Potentiometer

freddie

Hi guys
I am working on designing a floor lamp, and am hoping to use a Potentiometer (as far as i can find out) to vary the brightness. The Potentiometer must fit inside a 30mm tube. First of all is it possible to do this? and second, if it is possible could anybody explain the easiest way of doing it? If you could send me a link to a suitable Potentiometer in radionics, i would be really grateful!

Thanks in advance

freddie

tak

What wattage is your lamp to be ?

What sort of intensity variation do you want - e.g. from 100% down to 0% ?

What do you mean by within a 30 mm tube exactly ?
Some part of the potentiometer dial must be accessible for control.

Sketch out how you envisage it being enclosed.

In meantime look at the potentiometers on offer on

http://www.radionics.ie/cgi-bin/bv/rswww/searchBrowseAction.do?D=potentiometer%20&Nr=AND%28avl%3aie%2csearchDiscon_ie%3aN%29&Ntk=I18NAll&Nty=1&Ntt=potentiometer%20&Dx=mode%20matchpartial&Ntx=mode%20matchpartial&N=0&No=25&name=SiteStandard&obs=sObs&callingPage=/jsp/search/search.jsp&BV_SessionID=@@@@0295052330.1182873912@@@@&BV_EngineID=ccchaddlfiimkkicefeceeldgkidhgf.0&cacheID=ienetscape

and maybe imagine a suitable form at least.

freddie

Hi Tak

Thanks for your very quick reply! Sorry for not being more specific with my question, below are the answers to your questions

the lamp will be a max watt of 60 i guess.

I would like it to go from off position to 100%, Preferably smooth and not in steps.

See the attached image for what i mean, the ID of the tube will be about 30mm and the switch will be mounted on a cap, which will fit snug inside the hole. We will place the knob on afterwards.


I think something like stock Number 460-1837 (sorry I couldnt create a link) could be good, as I could easily connect the flex to it. If not, anything with the connections on the base and with a 20mm - 28mm OD that can do what I want will be fine!

bushy...

Thats a 1 watt pot. you picked out , it'll fry , it'd be hard to find one to carry 60w so a little circuit is what you need to take the load off it. Could you get away with an inline dimmer in the cable or would that spoil it ?

freddie

Did I mention that I don't know anything about electronics? If a 1 watt pot would fry, does that mean that I would need a 60w pot if I want to power a 60w bulb? The same Potentiometer is there only 50K (460-1865) would that work? unfortunately there is no chance I can use an in-line dimmer.

freddie

If it turns out that I do indeed need a small circuit, would it be difficult to make it as a prototype? It would also need to fit inside the tube, so 30mm wide by up to 80mm long. If i do need one, is there anybody here who could make it for me? PM me and we can discuss.

thanks a million
freddie

DublinDilbert

If you are just trying to dim an incandescent bulb you could strip down a dimmer switch and use the electronics from inside it... this will not work with a florescent tube....

I would not recommend messing with mains power, is there any way you could make a battery powered prototype? or maybe a prototype running off a 12V transformer?

freddie

Thanks Dublindilbert

I don't think i can make it battery operated, and i don't know what you mean by a transformer. Would it be simple to add a transformer? the other idea i just had was to use an 9w LED light bulb, that should do the trick if it needs to come to it. But i would really like to use a resistor. I had a look at the resistors on the back of dimmers and unfortunately they are too big to fit

themole

freddie wrote:

Thanks Dublindilbert

I don't think i can make it battery operated, and i don't know what you mean by a transformer. Would it be simple to add a transformer? the other idea i just had was to use an 9w LED light bulb, that should do the trick if it needs to come to it. But i would really like to use a resistor. I had a look at the resistors on the back of dimmers and unfortunately they are too big to fit

What exactly are you looking for?
Why do you need to build the lamp?
What is your use for it?

