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Thread: Zener + transistor to regulate voltage

  1. #1
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    Default Zener + transistor to regulate voltage

    I'm building a 12V multi-LED light and need to control the voltage so I don't overdrive the bulbs. Most people simply use a resistor, but that consumes energy. I'm off-grid and have only solar power, so efficiency is important, at least in the winter. Plus, my household voltage varies from 12.2V to 14.5V during the course of the day, so no resistor value will be correct for all voltage conditions.

    I used to know a simple circuit that uses a zener diode and a transistor to control voltage without consuming much energy and without much reguard to small changes in input voltage. But I can't remember it and can't find my old notes.

    Can anybody help? Thanks.

    Josey

  2. #2

    Default Re: Zener + transistor to regulate voltage

    In general, using a transistor (and related stuff) as a substitute for a resistor doesn't change the fact that "energy is being wasted". The trick is to use as much as possible to make light (maximize efficiency).

    In general, you should regulate *current* (not voltage) in LEDs. It's easily possible to run strings of 3 LEDs in series (I assume you're using white?) on a single regulator. You can also use parallel strings for more light. Such regulators will compensate for changing battery voltage, giving constant light output for all within range inputs.

    What is it you want to do?

    Doug Owen

  3. #3
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    Default Re: Zener + transistor to regulate voltage

    Thanks, Doug:

    I guess I could use a regulator to maintain constant voltage, it is just more expensive and consumes more energy than a zener/transitor. My understanding is that if I overshoot the rated voltage of the 3-bulb series string, I'll also increase current and shorten the life of the bulbs.

    I had purchased some 36-LED lights that used only a resistor (for each 3 LEDs in series)to control voltage, but the factory had made a mistake, installing 62 ohm resistors instead of 82 ohm resistors. The result was LEDs reaching 200 degrees F.

    The teacher who showed me the circuit said it was used by NASA on space flights to conserve energy.

    Josey

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    Default Re: Zener + transistor to regulate voltage

    HI - I am not sure how much current you need, but I recently purchased a simple board from cpf'r georges80. He can send you one with any constant current setup you want from small (say 50 ma) up to 1 amp. His setup can take up to 35 volts input voltage.

    I think he also has one model with dimming. I paid less than $ 20 for the board - around 1x 1 inch.

  5. #5

    Default Re: Zener + transistor to regulate voltage

    [ QUOTE ]
    Josey said:
    I guess I could use a regulator to maintain constant voltage, it is just more expensive and consumes more energy than a zener/transitor.

    [/ QUOTE ]

    For the same amount of power going to the LEDs, a zener/transistor combination will require more power than a linear IC regulator. If the voltage is enough higher than the LED requirement (which it sounds like it is), then a switching regulator will be more efficient than either of those choices.

    At a fixed voltage, a plain resistor actually takes less power than a zener/transistor that allows the same current to flow through the LEDs (because of the required bias current through the Zener, and the base current through the transistor if it's a BJT).

    [ QUOTE ]
    My understanding is that if I overshoot the rated voltage of the 3-bulb series string, I'll also increase current and shorten the life of the bulbs

    [/ QUOTE ]

    Actually, you have to watch the maximum current, not the voltage. You can't really regulate the voltage to the LEDs very successfully.

    [ QUOTE ]
    The teacher who showed me the circuit said it was used by NASA on space flights to conserve energy

    [/ QUOTE ]

    That's unlikely for any zener/transistor combinations I can think of, unless the input voltage is very close to what the LEDs will be running at.

    Look at it this way: three typical white LEDs might have a forward voltage of (say) 10V at your desired current. Let's say that current is 20mA, for example.

    Now your input voltage ranges from 12.2 to 14.5V, so a perfect linear regulator
    (i.e. one which takes no power to run itself) would be wasting between (12.2-10)/12.2 and (14.5-10)/14.5 of the incoming power, or between 18% and 31%.

    So the linear regulator, no matter how you make it (zener/transistor, IC, whatever) is at best 82% efficient in this application, and at worst only 69% efficient.

    Typical step-down switching regulators have efficiencies between 85% and 95%, for a <font color="red">reduction of between 3% and 26% in the input power</font> (and energy) required to run these LEDs at the same brightness.

    Whether it's worth it to you to spend the money on a step-down switching regulator is your choice, but you should do the math.

