Powering LEDs from 240 VAC

• Powering LEDs from 240 VAC
  Posted by calzap on 21 Jun, 2021 18:03
• This topic has been covered before.  For example, https://www.eevblog.com/forum/beginners/led-indicator-for-240vac/.  My intended solution is below.
  
  I need to power a few 5 mm LEDs from 240 VAC.  They are pretty standard: 20-30 mA, 2-3.5 V drop and will be operated independently.  Some are self-flashing.  I like them to be bright.   There’s the series-resistor-with-reverse-protection-diode approach.  But the required resistors will be consuming 80+ times the power of the LED and would have to be sized accordingly too. I don’t like to run resistors at much more than ½ rated wattage, so we’re looking at 5-10 watt resistors.  This approach is definitely out.
  
  There is the capacitor approach.  A capacitor (preferably class-Y safety) is needed along with reverse protection diode or full-wave bridge, surge protection resistor, and bleed resistor.  There are scary implementations on the net for the capacitor approach … capacitor isn’t safety rated, capacitor is rated less than peak voltage, no bleed resistor, no fuse, etc.   Isolation?  Forget it.
  
  Then there are mini AC to DC PSUs that put out 5 volts at 200-600 mA.  Current limiting resistors will still be needed but won’t use much power.  I favor this approach and want modules that are enclosed, have pins, have isolated outputs, surge protection, and meet UL specs.  Candidates are Mornsun LD03-23B05R2 (https://www.digikey.com/en/products/detail/mornsun-america-llc/LD03-23B05R2/13531047) and Zettler HP01S0500WI (https://www.digikey.com/en/products/detail/zettler-magnetics/HP01S0500WI/12093237).   Plan is to have one PSU per LED.  Having only one PSU for the group and a relay per LED is an option but seemingly more complex and expensive.  A fuse would be used to protect the group.
  
  The simplicity of installation, safety, and robustness of the mini PSU more than offsets any price advantage of a properly done capacitor solution, especially for my limited number of one- or two-off projects.
  
  Neon, argon and incandescent lamps are not options.
  
  Any suggestions?
  
  Mike in California
• #1 Reply
  Posted by ajb on 21 Jun, 2021 19:06
• Sounds like you're basically trying to make mains-voltage indicator lamps.  Are the existing commercial solutions for that problem not an option? 
  
  Are you intending to run the LEDs at their max current?  Modern LEDs are pretty efficient so that's rarely necessary unless you're multiplexing them (and need the higher on current to make up for <100% duty cycle).  If you can drop that from ~20mA to ~1mA they may be plenty bright enough and that cuts the dissipation needed by any current limiting resistor substantially. 
  
  The lack of isolation problem with the capacitor dropper supplies could be solved by designing the mechanical integration to not require isolation.  All of the other safety concerns still apply (like the bleed resistor and suitably rated parts) but if everything including the LED ends up sealed into a non-conductive tube or whatever then the isolation is unnecessary.
  
  Running the LEDs from an off-the-shelf AC-DC converter isn't an unreasonable solution if you do need isolation and want to minimize development cost, but one problem you may run into is delayed on/off, depending on how long it takes the converter to start up, and how long it takes any input/output capacitance to discharge via the load. 
• #2 Reply
  Posted by mariush on 21 Jun, 2021 19:48
• You could use a simple high voltage linear regulator like LR8 to output 12v (or more) at up to around 50 mA  (30mA max, but current limit is set somewhere at 50mA) and then put the resistors in series or parallel with a current limiting resistor behind it.
  
  You could reduce the input voltage by doing half wave rectification (a single diode) and use of a small enough capacitor so that the regulator always gets more than voltage it needs (output voltage + 12v) but not big enough to charge to 230v or whatever. Could be something like a 1uF 350-400v electrolytic or a few ceramic capacitors in series (for higher voltage rating)
  
  LR8 datasheet:  https://ww1.microchip.com/downloads/en/DeviceDoc/20005399B.pdf
  
  It's not isolated power though.
  
  There's digital isolators out there, but they're expensive and fairly inefficient... but I guess could be used if your power supply can't produce isolation or you can't fit a transformer inside the product... ex see adum5000 series : https://www.digikey.com/catalog/en/partgroup/adum5000-series/15568
  
• #3 Reply
  Posted by calzap on 22 Jun, 2021 14:50
• Thanks for the replies.
  
  First project will be a status indicator for a dual element electric water heater.  Three LEDs to show if power is present, and whether either element is getting power.  Will also have a digital thermometer.  The latter will require a bit of plumbing fun to install … threaded thermocouple protruding into the tank must be inserted at the hot water outlet.  It needs to be installed dropping down into the top of the tank, but not block flow.  Will use a T-adapter.
  
  Considering the simplicity of installation and features, and at US$5-6 each, I’ll try the mini PSUs.  I’d like some experience with these anyway.  If it were to be a production run, I might try something cheaper.
  
  Good point on timing.  Start-up time is less than a second, which is fine.  I don’t see anything in the specs regarding shut-down discharge time.  2-3 seconds to go dim is not a problem, but I’ll try it and see.
  
  Mike in California