There may be better ways of going about it depending on what you want to use it for.

freddie

Hi themole

I am designing this lamp for a design exhibition, with the aim of getting it mass produced. A major feature of this lamp is the fact that the controls are positioned so as to be easily accessible to the user. Because the lamp is designed already, therefore the electronics have to fit the constraints I outlined earlier. Not being an electronics engineer, I didn't realize it was going to be a difficult task to find a suitable potentiometer. If all comes to all we could just fit a switch but this is a last resort.By the way I am really grateful for all your help, and hopefully we can figure this out!

thanks
freddie

themole

freddie wrote:

Hi themole

I am designing this lamp for a design exhibition, with the aim of getting it mass produced. A major feature of this lamp is the fact that the controls are positioned so as to be easily accessible to the user. Because the lamp is designed already, therefore the electronics have to fit the constraints I outlined earlier. Not being an electronics engineer, I didn't realize it was going to be a difficult task to find a suitable potentiometer. If all comes to all we could just fit a switch but this is a last resort.By the way I am really grateful for all your help, and hopefully we can figure this out!

thanks
freddie

Ah, that explains a bit.

I would say that if the bulb is a mains bulb then there is indeed little chance of fitting a brightness control in the the tube.

As suggested by a previous poster this could be done using a lower voltage bulb and hence lower voltage electronics, which would be able to fit in the tube. If the lamp is to be run off the mains then this would necessitate the use of a transformer, which changes the mains voltage, 230V, to the lower voltage of the bulb, say 12v. You would need someplace to put the transformer, some thing like RS Stock No. 419-7518.

Do you have space in your deisgn to fit this?

freddie

Hi themole

No, unfortunately I don't think i can fit that in anywhere. Would it work if i used an LED bulb like the one below, or would i still need a transformer? Do you know if there exists a transformer that can be cable mounted? If i have to use a simple switch what are the main volts & wattage i need to search for?

http://www.x-tremegeek.com/templates/searchdetail.asp?productID=11880

cast_iron

A transformer like this would be suitable (though an 80W would be better).

Along with a bulb like this and the dimmer electronics described above, it should work out.

freddie

Is it possible to dim an energy saving bulb? I like your idea, and will probably have to go with something like that, but i don't want to lose hope in my original intention. Can anybody tell me why a dimmer like the one below can be so small and handle the voltage, yet a panel mounted one cannot? is it possible to take one of these apart and connect a potentiometer to it? as a backup, and before ye start getting bored of me and this subject Can anybody point me to a suitable panel mounted rotary on/off switch that will fit my purpose?

http://www.mr-resistor.co.uk/item.aspx?&t=603&r=604&i=5554

themole

freddie wrote:

Hi themole

No, unfortunately I don't think i can fit that in anywhere. Would it work if i used an LED bulb like the one below, or would i still need a transformer? Do you know if there exists a transformer that can be cable mounted? If i have to use a simple switch what are the main volts & wattage i need to search for?

http://www.x-tremegeek.com/templates/searchdetail.asp?productID=11880

That led bulb is 120Volts, porb made for the us which uses 120 mains Voltage. We use 230, so you would still need a transformer to use that bulb here.

themole

freddie wrote:

Is it possible to dim an energy saving bulb?

For most energy saving bulbs, no it is not possible to dim them. There are apparently some dimmable ones but make sure it is mentioned on the box.

freddie wrote:

I like your idea, and will probably have to go with something like that, but i don't want to lose hope in my original intention. Can anybody tell me why a dimmer like the one below can be so small and handle the voltage, yet a panel mounted one cannot?

The ones for mounting on your wall are stronger.

freddie wrote:

is it possible to take one of these apart and connect a potentiometer to it?

Maybe, this may be hard to do though.

If i were you i would go to the likes of woodies and look at the lamps there. Find one with a swtich that suits and electronics that will be small enough. Dismantle it and try to make the electronics fit into your lamp design.

freddie

Good idea, I will pop into woodies later and see what i can find. I picked up a small rocker switch in Maplin this morning, Its got 16A 125VAC / 10A 250VAC printed on it, please excuse my lack on knowledge, but would this work with mains power?

thanks so much again, and i will leave ye all alone soon

freddie

themole

freddie wrote:

Good idea, I will pop into woodies later and see what i can find. I picked up a small rocker switch in Maplin this morning, Its got 16A 125VAC / 10A 250VAC printed on it, please excuse my lack on knowledge, but would this work with mains power?

thanks so much again, and i will leave ye all alone soon

freddie

Ya, that would appear to mean is can handle 250 Volts of AC electricity, our mains supply is 230V AC. So the switch will handle about 10amps of current at mains voltage, which is more than enough for a lamp.

