LED Dimmer using a 5-6ohm Rheostat?

Discussion in 'The Projects Forum' started by SuperRA, Jan 15, 2016.

  1. SuperRA

    Thread Starter New Member

    Jan 15, 2016
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    Hello everyone, I'm new here and was hoping to find some direction on a possible project. I would like to make an LED dimmer that is controlled by a 5-6ohm rheostat. Here's what I'm doing. I want to change out bulbs in a couple of my cars instrument clusters to LEDs, but the stock dimmer circuit reportedly doesn't dim LED's well as it was just a simple rheostat in series with the incandescent bulbs. I want to keep the stock rheostat as a control, but somehow use it to control a more sophisticated circuit (like the 555 timer circuit) to dim the LEDs. Is this possible or is there some other method that would work?

    Thanks
     
  2. #12

    Expert

    Nov 30, 2010
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    Something like this, considering you gave me nothing about how much current your LEDs need.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    The stock dimmer is not likely a 5-6 ohm rheostat. Can you see or measure it? I'm sure there is a solution, but we need to know what we're starting with.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    It's possible he already has "12V" LEDs, or not, so it's tough to know. PWM should work either way but everybody likes simple.
     
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  5. #12

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    I can see how a 1950 wirewound/ceramic dash light dimmer might be good for 2&1/2 amps. I'm that old. As for a current driver or a voltage driver, I accomplished one interpretation which is a good starting idea even if we have to convert it to a voltage driver or PWM.
     
  6. SuperRA

    Thread Starter New Member

    Jan 15, 2016
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    Thanks #12. OK yes I do need to provide specs on the LEDs to nail down the circuit better. Wayneh, I'll confirm the resistance of the rheostat. I read an online spec somewhere that it was 5.5ohms.
     
  7. #12

    Expert

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    You are going to have to get your paws on it some day, so you might as well make it now. Measure it!
    "I read something somewhere" is not how you design circuits.
     
  8. SuperRA

    Thread Starter New Member

    Jan 15, 2016
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    I know, there's nothing like the assurance of measuring it yourself. However it isn't exactly an easy task to remove. I've replaced it before and I did look around for the old switch, but it's likely I threw it away. I did find this forum thread showing that the full resistance is 5.8ohms.
    http://www.jeepforum.com/forum/f11/...-your-headlight-dimmer-switch-97-xjs-1314610/
    14 pictures down, there's a picture of him measuring it with a meter. Although mines is the a wire wound type and not the resistor plate as shown in the picture, it should have the same full scale spec or at least something close to it. I would say that is the best I'm going to get without actually measuring it.

    Now for the LED current, I've measured the LEDs I've already bought (which by the way are the following)
    7 of these: http://www.amazon.com/gp/product/B00877SL2M?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00
    and 7 of these: http://www.amazon.com/gp/product/B00K85H9F6?psc=1&redirect=true&ref_=oh_aui_detailpage_o02_s00

    The large bulbs draw 63mA and the small ones draw 32mA. Since they are all wired in parallel, that's a total of 665mA. I'll have to put in a current limiting resistor as well as I don't believe these have any.
     
  9. #12

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    Yes, those are automotive 12V LEDs, built to plug into existing sockets. PWM is the only way to go. I converted the first stage to a voltage driver. Who has a PWM driver to tack on to it? (It's 4 am here and I'm about out of steam for today.)
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Your rheostat maybe wired in series with the bulbs, to control the dimming, you need to find out how its wired before you can use it.
     
  11. dannyf

    Well-Known Member

    Sep 13, 2015
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    The existing rheostat will dim the led, just barely, because the LEDs' on equivalent resistance is far higher than the rheostat's.

    It is possible but probably not the best solution as a typical 555 timer would require considerably higher resistance / resistance range. A easier way to go down the 555 path is to use a different potentiometer + 555 timer to drive the leds.

    One approach would be to use a mcu. A typical 8-dip mcu could detect the rheostat's position via adc and use a pwm generator to drive the led. It does require programming so if you are new, the learning curve can be steep.
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Can you post a link to the car's wiring diagram, showing how the rheostat and present lamps are interconnected?
     
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  13. #12

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    I am familiar with this circuit. If this was 1960, I would bet on it. The 5.8 ohms is in series with the lamps and on the high side (in the last car I worked on). That means I can detect the rheostat's position via analog amplifier and use a PWM generator to drive the LEDs. This doesn't require any programming, it just requires fairly good information to use in the design.

