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Zener clamp efficiency/drain

Discussion in 'General Electronics Chat' started by Gigaah, May 19, 2010.

  1. Gigaah

    Gigaah New Member

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    So as with most hobbies in my life they end up taking me down many other paths. My Saltwater aquarium hobby has me digging into electronics. I know enough to be dangerous. I've learned a lot in the past few weeks too.

    I'm looking to get around 32 to 50VDC from 110AC Cheaply..but I don't want super terrible efficiency but I'm looking for a cheaper way than an AC transformer. I'm looking for about 1.5amps. Voltage isn't critical as it will be regulated by an LM317 Later on. I did the math on bringing the voltage down with a bunch of LM317 but I'm looking at loosing 60watts by the time I hit 40-50 and Also I don't know how well the lm317 IC will take to unsmoothed FW Bridged DC. Further more its WAY cheaper to smooth the voltage after I get it down to where I need it at 50v vs 120v. Here is a rough example of what I was thinking of doing.

    I'm just looking for input as to how much waster power there is just in the converting the 120AC to 50DC like this. Is there a better method that is still cheap? What sort of drain or efficiency am I looking at clamping the voltage like this?

    Constant current supply.jpg
     
  2. dknguyen

    dknguyen Well-Known Member

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    Your LM317 also appears to be wired as a current source, and not as a voltage source. That aside...

    THe DC-average of a rectified 120Vrms sine-wave is:
    0.636 * 120Vrms = 76.632VDC.

    And losses from clamping/linear regulation is
    Voltage Wasted * Current = (76.632VDC - 50VDC) * 1.5A = 40W

    THat would mean that between the resistor and LM317 you would need to dissipate 40W. Which basically means a 40W resistor since LM317's heat dissipation is almost neglible compared to 40W. You also need a zener that can handle 1.5A which is hard to find. Use a transformer between the wall and rectifier to greatly reduce this difference.

    Also, having the smoothing capacitor after the voltage clamper causes the zener to constantly switch on and off (or rather, between blocking and breakdown). THat's why you have the smoothing capacitor before the voltage clamp. THe way you have it would make noise and other undesireable artifacts.

    I would realy add in a transformer for safety sakes and efficiency . It's also one of the best things about AC conversion to increase efficiency so you might as well use it. Use it to to step down your AC voltage so that when you rectify the wave, the DC average is much closer to 50V which would improve efficiency of the clamp circuits and regulator.

    0.636 * VACrms/n = VDC, where the transformer input : output turns ratio is n:1.

    (The transformer acts on the AC voltage before rectification which is why division by n only applies to VACrms, and not 0.636*VACrms as a whole.)

    A 1.4:1 or 1.5:1 transformer is about right to get close to 50VDC rectified (52VDC-55VDC to be exact) while still being high enough for the clamping circuits and linear regulator to deal with. It'd cut your power dissipation down to about 7-10W or so.

    Transformer ratio of 1:2 would get you around 40V. Ratio of 1:3 would give you around 25V.

    So what costs more? A 40W 100ohm resistor? Or a 100W transformer? The resistors are certainly rarer. Also consider that when when run at their power rating, resistors run hot enough to burn you and other things. One way around this is an resistor with an oversized power rating, but then you might as well spend the extra dough on a transformer and get better efficiency.

    What is your load anyways? DOes it even need good regulation? Or even DC for that matter? If it didn't need regulation, that would make your life a lot easier because cooling can still be difficult at 10W.

    EDIT: For example:
    http://parts.digikey.com/1/parts/778837-resistor-100-ohm-40-w-10-ww-12m40-100.html
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=241-8-56-ND

    I see what you mean by the cost for transformers is a lot more than resistors. But for me, non-isolation of the mains and the burning potential of the resistors is not acceptable. If they are acceptable to you, then you would have to first see if you can actually find such a resistor in low quantities.

    And it seems that considering the cost of the transformer, you might as well just get one of these:
    http://www.trcelectronics.com/Meanwell/gs90a48-p1m.shtml
     
    Last edited: May 19, 2010
  3. Hero999

    Hero999 Banned

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    If all you want to do is power a light bulb, a 50VA transformer will do.
     
  4. dave

    Dave New Member

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  5. Gigaah

    Gigaah New Member

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    The LM317 is indeed wired as a constant current driver for High power LED's.