Make sure you use a fuse and also that if the lamp is made of metal that it is earthed. Good luck, and post up a pic of the finished article.

freddie

Thats brilliant, and will teach me to design something before i know it works! I hate to lose the dimming capabilities , but sure maybe i will find something later down the line. I should have the lamp built in the next few weeks, and will post an image. Thanks everybody for your help, and if you think of anything else let me know

thanks
freddie

freddie

Ok, I promise this is the last stupid question, but how do i earth a metal lamp? Does it mean I will have to use 3 core flex? at the moment i am getting in 2 core, and the switch I have has only 2 terminals, so what do i do?

Michael Collins

freddie wrote:

Ok, I promise this is the last stupid question, but how do i earth a metal lamp? Does it mean I will have to use 3 core flex? at the moment i am getting in 2 core, and the switch I have has only 2 terminals, so what do i do?

Yep if the lamp is metal it will need an earth, and to do that you will need a 3 core flex. The earth goes from the plug to the metal of the lamp and must not be switched along the way...you only switch the live wire.

Regarding your dimming problem, what's wrong with the one you pointed out http://www.mr-resistor.co.uk/item.aspx?&t=603&r=604&i=5554 ?

Would another option would be to put the dimmer on the lamp itself somewhere?

freddie

Ok, 3 core it is then! As you will see from my earlier posts the main feature of the lamp is the position of the switch. This cannot change for both aesthetic and ergonomic reasons, and therefore in-line dimmers are out of the question! I think for now i am going to go with a simple rocker switch, and if a manufacturer takes it off my hands I'm sure they can figure it out.

themole

freddie wrote:

Ok, I promise this is the last stupid question, but how do i earth a metal lamp? Does it mean I will have to use 3 core flex? at the moment i am getting in 2 core, and the switch I have has only 2 terminals, so what do i do?

If the lamp is metal you earth it by using 3 core flex, where the green/yellow wire is connected to the earth pin of the plug on one end and the metal body of the lamp on the other end. The earth wire does not need to go through the switch.

I'm not 100% sure, but i think when using mains voltage any electrical device which has a metal case is supposed to be earthed.

EDIT: Just saw above post

fishdog

Speaking as an electrician I would advise you to think about the following:

Dont buy in Woodies, apart from selling low quality electrical components, they are expensive and have a poor selection. Try Kellihers or Eurosales or N.E.W.

Next, select your lamp carefully, some get very hot (eg. downlighter bulbs). Perhaps you can put the lamp in a shade or enclosure to prevent accidental touching of the bulb.

Energy efficient lamps are cold, but only very expensive ones can be dimmed.

LEDs are cheap, energy efficient, cold, dont blow and come in many colours. However expect them to be rated at about 0.25 watts. This can be a problem in itself, because most dimmers need a minium load to function. This will be written on the dimmer switch. Transformers also have a minimum load. IBL 60VA transformers are the ones I normally use for downlighters. They might suit your needs as they are sausage shaped and about 50mm in diameter.

A potentiometer would work, but you are making alot of work for yourelf. If it were me I would buy a dimmer switch.

I think the suggestion of using 12v lighting is a good one, because it makes it safer.

tak

Yeah. Safer and more efficient powerwise too, which is one of the 'selling' points of the lamp originally.

http://home.howstuffworks.com/dimmer-switch2.htm

freddie

Thanks guys

two quick questions

Would 12v lighting be bright enough for a floor lamp? and could i use a potentiometer to dim it?

Michael Collins

12 V can be effectively any brightness you like, it's the total power you use that's important. The current will just be higher - the power is determined by the current * voltage - so a 60 W 12 V bulb would use 60/12 = 5 amps.

