Mike Singer wrote: > ------- [OT]: =3D> [EE]: ------- > > I'm looking at a small incandescent lamp 60V-50mA, > lying on my desk. (Even don't ask me what device I took > it from :-) When cold it shows 120ohm, when under 60V, > I guess, it's current should be 50mA, so resistance > 60/0.05=3D1200ohm. > Assuming temperature under 60V is about 2000 C we > have (1200-120)/2000 =3D aprox 0.5 ohm per 1grad C. Or this > thing should double it's resistance at 250 C. > So needed current I =3D 5V / 250ohm =3D 20 mA. > Power dissipation P =3D 5V * 20 mA =3D 0.1 W much less then > 60V * 50 mA =3D 3W. > > So voltage across the lamp should change from ~2.5V > when cold to 5V when oven is heated to 250 C. > > Why not use this as "Temperature Sensor" ? > 20 mA current source is not a big problem, I think. > > Mike. > ------- > Correct me if I'm wrong, please. Lets especulate about it. Suppose you insert a small lamp inside the oven and measure its filament resistance, what can happens? The lamp has vacuum or an dumb gas into the bulb to avoid oxigen to oxidi= ze and burn the filament. Small lamps normally have just vacuum, it is cheap to produce that way. With vaccum, the temperature will have some difficulty to be transported from the oven internal radiation to the lamp= 's filament. I mean, the filament will receive just infra-red initially, later on the thermal vibration can reach the filament from the lamp's bod= y and glass posts and wires holding the filament. It means, the lamp resistence change will take some time to reflect the oven temperature, ti= me not fast enough to allow a good power control. Then suppose we build a circuit to measure the actual oven thermal heatin= g elements electric current consume during heating. A table could be built doing lots of measurements using a thermometer and lots of patience, what can happens? The resistance of such element will reflect the temperature of the element rubishing like a shy girl, not exactly the oven's temperature. I think that when the oven is at 200=B0C, the element red probably will be around 1000=B0C, when the thermostat turns it off and st= ays there, probably seconds before it turns on again its temperature should still much higher than the oven, a guess would say around 300=B0C or more. During this week I will try to measure it with the non-contact thermomete= r I should receive on Tuesday. If measuring the actual power consume of such thermal element, I guess it will not help much, since the change in current will not (I guess) be muc= h noticeable from a warm to a hot oven, when you feel 100% power to an element, it turns bright red, and this means very high temperature, for a cold or hot oven, I may be wrong. I remember seen a color table, meaning "color of the temperature". It was= a simple color bar printed on a thermal scale (like a ruller), from dark re= d, going to shine red, redish yellow to bright yellow to bright white in 6 t= o 7 inches. You can guess DARK RED is how your car's brake pads and disk go= es after a strong break at the freeway when you spot a police officer with a radar gun - and WHITE is simply hell's temperature of a star. By simply using this ruller in front of your eye and matching the color, you can sa= y what temperature the target is, of course, it should be at least dark red. I never found this scale for sale, if someone has any tip, it will be welcome. You can relate this issue to a range's spiral heater, the pan temperature could be around 300=B0F, but the spiral will be middle red, and it can represent something around 1500=B0F. When the thermostat turns off the heater, it probably stays at the pan temperature or a little bit higher, but it happens since there is a direct physical contact between the parts. If there was air in middle, as into the oven, I don't know what the heate= r temperature could be. Suppose the minimum it goes is exactly the oven's temperature, then that's ok, but it requires to cools off to oven's temperature before taking a valid resitance reading, and that time could = be a killing for any temp control process. By other side, any other exposed thermal element, as a simply AWG32 (very thin) nickel-chromium wire, or even a disconnected secondary heater eleme= nt could be used for such measurement. Wait for this week news... :) VV46NER -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body