  6. #6
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    Default Re: Zener + transistor to regulate voltage

    Thanks HarryN and BikerNomad.

    I tried a board like you mentioned and it failed. And I bought a commercial 12V LED light, but it was mismanufactured, and I've been waiting since the first of the year for the correct light.

    I'll do some more work like you suggested.

    Thanks. Josey

  7. #7
    Flashaholic* MrAl's Avatar
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    Default Re: Zener + transistor to regulate voltage

    Hi there Josey,

    Using a resistor and three LEDs in series,
    your average efficiency is about 75 percent.
    Using a general switching regulator you'll get
    about 85 percent, with a carefully designed one
    you might see 95 percent.

    What does this mean in terms of light performance?

    Normalizing to 75 percent efficiency:

    If your lights normally run for 5 hours:
    Going to 85% will give you 30 minutes more run time.
    Going to 95% will give you 1 hour more run time.

    If your lights normally run for 10 hours:
    Going to 85% will give you 1 hour more run time.
    Going to 95% will give you 2 hours more run time.

    If your lights normally run for 20 hours:
    Going to 85% will give you 2 hour more run time.
    Going to 95% will give you 4 hours more run time.

    This can help you decide whether or not to go to
    a switching regulator or not, and what type you'll
    want.

    Take care,
    Al

  8. #8

    Default Re: Zener + transistor to regulate voltage

    As I said before, you'd be better advised to try to regulate *current* not voltage.

    And as another poster has pointed out, there's little if any advantage of resistors over a regulator in terms of efficiency.

    If I understand your problem with the light you now own, it can easily be 'solved' with the addition of a series resistor to make up for the 'missing ohms'. In this case, you want 20/12 ohms or 1.7 ohms. It will need to be fairly high power, nearly a watt. Say six ten ohm, 1/4 watt resistors in parallel?

    That's still a lot of current per LED (60 mA), you may want even more series resistance.

    Doug Owen

  9. #9
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    Default Re: Zener + transistor to regulate voltage

    Let me summarize the three main options:

    1) Resistor. Wastes some energy. There is a trade-off. With a lot of LEDs in series and a large resistor, you get fairly constant brightness over a large voltage range, but the efficiency is poor. If you put as many LEDs as you can fit in series and use a small resistor, then you get better efficienty, but the brightness more with voltage.

    2) Linear regulator. Think of this as a "smart resistor." It is a resistor that adjusts it resistance to maintain the proper voltage or current. All you need it one adjustable regulator and two resistors to make this work. These need at least a volt or so of overhead in order to work, unless you get the LDO (low drop-out) version, but LDOs need caps and proper board layout to keep from oscillating.

    3) Switching regulator. This gives you be absolute best efficienty (if done properly), but is somewhat of a black art, and is not an ideal project for a beginner.

    If you don't mind spending some money for something pre-made, check this out: http://www.techass.com/el/versalux/ulm/ulm.php

  10. #10
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    Default Re: Zener + transistor to regulate voltage

    Thanks MrAl, Doug and Harrkey:

    I do hear you Doug, about current regulation. And Harrkey your points are great. I need at least 20 LEDs for a good reading light, so the cost at $50+/10LED cluster is steep.

    I'll take all of your advice and build a couple diffent types and measure the efficieny; I'll probably end up with results like what MrAl said.

    Thanks again. Josey

  11. #11

    Default Re: Zener + transistor to regulate voltage

    Josey,

    I'm not so sure you need 20 LEDs. I've been working on a reading/room light using 12 in 4 strings of 3 (with a 10 ohm resistor each) in parallel. After some fiddling, I ended up grinding the round end off each lens (dressing it with a file afterwards), the result is wide angle artifact free light entirely suitable for reading or other close in tasks. This is at 100 mA total (25 mA per LED). The LEDs and parts for the (three level) regulator cost about $6, more or less.

    BTW, the cicuit you couldn't recall is an emitter follower. It was neither 'invented by NASA' (that was the power factor controller, for AC not DC) nor really useful here.

    Anyway, send me a PM, I'll email you a couple of photos.

    Doug Owen

  12. #12
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    Default Re: Zener + transistor to regulate voltage

    Thanks Doug:

    Sounds like you created a bunch of sawed-off photon shotguns.

