Transformerless supply -- resistor?

The dropper may give you a jolt. That's not the problem.

The problem is connecting the DC output to the line voltage. That can kill you.

Reply to
Steve Wilson
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The app note is wrong. How does a parallel resistor R2 reduce EMI?

I think they meant the R1 resistor, if anything

Cheers

Klaus

Reply to
Klaus Kragelund

You need to know what you're doing if you're going to scope a live transformerless power supply (see below). One of the primary rules is: don't touch anything with your bare hands, especially not wet bare hands. But, you already know that.

Caveat emptor Inet websites. Some of your links refer to the "others" mentioned in my initial followup.

Thanks, but no thanks. Berkeley SPICE is my poison of choice. Besides, there's already a AN954 circuit on a breadboard packed away on a shelf somewhere in my lab. There's also a truly isolated AC supply available. Whenever the AN954 circuit piques my interest, it's far easier for me to just pull the breadboard down from the shelf and scope it. The open question (to me) of the function of the parallel resistor may just pique my interest enough to pull it down and poke around.

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

You need to know what you're doing if you're going to scope a live transformerless power supply (see below). One of the primary rules is: don't touch anything with your bare hands, especially not wet bare hands. But, you already know that.

Caveat emptor Inet websites. Some of your links refer to the "others" mentioned in my initial followup.

Thanks, but no thanks. Berkeley SPICE is my poison of choice. Besides, there's already a AN954 circuit on a breadboard packed away on a shelf somewhere in my lab. There's also a truly isolated AC supply available. Whenever the AN954 circuit piques my interest, it's far easier for me to just pull the breadboard down from the shelf and scope it. The open question (to me) of the function of the parallel resistor may just pique my interest enough to pull it down and poke around.

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

If you want to scope a CR PSU, run it connected to earth. In a lot of places this will require an iso transformer for practical or legal reasons. And don't overlook the obvious: the earth needs to connect to the output of the BR if it uses one.

Often easier to just substitude a traditional iso psu for testing.

NT

Reply to
tabbypurr

This is a fun circuit because it uses components in non-intuitive ways. Take the thermistor, for instance. SMPS commonly use it in series as a current surge arrestor. Yet, AN954 uses it in parallel. Its parallel configuration sure makes the thermistor look like a bleeder to me. YMMV. C2's yet another anomaly. SMPS normally use X1 capacitors in a parallel configuration. Yet, AN954 uses it in series as a dropper. Does R2 smooth out C2's ripple? Does that help suppress EMI?

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

During reverse recovery does diode D2 overshoot and ring? Does that create EMI? Does a parallel RC dampen that EMI?

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

The AN954 I remember has no thermistor - I think what you see is a voltage variable resistor aka varistor aka MOV. Overvoltage protection for the dielectric of the dropper cap. The surge current limitation is provided by the series resistor (typically 33 to 330 ohm).

piglet

Reply to
Piglet

No, I think you are over-thinking the circuit, app-note AN-954 describes the function of each part but their statement on page 10 about fig10 : "Adding R2 in parallel with C1 creates a filter that will attenuate EMI from traveling back onto the line." is just plain wrong for the series C dropper. My guess is goof in proof-reading - they show on the next page a resistive dropper version fig 11 with a suppression capacitor across the line which does help reduce EMI but even there the high value resistor across it is acting as bleeder not a filter. Maybe they copied that sentence into the earlier paragraph?

piglet

Reply to
Piglet

In your other followup you say "the AN954 I remember has no thermistor." You are correct. Figure 10's caption clearly says "A varistor, or MOV, provides transient protection." IIRC, some of the math shown on page 6 or thereabouts is wrong. So AN-954's reputation for factual statements isn't exactly stellar. It's probably best to stick with Figure 11 for the rest of this thread. The X1 capacitor across the line shown in Figure 11 is very familiar to me because that's how SMPSes tend to use X1 capacitors to suppress EMI. In Figure 11, isn't D2's reverse recovery ringing the primary source of EMI?

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

Rectifier reverse recovery snap EMI at line frequency can happen but is probably not the primary problem in this case. I think it more likely that the EMI concerns the AN writer was thinking about is that coming from the payload circuit making it past the bulk filter (C2) capacitor Esl and Esr.

piglet

Reply to
Piglet

Not a thermistor, just drawn that way.

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Reply to
Jasen Betts

That's an excellent question. "How does a parallel resistor R2 reduce EMI?" Figure 1 in US Patent 2012/0007567 A1, "Low Loss Discharge Circuits for EMI Filter Capacitors", shows a resistor in parallel with a capacitor and labels the combo as "prior art." Line 0009 of the patent says "FIG. 1 shows a schematic of a conventional EMI filter circuit. The patent acts like a parallel resistor and capacitor is common enough to be thought of as conventional. Does anyone know about this apparently common-as-dirt EMI filter?

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

Nevermind. US Patent 2012/0007567 A1 is talking about the common-as-dirt X capacitor in parallel with the line and neutral.

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

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