Hot!Transformer less Power Supply - Reliability

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2017/09/18 20:17:35 (permalink)
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Transformer less Power Supply - Reliability

Hi
 
I need to make a compact very reliable transformer less power supply for my infrared remote circuit.Please correct my attached drawing.
 
Thanks

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#1

45 Replies Related Threads

    karan123
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    Re: Transformer less Power Supply - Reliability 2017/09/18 20:31:47 (permalink)
    +3 (5)
    If it is student or academic level project.
    I suggest you to please use transformer based supply. Transformer less supply have no isolation from AC main. Thus Quite dangerous.

    Thanks
    #2
    Designer
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    Re: Transformer less Power Supply - Reliability 2017/09/18 20:36:53 (permalink)
    -3 (3)
    This is not what I expected from a technical forum.Please do not disturb my thread.
    #3
    qhb
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    Re: Transformer less Power Supply - Reliability 2017/09/18 20:59:08 (permalink)
    +3 (3)
    It is exactly the response you should expect on a professional forum if you give no indication what your own experience level is...
     
    #4
    karan123
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    Re: Transformer less Power Supply - Reliability 2017/09/18 21:36:21 (permalink)
    +1 (1)

    This is not what I expected from a technical forum.Please do not disturb my thread.

    Ok.. If you still want to make.. Below application note helps you.
     
    http://ww1.microchip.com/...en/AppNotes/00954A.pdf
     
    --
    Karan123
     
    #5
    PStechPaul
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    Re: Transformer less Power Supply - Reliability 2017/09/18 21:44:56 (permalink)
    +2 (2)
    I don't see any major problems. Here is an LTspice simulation of the circuit with 12V into 470 ohms:
     

     
    This works, but could be simplified:
     


     
    #6
    Nikolay_Po
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    Re: Transformer less Power Supply - Reliability 2017/09/18 23:08:43 (permalink)
    +1 (1)
    And even simpler. Remove D2. Move D1 in place of D2.
    And make sure your circuit will operate normally at +/-10% of mains nominal voltage which is possible and is valid.
    post edited by Nikolay_Po - 2017/09/18 23:13:55
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    Re: Transformer less Power Supply - Reliability 2017/09/18 23:51:33 (permalink)
    +1 (1)
    Excellent comments.Thank you very much.
     
    I redrawn the circuit with your comments.It looks likes the original microchip application notes circuit :O
     
    I have some doubts.
     
    1)Zenner diode with single diode circuit (last circuit) will give the same current as bridge version?
    2)I'm not sure with the 12V zenner,When it burns it will short circuit, Will it last longer? :(
    3)Is it ok to use a 78L05 in replace of a 5V zenner? I prefer 78L05 :)

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    PStechPaul
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    Re: Transformer less Power Supply - Reliability 2017/09/19 01:11:05 (permalink)
    +1 (1)
    A full wave bridge will give about twice the current into a load. Or, you can use a larger capacitor to increase current. Here is the FWB version:
     

     
    You might also look at one of the simple off-line buck regulator devices designed for LED lamps:
     
    http://www.microchip.com/wwwproducts/en/HV9921

     
    #9
    Nikolay_Po
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    Re: Transformer less Power Supply - Reliability 2017/09/19 01:36:24 (permalink)
    +1 (1)
    You can't use regulator without a Zener in front of it. Mighty enough dropper capacitor will increase regulator input voltage over the limits. Use as low Zener voltage as possible to decrease power dissipation at light load and high mains. About ((5V+5%+2.5V)+5%)+5%=8.5..10V nominal including 2.5V regulator drop, tolerances and filter capacitor ripple.
    #10
    PStechPaul
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    Re: Transformer less Power Supply - Reliability 2017/09/19 01:54:10 (permalink)
    +5 (5)
    You may also consider a fully isolated 5V 700mA switching supply for less than $2 and only 3 x 2 x 2 cm:
     
    https://www.banggood.com/5V-700mA-3_5W-AC-DC-Step-Down-Isolated-Switching-Power-Supply-Module-p-993723.html?rmmds=search
     


     
    #11
    CinziaG
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    Re: Transformer less Power Supply - Reliability 2017/09/19 02:21:01 (permalink)
    +1 (1)
    I agree with PStechPaul in all of his comments: I usually used his version 2 above (2 diodes, one zener) and one time also the bridge. They're usually ok, reliable... of course within the limited current available and taking care of safety.
     
    Then, nowadays switching supplies became so cheap and small that...