    PM sent

  13. #13

    Default Re: Zener + transistor to regulate voltage

    [ QUOTE ]
    Josey said:
    Thanks Doug:

    Sounds like you created a bunch of sawed-off photon shotguns.


    [/ QUOTE ]

    Interesting perspective.

    I'm sure after you work with this a bit you'll come to the same conclusion I did WRT beam angle. That is for reading or general lighting at close ranges we want a very wide beam angle and flat, artifact free illumination. If you look at the beam from the Attitude you can clearly see that each of the three LEDs contributes to the total. Looking at the SL 7 LED light tells us that overlapping light patterns can mask the artifacts from an individual device. This means you don't have to be too careful when you cut them back, small errors don't matter.

    So, I like your analogy. In fact, I'll go you one better (at least IMO), it's a *battery* of 'sawed-off photon shotguns'. Gets back to one of my pet peeves, the way otherwise knowledgeable folks refer to a single cell as a 'battery'. For the record, one cell (like one cannon) is just that. A *battery* is one or more cells (or cannons) connected together and working as a single entity. A box of 123's from Surefire is a dozen *cells*, even in the box (since they're not connected and functioning as one), they are not a battery. One alone is *never* a battery. The Arc AAA uses an AAA cell, not 'AAA batteries'.

    So that's my rant for the evening, care to share yours?

    Doug Owen

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    Default Re: Zener + transistor to regulate voltage

    A 9V battery is a proper battery. There's either a stack of raw flat cells encased in plastic, or 6 "AAAA" batteries.

    Power supply design, in general, is a black art to those not in the know. There's a lot to account for - capacitance, reactance, getting the exact right oscillation, saturation points on transistors, etc - an a heck of a lot of it is more involved than Ohm's law.

    The series resistors in-line with LEDs are almost universally referred to as "current-limiting" resistors, since most LEDs have a range of voltages that they can operate over. The forward voltagfe that's quoted is almost always typical. Drop below the minimum, and nothing will happen; exceed the maximum, and you'll release the magic smoke.

    A current supply will generally attempt to supply a given current. V=I*R, so it will adjust the voltage (within limits) until it supplies the current it's happy with.

    With your situation, you could almost use a simple voltage regulator (usually a largish 3-pin device, can't remember what the standard package is called) to regulate your fluctuating 12V to, say 9 or 6V. If you find a combination of LEDs whose voltage drop is just less then the rgeulator's output, you waste little other than the small drop across the resistor and the regulator's inefficiency (I confess that I don't know what the efficiency of a voltage regulator is).

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    Default Re: Zener + transistor to regulate voltage

    "A *battery* is ONE OR more cells (or cannons) connected together and working as a single entity"

    One or More? Don't you mean More than One ?

  16. #16
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    Default Re: Zener + transistor to regulate voltage

    Instead of sawing off LEDs, you can just lightly sand the surface. Less work, and it still gives a very diffuse light.

  17. #17

    Default Re: Zener + transistor to regulate voltage

    [ QUOTE ]
    Harrkev said:
    Instead of sawing off LEDs, you can just lightly sand the surface. Less work, and it still gives a very diffuse light.

    [/ QUOTE ]

    Yup. "sawed-off" was not my term, it actually refereed to shotguns when Josey used it.

    You'll note I said 'grinding', actually I used a small power sander, as a grinder would quickly foul with debris. I dressed the surface with a file (a worn 6 inch mill bastard for those taking notes) afterwards. As you suggest, the surface need not be perfect, in fact there seems to be an advantage to it not being so. The ones I polished didn't work as well in my modest testing.

    As it is, it takes only a few minutes to do the dozen needed for each array.

    Doug Owen

  18. #18

    Default Re: Zener + transistor to regulate voltage

    [ QUOTE ]
    Negeltu said:
    "A *battery* is ONE OR more cells (or cannons) connected together and working as a single entity"

    One or More? Don't you mean More than One ?

    [/ QUOTE ]

    Exactly right, thank for catching it.

    See what happens when you rant?

    OTOH, it's nice to know I'm being read......

    Doug Owen

  19. #19

    Default Re: Zener + transistor to regulate voltage

    [ QUOTE ]
    idleprocess said:
    A 9V battery is a proper battery. There's either a stack of raw flat cells encased in plastic, or 6 "AAAA" batteries.



