How to make resistance adjustable

Discussion in 'Homework Help' started by Dunkhan, May 23, 2014.

  1. Dunkhan

    Thread Starter New Member

    May 23, 2014
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    My question is basically this:

    How can a make a DC circuit with adjustable resistance?

    I was going to use a potentiometer, until I found out that potentiometers start at about 1kohm, and my circuit needs adjustment between 0 and about 5 ohm. I found some pots that low on some specialty electronics sites, but they cost like 100 bucks. Which leads me to a follow up question:

    Shouldn't it be cheaper to build a low resistance pot than a high resistance one?

    For those of you wondering how anyone can be so ignorant and why I am even asking, here is some background, don't feel obliged to read any of this:

    This is not homework per se, but I believe it is roughly on the same level of difficulty of highschool homework and I did't want to bother people in serious discussions with my noob question.

    I smoke an e-cigarette and I got sick of recharging batteries all the time, so I wired up a simple wall plug adapter. I got a 5v 1a cellphone charger, and connected the leads through a button to the vaporiser terminals. This worked quite well, an e-cig circuit is just simple element so my very limited knowledge of electronics was enough. The issue is that the power output of the adapter is a bit high, the standard for these devices is usually around 4 volts at about 500ma (at least that was what someone on the forum measured). This means it burns too hot sometimes. As the element ages it gets worse so this extra power is great for getting more life out of an old element, but with a new element it is just far too hot. I build my own elements so I can build some that will cope better with the higher power, but then when I am out and about and dont have a handy wall socket I have to use my batteries, and then it is too cold.

    My plan was to simply install an adjuster knob, but as I said above nothing I can get from an electronics store will do the job. I researched for hours on the web, and came up with nothing. I did manage to work out the calculations for the result of adding resistance to the circuit, using this very nifty simulator. Basically if I add 5 ohms to the circuit before the element it will be pretty much unworkably cold, 2 ohms will bring it onto the same rough level as my batteries. Using a 1k pot with a 270 degree turn would mean turning the knob 1 degree would pretty much turn off the device.

    I understand there is probably something very basic and obvious I am missing here but I started self teaching electronics a few hours ago so I hope it is understandable.
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
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  3. #12

    Expert

    Nov 30, 2010
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  4. inwo

    Well-Known Member

    Nov 7, 2013
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    I don't know how critical fine tuning is. You might try a double throw switch with one or two low fv diode drops. Or fixed resistors.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    Potentiometers come in a wide range of resistances, even lower than 1Ω.

    But there is more than one way to skin this cat.

    1) You can lower the voltage by placing another load in series with the 5V power supply. The extra load can be (a) a resistor (b) an incandescent lamp (c) one or more diodes such as 1N4001 in series.

    2) You can build (or ask someone to build) a pulse-width-modulated (PWM) circuit that outputs 5V pulses of adjustable widths allowing you to control the power delivered from 0-100%.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    I would recommend just putting a 1N4001 diode in series with the element. That will drop about 0.9V at 500mA putting it right about where you want it. The diode might get a bit warm as it will be dissipating nearly half a watt which will result in about a 20C to 25C temperature rise. You might put two in parallel. You'll still have a 0.8V drop but should only have a 10C rise.
     
  7. Dunkhan

    Thread Starter New Member

    May 23, 2014
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    0
    Thanks everyone for the ideas. You have given me a lot to think about.
    I am still left wondering why low resistance pots are so expensive though. The only reason I can think of is that demand is so low that there is no mass production of them.

    Once again do not feel obliged to read the wall of text, for those interested I will explain what I am going to try next. (I hate it when searching the web I find a person with the same problem who finds a solution but does not share it).
    One thing I failed to mention above is that the setup needs to be small, the device is built into a hand held housing about 13mm in diameter and about 100mm long. The potentiometers that were found on the net (thanks inwo) the first one was quite large at 24mm, and still $10 not including postage from Canada. The digikey one which I actually already found before posting was perfect, compact, low resistance, relatively cheap... oh the company has a minimum postage charge of $46. Actually while writing this I noticed that the postage is cheaper outside the US :)confused:) at $30.

    Still for that price I can probably get some kind of digital microcontroller if I only knew what to search for.
    Which i kind of know now thanks to MrChips' suggestion of a PWD circuit. I will look into buying/building one of these, it is at least something that will help me refine my electronics knowledge to build, and seems to be made of relatively inexpensive parts.

    The other solution is a multi stage switch. As inwo pointed out in his second post fine tuning is not critical, although a double throw switch might not be enough. I am going to look into what kind of multi stage switches are available and maybe just run a set of fixed resistors with something like 0, 1, 2, 3, 4Ω

    Thank you all I am very impressed with my first impressions of this community.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    A typical 555 timer circuit and a MOSFET will give you 50-100% power control using PWM.
    That should work for your application.

    Ask for help when needed.
     
  9. #12

    Expert

    Nov 30, 2010
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    I think a 2 position switch with one position connecting 1 ohm and one position with 2 ohms might work, but a PWM generator is small, uses cheap parts, and is about 99% adjustable. Chopping the current is exactly the same result as using a resistor, but wastes a lot less power.
     
  10. Dunkhan

    Thread Starter New Member

    May 23, 2014
    3
    0
    I just wanted to add something in case others searching for similar information found this page:

    As far as I can tell the idea of using a multi position switch and various levels of resistor is not viable for the following reasons:
    According to a random ohms law calculator my circuit running at 5v through the standard 2Ω heating coil will be running at 12.5 watts. Obviously with the resistors added it will be a bit lower but I will need a relatively low resistance setting, say 0.5-1Ω. At these values the circuit will be running 10 and 8 watts.

    My local electronics store has high powered resistors, but a 1Ω 12w resistor is about 5 euros, and is far to big (35x24x10mm) to fit inside the unit I am building. Not to mention that running at those maximum values it will probably be producing almost as much heat as the heating element itself, which given that it is inside the hand grip of the unit, means I will probably burn myself and melt various wires and components. All this without even considering how much power is being wasted.

    tl:dr
    for the purposes described in the OP the ONLY viable solution is the pulse-width-modulation system suggested by MrChips and others. Now I just have to learn how to build that :p
     
    Last edited: Jun 12, 2014
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