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    #12
    simong123
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    Re: Transformer less Power Supply - Reliability 2017/09/19 06:05:58 (permalink)
    +2 (2)
    In none of the abovee circuits is there a good, low rated, fuse on the Live 😱
    #13
    CinziaG
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    Re: Transformer less Power Supply - Reliability 2017/09/19 06:55:48 (permalink)
    0
    Or a 10ohm resistor Smile (like I've seen on some cheap switching charger out there)

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    Re: Transformer less Power Supply - Reliability 2017/09/19 18:24:04 (permalink)
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    Hi 
    I planned to use the below design.Even when short circuiting the zenner, output voltage will be stay at reasonable level.
    See the circuit
    https://drive.google.com/open?id=0B3RELHoBhPvpSDV6bU5WS3paNVk
     
    Regarding the fuse...!!
    Do I really need a fuse in this design?
    #15
    PStechPaul
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    Re: Transformer less Power Supply - Reliability 2017/09/19 22:29:07 (permalink)
    +2 (2)

     
    The zeners are only running at 50 mW or less, so should not be in much danger of failing. However, I had a design where the zener diode failed in the field, and my conclusion was that the supply must have been a PWM wave or having many high frequency spikes. Here is a simulation of that circuit:
     

    Here is my analysis:
     
    Voltage Relay S-B183 for PI-1600
    Failure analysis
    Rev 2 - 07/29/10 - PES
     
    The voltage relay failed with a shorted zener diode D1. This is a 1N4742A, which is rated 12 VDC and 1 watt.
     
    An LTspice simulation reveals that at 280 VAC input (400 V Peak), at 60 Hz, the power dissipation of the diode is 648 mW (with no load).
     
    The two series capacitors are 1.1 uF with reactance 2.41k at 60 Hz. The maximum current would be 280/2.41 = 116 mA.
     
    The limiting resistor R3, 15 ohms, 1 watt, dissipates 197 mW, but there is indication of overheating.
     
    With a 50 mA load, simulated with a 240 ohm resistor, the zener power dissipation is only 40 mW. There is 570 mW in the 240 ohm load. This should be about twice the maximum expected load.
     
    With a 480 ohm load, the zener dissipates 330 mW.
     
    The following apply to a 4800 ohm load, which should be worst case:
     
    If the input waveform is a 240 VAC (RMS) square wave, the zener dissipates 397 mW, but it experiences spikes of 30 amperes with a time constant of about 28 uSec.
     
    But the resistor has a current of 987 mA RMS, which means power dissipation of 14.7 watts. This is with a square wave of 50 nSec rise/fall time, which is unlikely.
     
    Rise time 100 uSec. Power in R1 = 4.07 W.
     
    With a more reasonable rise/fall time of 500 uSec, the resistor dissipation is only 944 mW at 250 mA, which is substantial but within the capacity of the resistor. Under this condition, the zener power is 382 mW, and current peaks are about 700 mA maximum.
     
    The conclusion is that the failure could be caused by very high frequency (fast rise/fall time) components on the AC line, such as possibly from an inverter or a generator with very bad brushes.
     
    If the input is connected to a higher frequency source, an overload would occur, but it would have to be in the order of 100 Hz.
     
    Another possibility is that one of the series capacitors might have exhibited sporadic shorting and healing, which would have applied about twice the normal current, or even more if both capacitors shorted. This would be more likely if the power source were noisy, as the voltage of a spike might not be divided evenly between the two capacitors.
     
    The only sure way to avoid this problem is to supply power to the VRLY with external 12 VDC, and provide an isolated voltage sense input, as implemented in the PI-4000B.
     
    Or an external dropping resistor could be used. R=2.4 kOhm at 116 mA or power of 32 watts.
     
    A small switching supply could be used to supply the 12 VDC power. If a DC current can be tolerated, a single diode to a capacitor could provide 400 VDC and then a MOSFET with a PWM signal could drop it down to 12 VDC.
     
    An extra large octal enclosure is available as Keystone #712 and is about $8 from Mouser. The 5V 5W power supply from Cincon, Mouser 418-CFM05S050should fit and costs about $17. It is 1.378" x 2.165".The enclosure is 1.75” x 2.38” x 2.5” high, and costs about $8.
     

     
    #16
    bosco
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    Re: Transformer less Power Supply - Reliability 2017/09/20 06:23:03 (permalink)
    +2 (2)
    Designer
    Regarding the fuse...!!
    Do I really need a fuse in this design?

    If something fails drastically, do you want the customer to have a smoke event or fire?
    #17
    Nikolay_Po
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    Re: Transformer less Power Supply - Reliability 2017/09/20 08:45:03 (permalink)
    +1 (1)
    In extremely cheap designs the fusible resistors are widely used as current limiting resistance. To ensure the arc will not go to other components the resistor may be covered by a thick thermal shrink sleeve. An about 0.5mm^2 power cord is limiting the short circuit current allowing the fusible resistor to extinguish the arc under the sleeve. Without of dedicated fuses.
    #18
    Gort2015
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    Re: Transformer less Power Supply - Reliability 2017/09/20 10:31:48 (permalink)
    0
    I've had an electric shock from the mains before and it is not a nice experience.
     
    Straight up my arm, felt like my fingers were glued together.  Took a second to re-act before I felt it.
     
    Take extra care with mains or you could die.

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    CinziaG
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    Re: Transformer less Power Supply - Reliability 2017/09/20 12:30:40 (permalink)
    0
    Actually it is grin to same extent: I usually work barefoot when upgrading my home automation and "often" there is mains, or some switching power supply up there...
     
    Of course, it's not nice to sell stuff like that.

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    #20
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