    [/ QUOTE ]

    Here I would have to disagree. We're starting out at a nominal 12 Volts, easily capable of running three LEDs in series, and you're suggesting regulating it down to 9 Volts, which while 'doable' will only drive two LEDs. This automatically means this system will use 50% more current (3 strings needed at 9 Volts to do what two at 12 could have done).

    I suggest four strings of three at 25 mA per string for a total 100 mA from the source. You propose six strings of two, for a total current of 150 mA for the same light.

    I think the solution lies in a driver that can take full advantage of the available battery. "Giving away" a third of the power just getting started seems like a bad way to go. I favor a current regulator, running straight off the battery driving an array suited to the task.

    Doug Owen

  20. #20
    Flashaholic* MrAl's Avatar
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    Default Re: Zener + transistor to regulate voltage

    Hi there,


    I've always said that a current regulator is better
    for LED's too. There is no advantage to using
    a voltage regulator over a current regulator that i
    know of. The LED is happy with a certain level of
    current maintained regardless of it's nominal voltage,
    so we regulate the current. The LED is not very happy
    with a certain voltage across it maintained, as the
    current will vary with ambient temperature, so we dont
    want to regulate it's voltage. The LED is not that happy
    with a small resistance in series with a regulated voltage
    either, because again variations in LED voltage with
    temperature will cause the current to vary (think of a
    small resistance in series with a voltage regulator still
    a voltage regulator, while a large resistance looks more
    like a constant current source).

    Sometimes it's nice to regulate current while not
    having a voltage overhead that is much higher then
    the LED nominal voltage. This means the battery can
    run pretty low before the LED current starts to drop.
    In this case a circuit that doesnt require any overhead is
    used like this one:

    http://hometown.aol.com/xaxo/page2.html

    The actual overhead voltage is something like 35mv
    for an LS 1 watter.

    If you have some two or more volts to spare, the
    usual LM317 circuit should work just fine. The
    supply voltage will have to be at least 2 or so volts
    above the LED voltage for this to work well however.
    (Someone may wish to look up the required overhead for
    the LM317).

    In any case, if you're going with linear regulation,
    go with current regulation over voltage regulation
    for LEDs. You'll be happier knowing your LEDs are
    much happier :-)

    Take care,
    Al

  21. #21
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    Default Re: Zener + transistor to regulate voltage

    National Semiconductor's literature usually refers to a 2 volt overhead for the LM317. Other manufacturers may be different. There are low dropout versions (LDO) that operate with less than 1 volt overhead. I've considered using one of those LDOs but I've heard there were disadvantages like problems with ocsillation. How much of a problem is it? I could easily add a small capacitor if that is all it takes. What else might be needed or would work? -RussH

  22. #22
    Flashaholic* MrAl's Avatar
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    Default Re: Zener + transistor to regulate voltage

    Hello there,

    Thanks for looking that up Russ, and since you mentioned the
    LDO version i decided to take a quick look at those myself.
    I found the LM1117, which has a dropout voltage of about
    1.2 volts. This means it could be used as constant
    current to drive an LED with one resistor and would
    operate down to about 4.7 volts before current decreased.
    Too bad they dont make a zero volts dropout version :-)

    Im going to look at these a little more but so far
    it looks like in order to meet stability requirements
    an external cap has to be connected across the input
    of the device of about 10uf. I guess that's not too
    bad.

    Take care,
    Al

  23. #23

    Default Re: Zener + transistor to regulate voltage

    Amen Brother Al! [img]/ubbthreads/images/graemlins/thumbsup.gif[/img] (Once again, you're spot on)

    Current regulation is the only way to fly with LED's. Every one that I've ever come into contact with hasn't behaved itself too much in the presence of heat. The hotter they get, the better they conduct, the better they conduct, the hotter they get... pretty soon your solder is melting.

    Maitaining a constant current prevents that ugly run away. Constant voltage assures disaster if you prefer to run all your goods up near the edge of the performance envelope... and that would pretty much be all of us.