    I'm thinking, 470 ohms 1 watt to develop 0.187 volts (max) at the rheostat. Start the power supply with a bit of protection against transients on the 12 volt line, regulate down to 5V, use 5V to run an op-amp and a 555 timer astable at 30 Hz to keep it under audible range, pour those into another op-amp to make the pwm function, and do a double invert so I have a pull down transistor to drive the LEDs from the high side.

    My only worry is that such low current through the rheostat isn't in its normal range. I have to depend on the wiper motion to keep scuffing a new surface on the rheostat windings. If there is already a 5V rail in the dashboard, I might use that because I need less than 0.1 amp.

    So...wiring diagram? You're going to need it to see where to place a resistor in series with the rheostat, whether the rheostat is originally connected to the power line or the ground line, and where to insert the PWM drive into the light bulb power line. While you're in there, look for a 5V power supply intended to run the instruments.

    Right now, I need to do physical work during the daylight hours. I have a sick car on the back porch that needs my attention.
     
  14. wayneh

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    I wonder how those LEDs respond to voltage. Do they dim and go out when they hit, say 6V? Or 10V? There could still be a simple analog solution.
     
  15. #12

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    They surely go out when you hit them with 2 volts, because they are LEDs! But PWM makes that irrelevant. Being in the white range, it's probably more like 4.5 volts of deadband on the bottom end. Still, hitting them with a 12V PWM makes the "dim" voltage deadband irrelevant.
     
    Last edited: Jan 16, 2016
  16. dannyf

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    Sep 13, 2015
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    Since they are automotive bulbs, if you really want to drive them linearly, you can put the rheostat in the source of a mosfet (or emitter of a bjt), and the bulb in the drain. This arrangement widens the "adjustability" of the rheostat -> the current varies over a wider range of turns of the rheostat.

    Its advantage is simplicity. Its disadvantage is that the heat dissipation on the mosfet can be significant.
     
  17. #12

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    @dannyf
    Please post your schematic. I can retire from this project when we see how well your circuit works.
     
  18. SuperRA

    Thread Starter New Member

    Jan 15, 2016
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    First off. Thank you to all (#12, wayneh, et.al.) that are helping. I do realize that this is not your regular job and daily life as I have these as well. So if you have more pressing issues, skip helping me, I can wait. I normally try to help out on automotive forums where my knowledge is the strongest. I am a mechanical engineer but I do have a little (and I stress "little") electronics knowledge. Talking about op amps without specific part #s or function is right at the limits of my knowledge so it may take me a bit to catch on.

    Now here is the wiring diagram.
    The rheostat is on the high side inside the headlamp switch. Keep in mind that it doesn't have to stay there. I can remove it from the circuit and use it as a separate variable resistor if needed. Also there is no 5v present in this diagram and I am highly doubtful 5v exists in the dash. This is a 1987 Jeep Cherokee if that helps. 20160116_132430.jpg
    I know there aren't quite that many lamps in the cluster, so I counted up 7 large bulbs and 7 small bulbs.

    This also brings me to a somewhat related question. When selecting a current limiting resistor, can I choose one large resistor able to handle the wattage and limit all the bulbs (of two different sizes, 63mA & 32mA, in parallel) to 600mA total? Or do I need a separate resistor for each set of bulb sizes? Or does each bulb need to have its own resistor?
     
  19. dannyf

    Well-Known Member

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    If those are indeed automotive leds, be careful using a resistor on them.

    In general, you want to match the current consumption of your leds strings with that of the bulbs they replace.

    The fact that those bulbs are wired in parallel doesn't help.
     
  20. #12

    Expert

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    The limiting resistor is inside the bulbs you bought. They are all pre-fabbed for use in a car. Want to check? Connect one to a car battery. If it lights up, I'm right. If it pops like a fuse, I'm wrong.:D

    Really. Try it. One actual measurement is worth a dozen theories.

    And yes, that diagram and the model year brings a boat load of information. If we design this good enough to pop a 5 amp fuse, we have it whipped.:cool:

    It looks like that dimmer is internally connected to the high side INSIDE the headlight switch. Not accessible. You are going to have to splice in at R and I to get me the reference points I need. I will start with a variation of the first circuit I posted, a voltage to current converter, then into a resistor. The resistor converts the current into voltage and we're off to the PWM driver.:p

    I still want you to unplug that switch module and measure the ohms from R to I as you spin the knob from dim to bright. That's the ONLY reference we have, so it is desperately important to know what's going on in there. The difference between 5 ohms and 6 ohms means nothing to a light bulb. It means everything to this conversion. Remember, One actual measurement is worth a dozen theories.;)
     
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