posts
55 photos

Here is a zero-cost solution using the existing 5 Ohm pot.

If its a three terminal pot, connect the two outer terminals together and connect to voltage input.

Then take the output from the middle (wiper) terminal. For a 5 ohm pot, this will give you a 0>1.25>0 Ohm pot.

e.g. at beginning and and end of pot travel, resistance will be 0 Ohm. At mid-span resistance will 1.25 Ohm.

You could restrict pot movement to half scale mechanically to give 0>1.25 Ohm span. The travel/resistance will be a little non-linear, but shouldn't be a problem in this application.

Hope that helps, Sparks

Don Fry18/04/2019 19:31:33
4557 forum posts
54 photos

Arrrrrrrrrr Peter, but you quite rightly say your NiXxx cell is close to voltage. There are some who do right voltage. Hence a dropping resistance.

Your Nixx cell is good (ish), if fully charged, for plugs designed for zinc carbon cells. But weedy for plugs designed for a single lead acid cell. And possibly ideal for modern fusion plugs, as modern cooks say.

We are blessed by tolerant motors. But one size does not fit all.

Doc Marten19/04/2019 10:02:11
692 forum posts
7 photos

At the risk of causing the debate I'm trying to avoid:

It's the Amps that usually get focused on in these discussions but without the right Voltage you can't acheive the Amperage, that's what Ohms law is about right? I've even seen discussion with agreement on this forum that Amps are not drawn but supplied, there's also a lot of conflicting opinion that it's the Amps that should be regulated whereas others state it it should be the Voltage, It all ends up in too much opinion for the layman when the Electronics/Electricians get their heads together.

This is the way I see it; An Ammeter, not a Voltmeter, is the standard issue on power panels and the absolute basic in a glow circuit as it's assumed that the supply Voltage will be static and correct so the Amps can be adjusted to suit, this being the reason why plugs are sold rated in Volts not Amps.

Edited By Doc Marten on 19/04/2019 10:03:01

Peter Christy19/04/2019 14:05:25
1868 forum posts

Oh, dear! Where to start?

Don: All I can say is that my simple little box with a single NiMH cell has never failed to start an engine that was not otherwise faulty. I use it with OS, Enya, Super-Tigre and even old Fox plugs (designed for 2V). It lights them all perfectly.

Remember, its not just the terminal voltage that is important. The internal resistance of a cell also has a bearing, limiting the maximum current it can supply. Modern "C" or "D" NiMHs have a *very* low internal resistance.

Doc Marten: You are half way there with your reasoning - you just need to follow it through! Glow plugs do not have a constant resistance. It varies with the temperature of the element. The hotter the element, the higher the resistance. So, if we assume a 1.5V plug for a moment and connect it straight across a low-impedance 1.5V supply (such as a "C" Nimh), it will momentarily draw a surge of current that will quickly settle to a lower value as the plug reaches the correct temperature. This happens to fast to see on a meter, but trust me: it happens!

It is the temperature of the plug element that is the important factor. Too hot and the element will be damaged. Too cool, and the motor won't start. The current drawn is an indication of the temperature, but it is a relative measure, not an absolute.

If you are adding a resistor in series with the plug, it may not be able to draw sufficient current when cold to clear a flooded condition. The current will be limited by the resistor. The danger then is the temptation to increase the current via the pot. This will clear the flood, but when it does, the plug will be over-driven until the current is reduced back down. By the time you see it, it is too late, the damage is done. The plug will still glow, and may well continue in service for a while, but the best case scenario is that its life will have been shortened, and the worst is that it will blow.

The same flooded situation with a low-impedance 1.5V supply will allow the plug to draw more current to boil off the excess, but as the fuel boils off, the element temperature will rise, and the current will drop back all on its own.

Ideally a plug should be driven from a low-impedance source of the correct voltage. It will then be self regulating as far as current is concerned, with no need for intervention on the part of the operator.

So what damages a plug, and why do panels not have voltmeters on them?

In days of yore, plugs used a thin piece of platinum wire. Then platinum became much in demand for car catalytic converters, and very expensive! These days, plugs tend to use platinum *coated* wire. The coating is very thin. Over-heating the element can cause the platinum to flake off, leaving a grey, powdery appearance to the wire. At this point the plug is effectively dead, even if it still glows! Why? Because the platinum is a catalyst which promotes combustion in the engine. Its not enough for the plug to be hot, it needs the catalytic element to promote ignition. Without it, then engine will run badly - if at all - and will be liable to cut suddenly when throttled back.

If you place a piece of platinum wire in methanol vapour, the vapour will immediately ignite - without any heat being applied at all! However compressing the vapour makes it harder to ignite, so we need to add heat as well in order to guarantee ignition in a piston engine.

Coming on to voltmeters, the important factor is the voltage *at the plug* - not at the terminals on the power panel! There may be quite a difference between the two if, for example, the connecting leads are thin or long, or if there is a poor connection somewhere.

