Variable resistors

[Former Member]

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[Denis Watkins]

What is your supply volts Doc ?

5 ohm is barely an adjustment, and going to 1 ohm almost makes the pot limited for useful adjustment

Is the pot really 5 ohm?, are the numbers printed on the case

In answer to your question, yes you can add fixed resistors to alter the pot range value

 

Edited By Denis Watkins on 18/04/2019 08:31:56

[Frank Skilbeck]

Yes the formula for resistors in Parallel is

1/Total Resistance = 1/R1+1/R2..............+ 1/Rn

On that basis if your R1 is 5 ohms and you want a total resistance of 1 ohm then R2 needs to be 1.25 ohms

[Former Member]

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[Former Member]

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[Don Fry]

I've got the same setup, cyclon battery, ammeter, variable resistance. The resistance I use is half ohm, 10 watts , capacity, and I got it from the Far East on e Bay. Can't remember what the resistance cost but not expensive

Good unit and set up for life.

[Former Member]

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[BackinBlack]

Forget variable resistors, unless you can find a 1 ohm 10 watt wirewound, a rare and expensive item. The paralleled resistor idea will not really work with the value of variable resistor you have.

You need fractional ohm resistors. For instance if your Cyclon is fully charged at 2.2V and the plug is rated at 1.5 volts 2 amps the resistance needed is (using V=IR, R=V/I) .7V/2 = 0.35ohms at 3 amps it would be .7/3 = 0.23 ohms. As the cyclon discharges at 1.9 volts, say 0.4/2 = 0.2 ohms.

Best answer is is to get four 0.1 ohm 2 or 3 Watt resistors, five banana plug sockets and wire the resistors from one socket to the next with supply from the cyclon at one end, you then have 0, 0.1, 0.2, 0.3 and 0.4 ohms available depending on which socket you use. No switches, just simple and reliable.

[Former Member]

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[Peter Christy]

Low value, high current pots tend to be fragile at best, and prone to burning out. If you need low value resistances, a length of wire from a broken toaster or electric fire element can be quite useful.

However, by far the best solution is to use a more suitable voltage source in the first place. A single NiMH "C" cell will provide plenty of oomph for either 1.5 or 2V plugs without the need for dropper resistors of any kind, and also eliminates the possibility of overdriving the plug.

--

Pete

[Former Member]

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[Chris Bott - Moderator]

Doc it's not what you're asking about but this might just be man enough for your needs.

It's not expensive to try and if you do trip the 2A fuse then it apparently reset's it's self.

Just a thought.

Amazon 2A PWM controller.

[Former Member]

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[BackinBlack]

Posted by Doc Marten on 18/04/2019 10:04:34:

I like that idea but I'd rather have the convenience to select the power by a rotary switch rather than have to plug in/unplug leads to different outlets to select, could this be done using a rotary stepped dial switch with the resistors soldered to the pins? is there such a switch available?

Reliable low voltage DC, high current switches are almost as rare as the potentiometers.

I would ask how often do you need to adjust the current?

Chris' suggestion looks worth a try and would work with a 1 or 2s Lipo, 6 or 12V lead acid gel battery as well. There are quite a few others in the links below Chris' suggestion in that large River place.

But, as peter suggests the NiMh solution is the simplest and based on my use of NiMh glow sticks is as simple and reliable as it gets.

Edited By BackinBlack on 18/04/2019 12:04:35

[Former Member]

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[Martin Harris - Moderator]

I've seen many PWM type glow controllers fail and give the full 12V from the source battery but the 2V cyclon cell would give your plugs half a chance if you notice the excessive current flow quickly enough. Normally, failure seems to cost 2 or 3 glow plugs depending on how quickly the victim realises what's happening!

[Chris Bott - Moderator]

Posted by Doc Marten on 18/04/2019 11:52:57:

That's really interesting Chris and worth a go for the few £ it costs. It's working voltage is from 1.8V , is that suitable down to 1.25V plugs? I'd like a range from 1.2-2v.

I would think that within the range of the knob you'd get get the equivalent of 0v to full battery volts. So you'd have control, but just like a variable resistor you could also turn it right up and pop a plug.

[Peter Christy]

Posted by Doc Marten on 18/04/2019 10:40:49:

It doesn't give any adjustability though Pete and a coil of resistance wire is not particulary neat or compact.

Edited By Doc Marten on 18/04/2019 10:41:03

Why do you need adjustability? Its totally unnecessary! If you feed the plugs the right voltage from a low impedance source, they are self regulating. (As they get hotter, the resistance rises, reducing the current.)

Different plugs draw different amounts of current. There is no "correct" current, only a correct voltage. Feed them the correct voltage, and they will draw the current they require, and no more. Even 2V plugs work perfectly on a "C" cell NiMH.

A lot of people seem to think its a good idea to raise the current to clear a flooded plug. It isn't! If the plug is flooded, the current will rise on its own, due to the lower temperature, and lower automatically as the excess fuel boils off. Cranking up the current just ensures that the plug will be over-driven for a while before the owner can react.

Trust me, I've been using this method since single cell NiCads of sufficient capacity became available! A single cell NiXX cell of adequate capacity is far and away the best form of glow driver. Add a meter in series and you also have a built in diagnostic to tell you if the plug is working, if the engine is dry or flooded. You really don't need anything more complex.

I would concede that I've had a lot of issues with glow-sticks! I prefer my own, home made box containing a cell and ammeter - nothing else.

--

Pete

[Former Member]

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[Peter Christy]

Different manufacturers (and types within a manufacturer's range) plugs all draw different amounts of current. There is no single correct current for all plugs. There is however a correct *voltage*, and a single NiXX cell is very close to the ideal.

Both cylons (or any lead acid) and LiPos need some sort of dropper to get the voltage down to the desired level, but the dropper resistor will be different for different types of plugs, hence if you go this route, the need for an adjustment.

BUT, low value, high current pots are a nightmare for reliability. Ask Geoff Sleath about convergence pots in old CRT TVs! They very quickly get very noisy and erratic, and are likely to suddenly pass too much current without warning (other than feeling "gritty"!). This is not good for the plug!

The alternative is a PWM dropper of the type suggested by Chris, but this is a lot of trouble and expense for something that is totally unnecessary! Also, they are easily blown by an accidental short circuit!

All you need to do is feed the plug from a single cell NIXX of adequate capacity (a "C" cell is fine), and the current will take care of itself - regardless of the make or type.

The meter is still useful, as once you know what current the plug normally takes, you can quickly tell if its wet, dry or blown.

For example, I usually use OS No8 plugs in engines that require a short reach plug. These normally draw close to 3 amps. If it goes up to 4, its flooded! If it stays at three, but rises slightly when the engine is cranked, it is dry. (The fresh, cold air cools the plug slightly while the engine is being cranked). If there is no reading, either the plug is blown, or the connector is not making good contact. If it bangs to the end stop, there is a short! If its right, then engine will just start!

In engines requiring long reach plugs, I usually use Super-Tigre plugs (though my stock is getting low!). These nominally draw about 2 amps, but otherwise, the same observations apply.

Also, because the plugs *never* get over-driven by this method, they last a very, very long time!

--

Pete

[Sparks]

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 Fry]

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.

[Former Member]

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[Peter Christy]

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?

--

Pete

[Peter Beeney]

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…

PB