Electronics > Beginners
Transformerless power supplies circuit, cheatsheet and questions
mikeselectricstuff:
If going for higher powers, check out the Power integrations Linkswitch series devices - costs may be comparable to larger capacitive dropppers and certainly smaller
waste:
--- Quote from: Zero999 on February 06, 2019, 01:54:42 pm ---A transformerless power supply is a current source if the LED forward voltage is low, compared to the mains.
My advice is to avoid them where at all possible. Use a switched mode power supply instead. The only reason to use a transformerless PSU is for cost, which perhaps makes sense at low power levels, but at more than about 50mA, a switched mode becomes more attractive, because decent, large capacitors aren't cheap.
--- End quote ---
I agree with you that for higher wattage, there are more attractive alternatives, but it's not only the cost that makes them attractive compared to the alternatives.
If you go for a 5V switching power supply then you will have to either use a transistor to current limit your leds, or something like the AMC7235 chip, which usually comes in 350mA increments. Don't get me wrong I love how cheap and simple is the AMC7235, but it's not exactly efficient in 5V (it's mostly designed for li-ion batteries) and then you are constrained to 350mA which for many applications where you want ambient light, is too much.
Then you can go for a switching current limited board like the chinese ones built around the BP2836 and it's many siblings that go around. They are cheap and give okeish stable output, but even the smallest ones are quite powerfull for ambient light and they are not exactly reliable .
Additionally both solutions are quite bulky compared to a capacitive dropper and not so versatile. With the capacitive dropper you just add LED's in series and you are done.
To sum it up, I agree that for higher wattage/current needs, other solutions are better and more suitable, but I came to love those capacitive droppers when I started using them 2 years ago (before I was kind of timid going around the 220V), that's why I want to make sure they will stay as reliable as my first designs that go on and on and on :)
--- Quote from: not1xor1 on February 06, 2019, 01:19:48 pm ---The datasheet specifies the maximum working voltage.
With higher voltage there is loss of insulation so you risk to burn all your leds.
Probably that is affected by environmental conditions (temperature/pressure/humidity) too.
--- End quote ---
So using a 1/2watt or a 1watt resistor makes sense as it can take more abuse from higher voltages, the issue can be with the 1/4 1MΩ resistor that discharges the capacitor. Did I get it correctly?
StillTrying:
I doubt applying some 320V square waves is a very accurate way to simulate a bouncing or arcing switch, but I've simulated it that way anyway, :) using the values as in the 1st post.
Green and Grey is with the 33u smoothing cap disconnected, Purple and Yellow is with it connected.
At least in this simulation a way to protect the LEDs from switch-on current spikes Yellow, is to double the 33u value to 68u Blue. YMMV.
Benta:
"So using a 1/2watt or a 1watt resistor makes sense as it can take more abuse from higher voltages, the issue can be with the 1/4 1MΩ resistor that discharges the capacitor. Did I get it correctly?"
No. The correct way is reading the resistor data sheet. That's responsible design methodology.
Example: the Vishay VR37 high voltage resistors I mentioned previously are metal glaze high voltage resistors that will withstand pulse voltages up to 3.5 kV. Just to demonstrate: the tolerance colour band is yellow instead of gold to achieve this.
Standard metal film resistors will accept 300 V peak, 75 V continuous voltage. Not what you want.
EDIT: the resistors I use are actually VR25, apologies.
not1xor1:
--- Quote from: Zero999 on February 06, 2019, 01:54:42 pm ---A transformerless power supply is a current source if the LED forward voltage is low, compared to the mains.
My advice is to avoid them where at all possible. Use a switched mode power supply instead. The only reason to use a transformerless PSU is for cost, which perhaps makes sense at low power levels, but at more than about 50mA, a switched mode becomes more attractive, because decent, large capacitors aren't cheap.
--- End quote ---
a 1µF 275VAC X2 capacitor is around 25-30 euro cents (a few months ago I bought 10 for 2.51€ from TME)
the main drawback is that unless you use some active circuit (e.g. a high voltage mosfet current limit) the surge current limit resistance resistor would add to the overall power loss and greatly affect efficiency
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