light bulb for roast control?

[zix]

I just had another wacky idea. Got lots of them right now. When those clever guys that design amplifiers want to test their amps with a little lower voltage/current but don´t have access to a variac (variable transformer) they sometimes use ´the old light bulb trick´.
I am not quite sure what it is, but I suppose they probably just put the light bulb in series with the live connection, and it will act like a high wattage resistor, dividing the voltage with the resistance of the transformer. This way their amplifier won´t have the full 230VAC and they don´t risk blowing things up. I have actually seen it used instead of a resistor/current source in a transistor amplifier output stage...

Now, what happens if we couple two or three bulbs in series with the live?Less current, right? Or if we could switch between say 15-25-40-60 W?
the resistance of those bulbs ought to be somewehere around 65/109/174/261 ohms. So: couldn´t light bulbs be used instead of a triac (or variac) for controlling electric roasters? I don´t know because I have no idea of the voltages inside a pop corn roaster, or how big its load is. Someone (a real electronics guy perhaps, unlike me? ) else here that knows?

[phil]

I would have thought a dimmer was easier!

See Simon's article on how to do this.

[simonp]

Yes you could do this, nice piece of lateral thinking . It would be rather bulky, and not half as easy as just adjusting a variac, or light dimmer. You would probably need to be able to switch the bulb(s) completely out of circuit to get enough heat for the end of the roast though.

The heating elements in most popcorn machines are around 1100-1200W so givenb 230V, 1200/230=5.2A, (this is ignroing any phase shift due to indcutance, but there is little in a popcorn machine) R = I/V so 5.2/230 = 0.0226Ohms is the resistance of the heating element.

At the end of the day though you are interested in loosing energy, which is in Watts, so you can best judge by the wattage of the bulb, i.e. a 100W bulb in series will drop you 1/10 of the heat energy.

[tisri]

The heating elements in most popcorn machines are around 1100-1200W so givenb 230V, 1200/230=5.2A, (this is ignroing any phase shift due to indcutance, but there is little in a popcorn machine) R = I/V so 5.2/230 = 0.0226Ohms is the resistance of the heating element.

My physics is a little rusty (I prefer not to count the years since A-levels) but I remember V=IR, so R = V/I, meaning your resistance is actually 230/5.2 = 44.2 ohms. Gut instinct says this is a more sensible figure than 0.02 ohms.

[simonp]

V=IR, so R = V/I, meaning your resistance is actually 230/5.2 = 44.2 ohms

Yep, fetch me the dunces hat, not only did I write the equation down wrong, but I then calculated it that way too To make it worse I have been an electronics design engineer for 18 years
In my defence I was barely awake this morning, I've been taking tablets that are making me very drowsey in the mornings. No excuses though, I have certainly never mad that mistake before. Crazy, I can design a 32-bit computer, but I can get Ohm's law wrong

[tisri]

Don't worry... it was a Saturday morning after all. I'll just put it down to you typing it in before having your morning coffee(s)

[zix]

Crazy, I can design a 32-bit computer, but I can get Ohm's law wrong

Hey Simon, absolutely anyone can make that kind of a mistake, especially when drowsy. Don´t put the silly-hat on yet!
However, I was after a low-tech solution (and cheap, too) for controlling the effect, and am having second thoughts. It´s the wiring. The good thing about the bulb idea is if you could buy parts for it almost everywhere, and wouldn´t need to build a chassis to make it work. But switching in and out several bulbs - could you do that in a convenient and safe way without building a chassis? It would have to work something like this:

and if it is supposed to be convenient, it would employ the usual lamp connectors that mount on the lamp wire, right? They can only break or make. This one needs switches that have one input and two out (whatever the english word is for that switch, I only know the swedish word) and aren´t those only available as chassis mount (the smaller ones available from Farnell and such companies) and wall mount ("stair switches" we call them)?
In any case, this would mean mounting in a chassis of some kind, and I would kinda like a solution that avoids chassis. Because I am lazy, mostly, but also because that means cheaper build and it is easier to make it tampering-safe, with the mains voltage and all.
Of course, with only one light bulb it could be done without the extra box. One step might be enough, sure...[/img]

[zix]

Don´t put the silly-hat on yet!

ehhrrrmmm, that goes for me too. Thinking a little bit more here: how about this, that would work just about as well, right? Provided only one switch is on at a time.

[simonp]

Yep the second one would work, but if you close more than one you get the lamps in parralel,and would overload your wire/fuse potentially.

The first one would benefit from another switch so that you acn have no lamps in circuit.

Anything you do would need to be protected in some way as it is mains, so mounting the lamp holder in a box would be a good idea.

[Steve]

Now I'm deffinatly lost anyone want to talk football

[zix]

Hey, Eeffoc! Welcome to the pleasuredome. I promise, if I get this straight I´ll build it and take a picture of me holding it in my left hand, with a football in the right hand. Then I can safely say that my left hand does not know what my right hand does .
For now, I am extremely glad that simonp and tisri are here to discuss this with me, because saying that I know a lot more about electronics than about football is (a little bit) like saying that a squirrel has a much greater chance of solving a 12 piece puzzle than a snail does.
TMC is great! Thanks guys!

Yep the second one would work, but if you close more than one you get the lamps in parralel,and would overload your wire/fuse potentially.
The first one would benefit from another switch so that you acn have no lamps in circuit.

Aha! The first one actually has a switch in front - I see it isn´t very obvious though, the way I drew it... Yes, I see your point about parallelling the light bulbs - I will have to make sure that the output from the light bulb package can only go to the roaster´s live input, not straight to 0V.

[tisri]

For now, I am extremely glad that simonp and tisri are here to discuss this with me,

For what it's worth (my electrical knowledge is fairly limited) you could look at having some kind of twisting switch, where current comes in and goes through all the light bulbs in turn, but you've got a single connector to tag on somewhere along the chain. The connector effectively cuts off all the remaining light bulbs, so at one end you get none of htem and at the other end you get all of them. Sort of like a rheostat but with less control.

Truth be told, I still don't see why you don't just use a dimmer switch. Small, convenient, and readily available with minimal wiring from your local electrical retailer.

[zix]

Truth be told, I still don't see why you don't just use a dimmer switch. Small, convenient, and readily available with minimal wiring from your local electrical retailer.

A dimmer switch for 10A? (thats what I need, 6A will be too little for me) I want to do it this way mainly because I want to save money this month. Yes, it is also cheap to modify an existing dimmer, just like simonp did too, but these parts should be even easier to find. I hope... oooops, the time... good night for now. More of this later on.
Thanks again, fellow members!

[tisri]

In the UK a dimmer switch (admittedly for 6A) is less than £5 for a cheap one. By the time you've wired in lightbulb holders, added lightbulbs (and lightbulbs have a finite life) and made it electrically safe you'll probably have spent more than that, and also spent a lot more time to end up with a control that isn't as good

I'll be interested to know how you get on.

[simonp]

In the UK a dimmer switch (admittedly for 6A) is less than £5

The ones you will get for £5 are only 400W ones, and therefore will only handle less than 2A, my modified one will cope with around 6-8A, for 10A you would need a beefier triac, and an even bigger heatsink, but the problem will be finding an inductor of the right value to carry 10A.