Since isolation is not required, you can consider using a "series capacitor (about 680nF), rectifier, zener, fusible resistor" type of power supply. These have been discussed in this forum over and over again. There are also simple circuits with a series switch that only conducts near line zero crossings, with enough storage capacitance to keep voltage up for the 10ms in between. Among many switchmode chips, look at Power Integrations TinySwitch and LinkSwitch. Paul Mathews
Cor Jansen wrote:
Hello All,
I need to power a smal circuit (about 3 Volts, 30mA). This circuit need to be powered from the powerline (220VAC). The circuit need not to be isolated from the powerline.
I searched the internet for a simple solution for this, but could not find it. I hope there are special chips that do just this.
Does anybody has an idea?
Any help is welcome.
Thanks and greetings,
Cor Jansen.
I'll throw in a word for Power Integrations too. Super support, excellent app notes, free samples and a neat CAD program of their own that'll do 90% of the work for you ( esp with regard to the magnetics ) .
Graham
I would use something other than a switching circuit or anything involving a special IC for this.
If a couple of watts of losses can be tolerated, the usual solution is to get a DC "wall wart" and add an adjustable voltage regulator set to 3 volts.
If you need less loss and/or less weight/less bulk, then:
Put a .47 microfarad capacitor with a suitable actual AC voltage rating in series with one of the AC legs of a bridge rectifier. Put a resistor across that capacitor to discharge it to a safe voltage within .2 second after unplugging whatever this is - which would be 120K, bad case 100K ohms, power rating 1 watt, actual dissipation around half a watt.
Put a zener diode, LED, or some diode combination across the output of the bridge rectifier that sunt regulates the voltage to the desired voltage. Put a big filter capacitor across this. For a regulated voltage, put a filter capacitor and a zener diode of voltage 6-12 volts or so across the output of the bridge rectifier, and add a regulator to regulate down to 3 volts.
Should the load be an LED requiring 30 mA, then put the LED across the output of the bridge rectifier, and maybe parallel it with a big filter capacitor of at least 200 microfarads. Use only .33 microfarads for the AC capacitor to get LED current around 25-26 mA at 60 Hz, 21-22 mA at 50 Hz. The resistor to put in parallel with the capacitor for safety can then have its value increased to 150 K-ohms, and power dissipation is then reduced to a little under .4 watt (I would still use a 1 watt resistor for good reliability). Use .47 microfarad if you need the current to slightly exceed 30 mA at 50 Hz.
- Don Klipstein ( snipped-for-privacy@misty.com)
Look at this.
In article , Jeff Thon wrote:
Requires a bridge rectifier fed by a current-limiting capacitor that must withstand the line voltage. This capacitor is shown as .82 uF for
220V at 60 Hz, 1 uF for 220V at 50 Hz, and 1.5 uF for 120V 60 Hz.This appears to me to be an improved regulation version of what I suggested in my earlier posting. If requirement of regulation is not great, you can omit this IC. However, it is a good idea to see the above-mentioned datasheet for any whatever guidance for usage of current-limiting capacitors in series with one of the AC leads of a bridge rectifier in order to get low voltage DC at low current with less loss and/or less size and/or less weight than is achievable in schemes using a "wall wart".
Also shown, which I forgot about in my earlier post about a bridge rectifier with a current-limiting capacitor in series with one of the AC leads of the bridge rectifier: Add a resistor in series with the current-limiting capacitor to tame peak currents that can result from connecting the line when the instantaneous line voltage is high. Maxim recommends in the above datasheet 47 ohms with 1/2 watt rating for 120V, 100 ohms with 1 watt power rating for 240V. The resistor needs to survive an inrush pulse that suddenly charges the current limiting capacitor to a voltage almost 1.4 times the line voltage. The value of this resistor needs to be high enough to limit peak inrush current to a reasonable level, and I think around 10-15 amps (ballpark for a cold 60-100 watt incandescent lightbulb) should be OK. If you go by my earlier post and use a bridge rectifier fed through a current-limiting capacitor and shunt regulation (or directly feeding an LED), then this resistor can have a lower value to reduce losses. I would suggest 10-15 ohms for 120V, maybe around 22 ohms for 220-240V. As for type - I would use wirewound (a cheap and convenient form of these is the "sandstone" rectangular type rated 5 watts) in order to have good safety margin in ability to withstand a current surge resulting from applying power during a voltage peak. However, carbon composition types of lower wattage, maybe as low as 1/2 watt, should be OK. What I am most afraid of for this resistor is carbon film or other film type that has less ability to handle brief severe overwattage.
- Don Klipstein ( snipped-for-privacy@misty.com)
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