Using resistors or halogen light to make a finely adjustable heater?

Discussion in 'General Electronics Chat' started by RogueRose, Dec 23, 2015.

  1. RogueRose

    Thread Starter Member

    Oct 10, 2014
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    I'm looking at ways to make an adjustable heat source for an oil bath. It needs to have a very wide range of power output but will never really get above 300-400 def F (don't know total wattage needed though). I plan on using mineral oil as the oil for the bath, so it shouldn't short any connections

    I was thinking of using a number of 2 or 3 watt resistors to get very low heat by turning each one on as needed up to maybe 20/21 watts.
    When more heat is needed I would use 10, 15 or 20 watt resistors the same way as the smaller ones. Maybe add some 40-50 if plausible or needed.

    I'm wondering if it may be possible to use a halogen bulb instead of the resistors in the bath. The light from the bulb would actually be very useful at times so that is a good reason to try this. I was thinking that using a dimmer switch would allow for good control over power output. I would think that the bulb glass should have too much thermal stress as any heating and cooling would be slower than with air (which I think should help make it last longer...?). It seems the bulb idea would be the simplest to make and probably least expensive.


    I also need to make a low temp, low humidity oven that can run from 120-200 degrees, mainly for dehydrating things. I was thinking the light on a dimmer w/ fan would be the best solution but am open to other ideas if there are better ones.
     
  2. bertus

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  3. wayneh

    Expert

    Sep 9, 2010
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    I agree that the first place to start is with an estimate of the wattage you need. It will depend on insulation, how quickly you need it to heat up, and so on.

    I'm a big fan of lightbulbs as heaters because they solve a few problems. You get visual feedback, they are designed to dissipate heat safely, mostly, and they're easy to find and switch out as needed.

    But in the case of an oil bath, and assuming you need a few hundred watts or more, I think I'd lean toward an actual heating element. You can get fine control over it with a thermostat, or a PWM circuit, or a light dimmer. There are many immersion heaters to choose from.
     
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  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    To heat the oil bath evenly, a halogen bulb is the opposite of what you need. 20 W is 20 W, but if it is spread over the few square inches of a power resistor it makes much better contact with the oil than the same heat concentrated into the small space of a halogen bulb. High temperature/small contact area requires stirring to prevent localized overheating of the oil. Multiple small resistors will give much better results.

    ak
     
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  5. SLK001

    Well-Known Member

    Nov 29, 2011
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    Scrounge a heating element from an old electric water heater. It is just a resistor, and it is already shielded from the oil. Plus, you may be able to actually achieve your temp goals. Plus plus, you can easily control it with a light dimmer. In the US, they are built for use at 220V AC, but being a resistor, they will operate at any voltage, AC or DC, under 220V.

    If you can't scrounge one, they sell them as replacements at the box stores.
     
  6. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    A real PID temperature controller should also be used. How finely you can control depends on the controlled element. Look up phase angle fired, PWM etc.
     
  7. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Buy a heater rated for the temperature.

    The materials the heater is made of is what counts, using resistors would work, but they will break down and fail quickly
    at those temperatures.

    Light bulbs in oil sounds like a fire hazard?
     
  8. Tesla23

    Active Member

    May 10, 2009
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    You may want to rethink that. My understanding is that in your halogen bulb the filament is running much hotter than in a normal incandescent and so evaporates more quickly, the halogen atmosphere in the bulb helps the evaporated tungsten redeposit on the filament, giving you back a reasonable lifetime. In order for this to work the glass bulb has to be much hotter than a normal bulb or the tungsten will deposit there - and it is made hotter by making it smaller. If you cool the quartz bulb you will expedite the evaporation of the tungsten filament and reduce the bulb lifetime.

    Normal incandescent bulbs make great heaters, and last a reasonable while if you run them below specified wattage.
     
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  9. tindel

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    Sep 16, 2012
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  10. wayneh

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    I think we all need a better understanding about what the thread starter is doing, especially the scale and the expected wattage, but also the configuration. I'm picturing a small (1L?) vessel sitting above a lightbulb. (I do NOT like the idea of immersing any light bulb.) Or maybe it's big enough to cook a turkey, and a available commercially immersion heater would be suitable. Or maybe he's deep-frying manatees, I don't know. He hasn't said anything about the precision of the temperature control either. You don't need PID control to achieve 350°F±50°.
     
  11. sootydog

    New Member

    Jul 23, 2011
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    The resistance of a resistor is pretty constant with temperature, the halogen lamp's resistance will vary a lot so consider that when designing the controller if you choose to go that route.
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I would use a 500 W halogen bulb (designed for 120 VAC) but run at 12V DC. It should have about 3 ohms resistance when cold. Running 12 v through it will limit output to about 50 watts. Much more than your resistors and it will not get white hot. Should be easy to control.
     
  13. pilko

    Active Member

    Dec 8, 2008
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    @GopherT
    At 12 volts he may only get 5 watts?
     
  14. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The way you calculate, you assume the bulb heats to the same temperature and the resistance increases to the same level as when 120 volts are used.

    The key piece of information in my post is that the COLD resistance of a halogen bulb is about 3 ohms. Stone cold, the lamp will have just under 2 ohms of resistance.

    Power = (12^2)/R = 72 watts (assuming limited or no resistance change with heating at 12 volts). I think resistance could go up to 3 to 4 ohms, so I estimated 36 to 48 watts.
     
  15. pilko

    Active Member

    Dec 8, 2008
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    you are correct GopherT . I had a resistive element on my mind.
     
  16. RichardO

    Well-Known Member

    May 4, 2013
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    Thanks for the link. Lots of further links there that I intend to follow. :D

    I went to National Semi Analog Design seminars just to hear Bob Pease speak.
    Once he talked about 2nd, 3rd and maybe 4th order corrections used in bandgap reference diode circuits. I wish I had understood all he said and could remember what I did understand.
     
  17. RogueRose

    Thread Starter Member

    Oct 10, 2014
    189
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    Thanks for all the replies - I'll make the usage of the device a little more clear. I would say that the oil volume could be from 1-2L with a 1.5L average. I am going to have a circulation pump that will allow for running something opposite a condenser. The vessel will most likely be something like a stainless steel pan 6-7" diameter with some kind of insulation around the outside, maybe 2" of vermiculite or perlite, contained in a larger pot (possibly aluminum).

    As for heat requirements I really don't know what is needed as it will vary greatly as per the application at the time as it may be used as an oil bath to heat various vessels or it may be used as the heat source for a copper tubing setup (reverse from condenser).

    I have no problem using a heating element especially if it can be controlled with a dimmer, PWM or PID controller. I'm I think it may be beneficial to use 2 heaters, something of 200 watt and a larger unit, one for initial heating and the other for continued heating so I'll have to find out what the exact heating energy is needed.
     
  18. GopherT

    AAC Fanatic!

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    The lower the mass, and less heat capacity of the heater the better. Otherwise, the thermal time constant can be quite long to wait for a heater to start passing heat to the oil (takes a while to heat itself up), and then there can be a lag in cooling or hitting the set point with a soft landing without overshoot of mass is too high and not using a properly tuned PID specific to the heater.

    GOOD LUCK.
     
  19. wayneh

    Expert

    Sep 9, 2010
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    I really think you need a thermostat in there.
     
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