Transformerless supply -- resistor?

I have a transformerless supply that shows 220vac going through a dropper c apacitor (1.5uf) which feeds a bridge rectifier. There is also a 330k resis tor that is in parallel with the cap. Could someone please tell me, what is the purpose of that resistor? -- thanks

Reply to
mkr5000
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Surge current limiting... USUALLY.

Though 1.5uF seems high.

You ordinarily size the cap value to handle the max expected load, then add a surge-limiting resistor for the turn-on "blast". ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Draining the cap when disconnecting power source.

Reply to
Sjouke Burry

To stop any charge remaining on the capacitor for too long after it has been disconnected from the mains. Otherwise it could bite back sharply. -- Regards, Martin Brown

Reply to
Martin Brown

Oooops! I missed the "parallel" :-( ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

per capacitor (1.5uf) which feeds a bridge rectifier. There is also a 330k resistor that is in parallel with the cap. Could someone please tell me, wh at is the purpose of that resistor? -- thanks

but hopefully there is also a series R. If not the parts are surely getting abused.

NT

Reply to
tabbypurr

Yep. I've designed several transformer-less supplies for appliance (aka cheap-ass) applications. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

*Parallel* resistor. How's that going to reduce surge current?
Reply to
Clifford Heath

You missed my "oooops" post back some five hours ?>:-} ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Yes, sorry, it was further down the thread and I couldn't cancel.

Reply to
Clifford Heath

dropper capacitor (1.5uf) which feeds a bridge rectifier. There is also a 330k resistor that is in parallel with the cap. Could someone please tell me, what is the purpose of that resistor? -- thanks

It stops the user from jumping.

Else after unplugging it you might find a voltage on the plug pins with your finger.

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Reply to
Jasen Betts

It creates a filter that attenuates EMI from traveling back onto the line. See Figure 10 in Microchip's "AN954, Transformerless Power Suppies: Resistive and Capacitive."

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

DO NOT build transformerless power supplies that way. You will kill yourself.

The resistor in parallel with the cap is to discharge the cap when you pull the plug out of the socket. This prevents getting a shock if you touch the two prongs.

Reply to
Steve Wilson

You (and others) erroneously conflate the DC filter capacitor in this particular supply with its AC dropper capacitor. The DC filter capacitor holds a charge. The AC dropper capacitor does not hold a charge. The AC dropper only provides a reactance that opposes the change in voltage. A change in voltage that happens 60 times a second. It's never given enough time to charge up.

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

The dropper capacitor holds the difference between the line voltage and the dc ouput voltage. How else could it drop the line voltage? Why is it called a dropper capacitor? Put it in LTspice and find out.

The Microchip's AN954 that you recommended shows lethal designs for transformerless power supplies. Since you have no ability to see this, you have no business working with line voltage supplies. Until you learn some basic electronics, you have a good chance of killing someone.

Reply to
Steve Wilson

Let me get this straight. You feel that the AC dropper capacitor stores a lethal AC potential after the plug's pulled? And the lethal AC potential that's still stored on the AC dropper capacitor can kill you when you touch the prongs after you pull the plug? Is that your argument?

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

You are thinking about the steady state situation only.

For simplicity assume a capacitor is connected directly between Line and Neutral of an extension cord.

When connected into a socket, the capacitor voltage will vary between

-Vpeak and +Vpeak. Pull the plug at 0 or 180 degrees when the mains voltage is zero and the disconnected capacitor is at 0 V. However, if you disconnect the plug at 90 degrees when the mains voltage is at

+Vpeak, the capacitor voltage is +Vpeak (assuming no arcing in the socket during disconnect). If there is no leakage, the capacitor voltage remains for a long time and could potentially cause an electric shock when touching the extension cord plug.

The other question, can this be lethal, is a different matter. If the capacitor is big enough and the discharge current from one hand to an other is over 50 mA for several seconds, this can cause heart problems and locally also burns.

Even minor shocks can cause a surprise and a person drops something or falls from a ladder etc.

A 1 uF series capacitors are unlikely to cause much harm at 120 Vac, but big cos(phi) compensation capacitors for big motors are a different matter, especially at higher mains voltages. Thus some bleeder resistors are need to be connected in parallel with the capacitor to bleed of the charge, if the capacitor can be completely disconnected from any external circuit.

Reply to
upsidedown

Nobody said it is AC. The capacitor may hold a charge correspondig to anything from zero to peak AC voltage over the cap at disconnecting the supply.

As a ham, you should know about bleeders. This is one.

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-Tauno Voipio. OH2UG
Reply to
Tauno Voipio

Of course it holds a charge depending exactly where on the mains cycle it find itself disconnected with anything from zero to peak voltage.

On weedy 110v US mains it probably won't kill you but would certainly hurt. On UK 240v or 440v it could really give you a very nasty nip if it didn't have a bleed resistor.

It would without the RC time constant to discharge it. I think you would have to be very unlucky to get killed by one but it is considered to be very bad form to make something which can have a high voltage on its power plug *after* being disconnected from the mains.

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Regards, 
Martin Brown
Reply to
Martin Brown

Fair enough. Let's talk ham-to-ham and chew the rag. As others note, a DC potential, an artifact of the AC at the time the plug was pulled, remains on the dropper. You contend that the manufacturer is wrong about the reason they put the parallel resistor into their example circuit. The manufacturer says it's an EMI filter. But you know better and claim that it's actually a bleeder resister. (Because you never know when someone will pull the plug and touch the prongs.) A 1.5uF capacitor seems small fry (so to speak) to me. Yet you and others reckon that the little rascal potentially packs a lethal punch. 220Vrms is about 310Vpeak, which is the maximum voltage that can be left behind as an VDC artifact on the dropper if the plug is pulled at an extreme excursion of the AC cycle. 310VDC remains behind on the dropper to do its worst after the plug is pulled. "Volts jolt, but mils kill." So we need to ask ourselves, does the

310VDC left "as is" on that tiny terror (AKA Dropper the Killer) actually pack enough punch to the deliver the mils that kill? (BTW, allow me to thank everyone for the dog pile that they laid on me. It's terribly fun to talk this through, pontificate, and what not. It's also therapeutic for my other troubles in life. You make my day.) Enough of that. We're left with 310VDC on a 1.5uF capacitor. That's about 72mJ of energy. Hmmm. That seems a shade on the small side. Is it conceivable that Dropper the Killer's bark is a whole lot worse it's bite? Evan Mayerhoff, at High Voltage Connections, Inc. claims that it takes at least 50J or more to induce ventricular fibrillation. But Dropper the Killer only delivers 1/700 of the 50 bare minimum Joules that /might/ kill someone of faint heart. Mayerhoff covers all bases. Dropper the Terrible can still kill you if he jolts you enough to fall backwards and break your neck.

Note.

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Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

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