Variable 'potentiometer'? to control a small 12v bulb

[Piers]

Variable \'potentiometer\'? to control a small 12v bulb

Anyone know where I can buy a neat potentiometer for a small 12v map reading bulb?

 

[Refueler]

Re: Variable \'potentiometer\'? to control a small 12v bulb

Under the bench in my lab - have a large box full !! No use to you though !!

Why not nick a volume control from a broken radio ? Might even have on / off as well !

 

[John100156]

Re: Variable \'potentiometer\'? to control a small 12v bulb

RS Components or Maplin!

When I did my electrical theory many moons ago (1971), I recall that using a pot to control the voltage will dim the lamp but you need to be sure it can take the power it needs to absord when the lamp is dimmed - for instance say:

POT SET AT ZERO RESISTANCE
Bulb Power Required: 4.8W
Voltage: 12V
Current Therefore (I=P/V): 4.8/12 = 0.4A
Bulb Resistance Then (R=V/I): 12/0.4 = 30 Ohms

USING SAY A 100 OHM POT
Total circuit resistance now: 100 + 30(bulb) = 130 Ohms
Circuit current (I=V/R): 12/130 = 0.09A
Volt drop across Pot now (Vd=I x R): 0.09 x 100 = 9V
Power dissipated by Pot (heat! P=IxV): 0.09 x 9 = 0.81W

Voltage across bulb now 12V - 9V = 3V
Hence power now to bulb P=IxV = 0.09x3= 0.27W (Dim)

You could try a Pot say 0-100 ohms rated at 1W or even a higher resistance to dim further!

All depends on bulb power of course - I would probably go for something like:

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4867065

Or even double up on resistance and power of pot!

Others will no doubt comment - voltage regulators?

Anyway hope this helps!

 

[Refueler]

Re: Variable \'potentiometer\'? to control a small 12v bulb

Of course if it has any power - then there real neat model train single controllers .... that take a 12v feed of the main control box.

 

[VicS]

Re: Variable \'potentiometer\'? to control a small 12v bulb

The obvious answers are as already given Maplin and RS
BUT
you will need a wirewound one to handle the current/power and you will need a fairly low resistance compared with most electronic applications. Depending on the bulb power but not likely to be more than 100 ohms.

Nothing along those lines available from Maplin.

RS components have 10, 25, 50, 100, 250 ohm (and higher) wire wound pots rated at 1 watt and also 50, and 100 ohm and higher rated at 1.5 watts.

You might get away with a 2 watt bulb and a 50 ohm 1.5watt pot but I am not sure that a 100 ohm pot will withstand the current running at near full brilliance.

I see John 100156 has found some 2 watt pots but they are multiturn ones.

However they have a range of what they call Wire wound rheostats with much higher power ratings.

These are a bit of an overkill though. I think I might try a 100 ohm one with a 2.2 watt bulb or a 47 ohm one with a 5 watt bulb, or 22 ohm with a 10 watt bulb if you need a bulb as powerful as that. The 25 watt series will be adequate for all of these.

 

[shmoo]

Re: Variable \'potentiometer\'? to control a small 12v bulb

You may consider a switch that puts a resistor in series with the lamp. Only gives you two settings - bright and dim but I suggest that might be all you need. Advantages are that its cheap, simple (always good), and the wiper contact won't deteriorate with age the way variable resistors always seem to do.

 

[matelot]

Re: Variable \'potentiometer\'? to control a small 12v bulb

why do you want a potentiometer? If its cos the lamp is too bright, then OK. But if you are thinking that turning down the lamp with a potentiometer would save leccy - forget it. You will simply lose the leccy as heat instead.

 

[John100156]

Re: Variable \'potentiometer\'? to control a small 12v bulb

Yep - I like the idea of buying a string of resistors (very cheap) and say a three position switch - you can then have on/dim/off if you wish and can set the dim resistance accordingly to suit (2 x 100 ohm resistors in parallel give you 50 ohms, 4 give you 25 ohms etc and you can have series and parallel say 100ohm then two 100 ohm in parallel giving 150 ohms, etc, etc.) You can have play.

Mind you, the 100 ohm 2W wirewound I gave in the link above I believe has about a ten turns! You can shop around but you should now have all the info you need from us to select the right pot!

Not sure if you know this; often pots have three pins, one end of the resistor, a slider usually in the middle and the other end of the resistor.

Usually a good idea to connect the slider and one end of the resistor together, to prevent an open circuit if the slider fails!

 

[Piers]

Re: Variable \'potentiometer\'? to control a small 12v bulb

You have all been so helpful. A potentiometer it is, and the 100 ohms job sounds spot on. As I said, it's for the chart light which needs variable power given the different states of ambient light.

Again, thanks a million.

 

[Beadle]

Re: Variable \'potentiometer\'? to control a small 12v bulb

I can't help thinking that using a dimmer for a 2 watt bulb is something of an exercise in futility - I mean - how dim can a bulb get?

If essential how about a second bulb in series with a switch across it for half power or full brilliance.

I fear that a potentiometer is not a good idea - all the power you take from the bulb will be dissipated in the pot and there will be turned to heat. I would have an ongoing concern of it starting a fire if left in the dimmed position.

