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Variable resistors

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Doc Marten18/04/2019 07:59:08
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Is it possible to reduce a 5 Ohm VR/Rheostat/Potentiometer down to 1 Ohm by using fixed resistors in Parallel?

I ask because 1 Ohm components are impossible to find or stupid expensive and in the range of 25 Watts or am I not looking hard enough?

5 Ohm 5 Watt are not too common but are available, they're still quite pricey in comparison to higher resistance components though, a 5 Ohm one works but all the adjustment is a very thin band at one end, the circuit is a 2v Cyclon with a 5 Amp Ammeter to power glow plugs, I don't want a metre of lead to drop the voltage, I want an adjustable supply with an LED indicator to show that there is power even with a blown plug so that if I see zero on the meter but a glowing LED I know it's the plug that's blown and not a flat Cyclon.

I realise the potential here for a very long thread with, "This is all I do....You don't need to do that.....KISS....use a glow stick" etc, but that's all been well covered on the forum. 

Thanks

Doc

 

Edited By Doc Marten on 18/04/2019 08:07:47

Denis Watkins18/04/2019 08:26:52
3559 forum posts
166 photos

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 Skilbeck18/04/2019 08:31:59
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4340 forum posts
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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

Doc Marten18/04/2019 09:18:05
172 forum posts
2 photos
Posted by Denis Watkins on 18/04/2019 08:26:52:

What is your supply volts Doc ?

2v from a 2.5 mAh Cyclon so about 2.2v fully charged NLV.

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

5 Ohm gives me a 2-ish Amp reading on the Ammeter through a cold 1.5V plug.

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

I bought it as a 5 Ohm, 5 Watt VR but I will check.

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

 

Edited By Doc Marten on 18/04/2019 09:23:54

Doc Marten18/04/2019 09:19:35
172 forum posts
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Posted by Frank Skilbeck on 18/04/2019 08:31:59:

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

Thank you Frank.

Are there any drawbacks to doing it this way (not that I have much choice)?

Don Fry18/04/2019 09:40:39
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3388 forum posts
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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.

Doc Marten18/04/2019 09:47:28
172 forum posts
2 photos
Posted by Doc Marten on 18/04/2019 09:18:05:
Posted by Denis Watkins on 18/04/2019 08:26:52:

What is your supply volts Doc ?

2v from a 2.5 mAh Cyclon so about 2.2v fully charged NLV.

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

5 Ohm gives me a 2-ish Amp reading on the Ammeter through a cold 1.5V plug.

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

I bought it as a 5 Ohm, 5 Watt VR but I will check.

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

 

 

This has got me thinking now. I had to put a .2 Ohm ceramic resistor in the circuit to get it to work the way it does.....Hmmmm

img_20190418_093250.jpg

img_20190418_094149.jpg

The pot was from a UK eBay seller who has from 5 to 1K Ohms, I chose the 5 Ohm option. My pot has the following markings: 1722CTS 026TB332R5A0B1A.

This is the from part of the description: 

 

5 Watt Wirewound Rheostat / Potentiometer / Variable Resistor / Pot

This is one item of new 5W wirewound pot as shown, available in various different resistance value variations.  The taper is linear.  

The final picture shows the 25 Ohm version where 25 Ohms is represented by the 25 in the 026TB32R25B1A11217ZCTS engraving, that being being twofive and zero multiplier.  The spindle is a standard round 6.3mm / 0.25 inch type.
 
 

Edited By Doc Marten on 18/04/2019 09:59:53

BackinBlack18/04/2019 09:56:26
86 forum posts
1 photos

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.

Doc Marten18/04/2019 10:04:34
172 forum posts
2 photos

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?

Peter Christy18/04/2019 10:13:56
1316 forum posts

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

Doc Marten18/04/2019 10:40:49
172 forum posts
2 photos

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

Chris Bott - Moderator18/04/2019 11:06:31
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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.

Doc Marten18/04/2019 11:52:57
172 forum posts
2 photos

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.

BackinBlack18/04/2019 12:02:43
86 forum posts
1 photos
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

Doc Marten18/04/2019 12:23:04
172 forum posts
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Why is a NiMH or LiPO a better source than the Cyclon? I've got 2 glow sticks.

Martin Harris18/04/2019 12:47:07
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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 - Moderator18/04/2019 12:49:44
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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 Christy18/04/2019 13:52:09
1316 forum posts
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

Doc Marten18/04/2019 17:40:04
172 forum posts
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Posted by Peter Christy on 18/04/2019 13:52:09:
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.

Why do you need adjustability? Its totally unnecessary!

This is what I didn't want to start getting into.

Why is a NiCad, NiMH, LiPO better than a Cyclon for firing up a glow plug? I am putting a meter in the circuit and an LED to show me that I have power in the cell so if there is no meter reading and a lit LED I know it's the plug that's the problem, not the cell.

 

 

Edited By Doc Marten on 18/04/2019 17:41:48

Peter Christy18/04/2019 18:25:33
1316 forum posts

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! wink

--

Pete

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