    Ordin

  24. #24

    Default Re: Zener + transistor to regulate voltage

    Mr. Al, 10uF?? What's a few uF's among friends anyhow? Awww, they come in 0603 pkgs with X5R dielectrics these days. They aren't as stable over temperature as an X7R or NPO would be. As long as it's sized so that at the temp extremes the device is likely to be exposed to it still has enough uumph to keep the LDO from oscillating, it will be just fine.

    Those new-fangled high capacitance ceramics are wonderous little devices. Cheaper than tantalums, smaller than tantalums, none of the "surge surprises" that tantalums are so famous for, and miles less ESR. Better'n sliced bread, they are.

    Ordin

  25. #25
    Enlightened
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    Default Re: Zener + transistor to regulate voltage

    Josey,

    look in your inbox(email).

    9X25

  26. #26
    Flashaholic* MrAl's Avatar
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    Default Re: Zener + transistor to regulate voltage

    Hi there Ord_Guy,

    Very good point about the possible thermal problems!
    Also good point about the 10uf caps not being as big
    as they used to be :-) No longer need a big fat
    cylindar sticking up towering over the tiny SM parts.

    Another good point about using constant current over
    voltage regulation that i forgot to mention is that
    it takes care of variations from the manufacturing of
    the LED too. Any variation in voltage at the given
    current (such as 20ma) wont matter using current
    regulation. This means each circuit doesnt have to
    be tweeked individually for a particular LED.

    I guess it would be worth mentioning that this would
    handle every color too without any changes to the
    circuit.

    Take care and thanks for the interesting points,
    Al

  27. #27
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    Default Re: Zener + transistor to regulate voltage

    MrAl and OrdMan, thanks for the info. I'll have to look for some of those 10 uF ceramics. I'll be ordering some 317s again soon &amp; I'll get some 1117s (or cousin LMS8117A- I need about 1 amp) to play with, too. I'll have to get one of the larger packages. -RussH

  28. #28
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    Default Re: Zener + transistor to regulate voltage

    Thanks Christian:

    I appreciate that design and will be trying your circuit.

    Thanks Doug:

    Your filed down LEDs look a lot better than I had imagined. I'll be copying your work. Current regulation it is.


    And thanks everyone. I've got a ton of ideas running through my head.

    Josey

  29. #29

    Default Re: Zener + transistor to regulate voltage

    [ QUOTE ]
    Josey said:
    Thanks Doug:

    Your filed down LEDs look a lot better than I had imagined. I'll be copying your work. Current regulation it is.



    [/ QUOTE ]

    You're very welcome.

    I know it sounds crude, but the results are excellent. The 12 LED array ends up with just over 90 degrees of beam angle, that is it will light up two feet of wall (or newspaper) from a foot away. Even, artifact free light. Plenty bright enough for reading, at least so far.....

    Doug Owen

  30. #30
    Flashaholic* idleprocess's Avatar
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    Default Re: Zener + transistor to regulate voltage

    [ QUOTE ]
    Doug Owen said:
    [ QUOTE ]
    idleprocess said:
    A 9V battery is a proper battery. There's either a stack of raw flat cells encased in plastic, or 6 "AAAA" batteries.



    [/ QUOTE ]

    Here I would have to disagree. We're starting out at a nominal 12 Volts, easily capable of running three LEDs in series, and you're suggesting regulating it down to 9 Volts, which while 'doable' will only drive two LEDs. This automatically means this system will use 50% more current (3 strings needed at 9 Volts to do what two at 12 could have done).

    [/ QUOTE ]
    I was just addressing the tangential issue of "battery" vs "cell" in that statement.

    I only mentioned voltage regulators because I'm familiar with them and they're easy to use. I don't know how efficient they are, but I'm fairly certain they're not as bad as resistors (if they are terribly inefficient, I'll stand corrected). Since this isn't a portable operation, there could be a case for simplicity at the expense of some efficiency. 1-2 amp voltage regulators are dirt-cheap and don't take a lot of know-how to use. Also, because they're usually "power" components, they're a bit sturdier (mechanically and electrically) than many of the components that I've seen used in more complicated power supplies.

    A voltage regulator p/s also assumes some sort of current regulation - likely a small resistor that's dropping a fraction of a volt due to a convenient output voltage.

    Again - I'm shooting from the hip here, not really knowing a whole lot about voltage regulator mechanics.

    A current supply is a good way to go, since LEDs regulate their own voltage handily if current is restricted.

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