    Attached is an Oscope of the DC wave VS the root AC wave. It doesn't look to me like I'd be dipping below the zener voltage at all and the wave looks fairly smooth. Am I missing something?

    After looking you are right a 1.5a 50v zener is nearly impossible to find. I would have liked to get 50v. I could deal with 1 amp 36v. I just makes it so I need more 36v 1a zeners and lm317's both of are fairly cheap. I will only be able to run about 9LED off each zener/LM317 but I can deal with that.

    My biggest issue with using a transformer is output. The cost of a transformer that will put out enough amperage isn't cheap. I didn't realize the resistor would have to be that large. If I didn't need the resistor then I'd be set. Anyway to do that?

    Unless the 36v zener idea is reasonable I'm probably just going to re-purpose my old computer power supply and just run run 3 LED from a LM317 the 12v will be clean tho. I don't want to. thats alot more wiring BS than I cared for. Anyway here the oscope of the DC voltage compared to the incomming AC voltage
    WAve form current supply.jpg
     
    Last edited: May 19, 2010
  6. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    If you use a linear regulator like that you will always loose a lot of power, and the loss of power isnt always the issue yes, but the heat buildup because of that loss of power is almost always a nasty issue to deal with. Heat sinks, fans, air holes, etc., often required for good cool operation.

    Another idea is to use an offline switching regulator. This is a circuit that first rectifies the input line, then uses a high voltage switching regulator to buck the voltage down to the required level. Efficiency is often very very good too, such as 80 percent. You wont get isolation from the line however unless you use a special transformer (smaller than the typical AC line type). Typically an inductor is used for the energy conversion.

    You might try a very large AC cap but im not really sure if this would work or not or how long it would last at that high current of 1.5 amps.
    Typically these kinds of supplies are made for light current loads like 50ma or so. This design would have to be done very carefully too.
     
    Last edited: May 19, 2010
  7. Gigaah

    Gigaah New Member

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    I was looking at buck regulators. To be honest they all have like 8+ pins. I don't have the foggiest idea how to wire them properly. I can make sense of the lm317..thats pretty simple. They make 100v bucks.
     
  8. dknguyen

    dknguyen Well-Known Member

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    Yeah, buck regulators ICs can be overwhelming if you aren't accustomed to LCR circuits and ICs.

    They do have buck regulator modules though where the IC, inductor, capacitors, and power switches are all done for you. THey cost more, but the work is taken care of. THeir cost is around that of a transformer or the packaged adapter:

    Digi-Key - 102-1938-ND (Manufacturer - VGS-50-48)
    Digi-Key - 102-1338-ND (Manufacturer - VOF-65-48)
    Digi-Key - 102-1944-ND (Manufacturer - VGS-75-48)
    Digi-Key - 102-2031-ND (Manufacturer - VOF-80-48)

    WHat I'm referring to is not a matter of whether it works or not. It's a matter of the quality of the output is all. Think about it. A rectified half wave going into a zener voltage clamp without a smoothing capacitor will fall below the zener voltage every time the AC signal crosses zero. I don't know if the simulator accounts for finite time it might take for the zener to change operating modes. It's just something to keep in mind.
     
    Last edited: May 19, 2010
  9. Gigaah

    Gigaah New Member

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    Yeah for that price I can buy a regular power supply via ebay. National semiconductor has a nice web bench. I think I can get it to create me a 100v buck regulator for like 7$ and it'll tell me how to wire it and what to puchase. If someone could tell me how to adjust the voltage and amperage manually per application. If I did that would it be constant voltage? or just voltage limiting? I need constant current..but I don't know what would happen running an LED with a constant current and voltage supply. Initially I was thinkingMy understanding of electronics is probably limited comparativly to most of you but I've come a long way. Multisim has made learning a fun and enjoyable experience.

    So Ideally I just need a constant current supply that rectifies and smooths AC. As long as the amperage is limited thats all thats required.
    I don't understand electrically why when using web bench when I raise the amperage the efficiency goes up a lot. Would it be better to feed the buck into an lm317 to regulate current but keep the input voltage with in 3v of the expected output voltage? I'm just trying to figure this stuff out. bare with me.
     
    Last edited: May 19, 2010
  10. davidbear

    davidbear New Member

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  11. dknguyen

    dknguyen Well-Known Member

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    Most definately. Linear regulators work by burning off excess voltage as heat (whether it's to keep the voltage at a desired level, or whether to keep the current at a desired level. Less excess voltage means less heat.
     

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