An ammeter is a useful diagnostic tool, but only as a relative measurement - not as an absolute. As I pointed out earlier, an OS No8 draws about 3 amps at the correct temperature and voltage. A Super-Tigre only draws about 2A. So you need to know what the ideal current for your plug actually is, and this is not something you will find on the packet! However, if you feed it the correct voltage, it will sort itself out quite happily, for the reasons stated above.

I switched to using a single NiXX cell about 20 years ago, moving from a 2V lead/acid battery. I can't remember the last time I had a plug die on me! They just seem to last forever. And a good quality NiMH of size "C" or larger will even provide enough juice to light a 2V plug without any difficulty.

Its simple and fool-proof. What's not to like?




Peter Beeney19/04/2019 14:16:40
1593 forum posts
59 photos

I just happened to choose the 2V lead acid cell route for energising glow plugs, simply because in the beginning, circa early nineteen seventies, they were easily available. I simply use a slightly longer supply cable and I’d do this anyway, it’s a very necessary safety point with me so it’s ideal situation. I use an 8Ah Cyclon these days so if for any reason I wanted to reduce the distance from battery to plug I’d probably wrap the cable around the battery and then hold it in place with a turn or two of insulation tape. This little system has always served me very well, to be honest I can’t think that I’ve ever blown a plug but it’s certainly rescued a few fellow aviators with reluctant engines over the years.

A long time ago I was wasting time as usual and I collected 5 assorted unknown plugs from the squirrel stock (bits and pieces box) and tested the current flow value of each one. They were all still working, quite remarkable really, and the current requirement varied from 2 and a little bit amps up to 5 as I remember; but certainly they were all different. I have to stay firmly within the KISS (Keep It Strictly Simple) principle otherwise I quickly get somewhat confused…

As with just about everything else there are often a number of ways of doing something; driving a glow plug is no exception I guess. Some folks use slightly more involved systems with power panels, ammeters and variable current devices, down to a remote cell and a glow clip or the all-in-one nickel cell and plug clip. All types certainly start the engines ok, although over the years the power panel has been know to occasionally malfunction, to say the least. I’d never bother with an ammeter in this circuit, for one thing it’s just adding a bit more impedance.

I would consider the terminals of a battery to be an unregulated supply, depending on the resistance of any component connected to the terminals a given amount of current will flow, if the resistance is very low indeed a large current will flow. If however we connect a voltage regulator across the terminals this device will regulate it’s voltage output to a given value, which is on the spec; it’s also a current regulator, again on the spec. This would then be say 5V at 200mA, 500mA, 1A and so on. If you have a particular piece of equipment you wish to power with a known maximum predicable power requirement you might use one of these. It maintains a constant 5 volt supply until a variable load becomes too great (resistance gets too low) then it just shuts down. But it will reset itself.

A constant voltage is used for some situations whereas a constant current is a requirement for others…


robert chamberlain14/08/2020 00:56:02
160 forum posts

Hello all, I was thinking of a back up source of 1.5V for my glow plug. Of course I could just buy a second Ni Starter glow stick , but as I already have 12V batt in my flight box, how would I drop the voltage? the resistance in a long wire could do it , I suppose or maybe a resister of ample Amps (5 Amps?).

While I am here, any simple way to re-charge the glow stick Ni Starter from my 12V batt in the tool box? I do not have a power panel, just 12V batt in the tool box? Thanks in advance. Bob C in Kansas , USA

Denis Watkins14/08/2020 08:04:44
4621 forum posts
129 photos

This is using conventional experimenters parts Robert, it is worth a " price up " 1st

As some online ready builds are very low cost

T1 and T2 are BC184LC or similar transistors with T3 being a BUZ11A or similar power mosfet. The LED will illuminate when the plug lead is connected to the glow-plug, providing the plug element is intact. The LED should be a high brightness type. T1 and T2 must be high gain type, preferably better than 400 at around 2 mA.

Denis Watkins14/08/2020 08:06:35
4621 forum posts
129 photos


Nigel R14/08/2020 09:02:02
4066 forum posts
693 photos

"While I am here, any simple way to re-charge the glow stick Ni Starter from my 12V batt in the tool box?"

Several commercial answers for you:



But, a second glow stick seems like much better value for money.

Personally, I would be a little wary of putting money into kit based around the old 12V gel cell - having dumped the gel cell and gone to using an old 3 cell lipo for my starter and a second to run the fuel pump (that way I get a backup for the starter if I flat it), I'd say that was the single best thing I did for the flight box!


Edited By Nigel R on 14/08/2020 09:06:34

Doc Marten14/08/2020 11:05:17
692 forum posts
7 photos
Deleted. Misread the post. embarrassed

Edited By Doc Marten on 14/08/2020 11:30:28

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