 

[William_H]

Re: Variable \'potentiometer\'? to control a small 12v bulb

We can't let Matelot get away with a wild statement that a rheostat (pot) won't save electricity. The total current of the circuit will reduce with the added resitance. So a resistor (pot) equal to the bulb in resistance will halve the current drain but the power in the light will only be 1/4 that of full voltage and current. So yes you waste power in the resisitor. But total power is reduced.

The more modern approach to power control both in DC and AC circuits is to pulse modulate the power. Or switch it off and on at a high rate but with a variable mark space (on off) ratio. This means essentially that no power is wasted in the switch so no heat loss.
You would need a 555 timer chip and a lot of components. Yes Pot is easier.
regards olewill

 

[Bloater]

Re: Variable \'potentiometer\'? to control a small 12v bulb

err, a 2W bulb designed for a 12v supply should have an operating resistance of 72ohms, unless my maths is wrong.

Therefore to achieve 50% brightness at a half turn then the 'pot' should have a resistance of around 150ohm, say.

Therefore the 'pot' will pull 0.08amps continously no matter what the setting of the armature is (1W actualy). Therefore no matter what the 'pot' setting you will be wasting 1W (even with the bulb fully dimmed). That's not saving power is it?
----
Just to add to that... why not use a lampshade?

 

[philip_stevens]

Re: Variable \'potentiometer\'? to control a small 12v bulb

12v dimmer switch

or build yourself one

 

[VicS]

Re: Variable \'potentiometer\'? to control a small 12v bulb

[ QUOTE ]
unless my maths is wrong.

[/ QUOTE ] Afraid it is!

Yes a 2watt bulb will have a resistance of 72 ohms and it will take 0.167amps .

If you stick a 150 ohm resistance in series with it the total resistance will be 72 +150 = 222 ohms. That means the current will then be 12/222 = 0.054amps. Substantially less than the original 0.167amps!

In fact not really true because the bulb will be so under run that its resistance will be rather less but never the less the current will still be less than 12/150= 0.08amps, half the original.

BUT if the current is halved the power dissipated in the bulb will be quartered and the brightness will be even lower I suggest.

To reduce the power dissipated by the bulb by a half to 1 watt (ignoring the effects or a change in operating temperature of the filament on its resistance) you would have to reduce the current to 0.12 amps.

<span style="color:white"> ............. </span> P=I²R, therefore I²= 1/70, therefore I= 0.12amps

This will require a total circuit resistance of 12/0.12 = 100 ohms.

Therefore to reduce the power of the bulb to 1 watt will only require a series resistance of 100 - 72 = 28ohms !!!

The power loss in the resistance will be given by
<span style="color:white"> ............................. </span> P= I²R
and is therefore
<span style="color:white"> ............................. </span> (0.12)² x 28 = 0.4 watts rather less than you suggest

I have to repeat that this is all ignoring the changes in filament resistance with temperature. It also is based on the power dissipated, mostly as heat, not on the brightness of the bulb, which will be less than that of a 1watt bulb!

It's all academic anyway becase the original question was not about saving power but about dimming the bulb!

 

[VicS]

Re: Variable \'potentiometer\'? to control a small 12v bulb

[ QUOTE ]
Or build one youself

[/ QUOTE ]

To correct the link for you http://www.edutek.ltd.uk/Binaries/Circuits/12v_Light_Dimmer.pdf

Sods law of forums: If you dont check a link in preview it's wrong! /forums/images/graemlins/smile.gif

 

[Bloater]

Re: Variable \'potentiometer\'? to control a small 12v bulb

Hi VicS,

We seem to be talking about different circuit layouts. The original post was asking about using a potentiometer, not adding a variable resistance in series.

You are quite right that any resistance added in series will reduce the current flow and hence the 'power' drawn. I was talking about a circuit where the bulb was connected to the armature of the pot and therefore as a potential divider.

I still think the best solution is a lampshade.

 

[VicS]

Re: Variable \'potentiometer\'? to control a small 12v bulb

[ QUOTE ]
different circuit layouts. The original post was asking about using a potentiometer

[/ QUOTE ] Fair comment. Usually people just use a potentiometer as a variable series resistance. I assumed that was what the original poster was proposing. No reason as far as I can see to do anything differently.

I think I'd use a fixed series resistor and a switch. Then it'd be just bright or dim.

IIRC there used to be lights that had two polarising filters. Full brightness when aligned, dimmed as they were crossed.

 

[philip_stevens]

Re: Variable \'potentiometer\'? to control a small 12v bulb

Thanks once again Vic /forums/images/graemlins/blush.gif

 

[Beadle]

Re: Variable \'potentiometer\'? to control a small 12v bulb

The original post said a "small" bulb, it didn't specify 2 watts or anything else.

Personally- I'd go with the shade as well

 

[John100156]

Re: Variable \'potentiometer\'? to control a small 12v bulb

[ QUOTE ]
I fear that a potentiometer is not a good idea - all the power you take from the bulb will be dissipated in the pot and there will be turned to heat. I would have an ongoing concern of it starting a fire if left in the dimmed position.

[/ QUOTE ]

Use a hot bulb burning continuously at full brightness and dim it with a shade? Non-combustible I hope!

I'll stick with the pot but hey, if you like that sort of thing, I think you can still buy thermionic valves to replace transistors! /forums/images/graemlins/smile.gif