Whether you can use a potentiometer to dim it or not depends on
1) The bulb wattage and type you're using
2) The power rating of the potentiometer itself

freddie

Ok lets see if i get it

If I use a transformer like this

Wired up as normal to the lamp holder and 60W bulb, with a potentiometer rated 5W it should work? Or have i got the potentiometer part wrong, what should i be looking for?

Michael Collins

Yeh that transformer looks grand, and quite cheap too I think. As for the potentiometer part, it's hard to know, it depeneds on the bulb you're using. Chances are that it would work OK, but the only way to know for sure is to build it, then check the power dissapated by the pot. and make sure it's under 5W.

freddie

Cancel my genius idea, searched all of google and cannot find a 5w pot. Only 3w everywhere! I might just stick with a rocker switch and mains.

Michael Collins

Actually, I think you might be getting confused from my post above (at 12.22pm). I was just trying to show you how the voltage doesn't necessarily dictate the brightness of the bulb. I wasn't saying anything about the required rating of the pot. in my calculation...sorry if that was confusing!

freddie

Sorry, Electricity is really confusing to me, I just cannot make sense of it! I feel like a 5year old learning maths again! Do you think if i use the setup above with a 3w potentiometer it has a chance of working? That is a 230Va to 12va transformer through a 3w potentiometer to a 60w bulb.
thanks so much for bearing with me!

Michael Collins

Do you think if i use the setup above with a 3w potentiometer it has a chance of working? That is a 230Va to 12va transformer through a 3w potentiometer to a 60w bulb.

Yep I do think there's a chance. But you must test it first, the testing part is easy really, I can show you how to do that if you like.

thanks so much for bearing with me!

No problem. I'd be interested to get this working myself!

freddie

Thanks a million

I will order a few components from radionics on Monday, and fingers crossed something will work! Maybe if steorn will get their free energy device working i can fit one in somewhere! I will keep you updated!

freddie

just on Radionics now and just wondering which of these should I order? Not sure what the R, K and M stand for.

Michael Collins

That link ain't working Freddie!

freddie

Sorry about that, Radionics dosnt seem to like links being sent! the product number is 188-5521. there are a few different types with numbers following them like 470K, 1M, 220R etc. I just dont know what they are/mean. Thats the type of Pot I hope to use, with the connections on the base.

freddie

Ok guys
I tracked this pot down, and to me looks like it could be used on a mains line 10A@250VAC am i right or wrong?


http://www.farnell.com/datasheets/89365.pdf

fishdog

I looks like it can switch 10A at mains voltage.

The problem is I can not see how many watts it is designed to dim!
The technical information talks of a track rating of 0.2 watts, which appears to be the overall rating of the dimmer, but this would seem a bit strange seeing as though the switch part can switch 10A.

The other problem is that you would have to solder the wires on to this dimmer. This seems like alot of hassel.


The first thing you have to do is work out how many watts is the lamp you are using.


MK make a simple 250 watt dimmer module for a grid switch with terminals on it that do not require any soldering, just a screwdriver. You can buy them over the counter in Kellihers, Eurosales or NEW. They all have sevral branches across Dublin. T'OReillys also would have this, but they are VERY expensive.

If you have any more questions PM me and I will give you my mobile number (or PM me yours). When I talk to you on the phone I garantee you I will be able to sort out any problem you have with this. I am an experienced electrician and have been at this sort of thing for a long time now.

freddie

That's brilliant fishdog, a great answer! I was supposed to give an update earlier but didn't get a chance. I have sourced a touch sensor dimmer module form these guys which will fit what I need.
It seems like a great, easy to set up technology. I just have to put a plastic cap on the tube and run a sensor wire to it. Only problem is that it is very expensive, working out at €60, but thats ok for a prototype, and i'm sure I can get them cheaper somewhere, probably China.
Have you ever seen these in action? just wondering if they work as well in practice as they do in theory? Thanks a million for your help, and offer of assistance, I might have to take you up on that offer with future projects.

freddie