Forums

Transformerless power supply

Started by mkr5000 January 25, 2013
I have a simple circuit on a breadboard that has been doing fine 24/7 for months which is --

120vac, hot into a 200k 1/2 watt that feeds a dropping cap (.47uf, 400v) that then feeds a bridge rectifier. The output of the bridge has a zener and a filter cap which supplies my DC.

I breadboarded it because I really never used a transformerless design in a project and just wanted to see how reliable it was. (so far -- very reliable).

My question is about the 100 ohm resistor (or around there) that is used in the neutral leg of the AC feeding the bridge that you often see in this circuit.

I'm not using one, just tying the neutral directly to the bridge.

So what is the purpose of this resistor? A safety precuation? Just to provide "some" isolation from ground (neutral 120vac?) Or maybe it acts as a cheap fuse?

Of course, you'd be stupid to not have the ground isolated from the chassis or anything that you could touch (like they did in the 50's and 60's).

I'll have to say that it's been very reliable on the breadboard anyway -- but I'm only drawing 25ma or so with my load.



On Jan 25, 1:51=A0pm, mkr5000 <miker...@gmail.com> wrote:
> I have a simple circuit on a breadboard that has been doing fine 24/7 for=
months which is --
> > 120vac, hot into a 200k 1/2 watt that feeds a dropping cap (.47uf, 400v) =
that then feeds a bridge rectifier. The output of the bridge has a zener an= d a filter cap which supplies my DC.
> > I breadboarded it because I really never used a transformerless design in=
a project and just wanted to see how reliable it was. (so far -- very reli= able).
> > My question is about the 100 ohm resistor (or around there) that is used =
in the neutral leg of the AC feeding the bridge that you often see in this = circuit.
> > I'm not using one, just tying the neutral directly to the bridge. > > So what is the purpose of this resistor? A safety precuation? Just to pro=
vide "some" isolation from ground (neutral 120vac?) Or maybe it acts as a c= heap fuse?
> > Of course, you'd be stupid to not have the ground isolated from the chass=
is or anything that you could touch (like they did in the 50's and 60's).
> > I'll have to say that it's been very reliable on the breadboard anyway --=
but I'm only drawing 25ma or so with my load. I don't know about the 100 ohms. But is this a one of? You're using the AC 'neutral' as your circuit ground? (That's a bit scary if so.) I'm also having a hard time adding up your numbers... the 200k ohm resistor is in series with the cap and then feeds the bridge? (25mA * 200k ohm =3D ~5kV?) George H.
On Fri, 25 Jan 2013 10:51:35 -0800 (PST), mkr5000 <mikerbgr@gmail.com>
wrote:

>I have a simple circuit on a breadboard that has been doing fine 24/7 for months which is -- > >120vac, hot into a 200k 1/2 watt that feeds a dropping cap (.47uf, 400v) that then feeds a bridge rectifier. The output of the bridge has a zener and a filter cap which supplies my DC. > >I breadboarded it because I really never used a transformerless design in a project and just wanted to see how reliable it was. (so far -- very reliable). > >My question is about the 100 ohm resistor (or around there) that is used in the neutral leg of the AC feeding the bridge that you often see in this circuit. > >I'm not using one, just tying the neutral directly to the bridge. > >So what is the purpose of this resistor? A safety precuation? Just to provide "some" isolation from ground (neutral 120vac?) Or maybe it acts as a cheap fuse? > >Of course, you'd be stupid to not have the ground isolated from the chassis or anything that you could touch (like they did in the 50's and 60's). > >I'll have to say that it's been very reliable on the breadboard anyway -- but I'm only drawing 25ma or so with my load. > >
The "100 Ohm resistor" is usually there just to limit the amplitude of the surge current. Post your circuit in ASCII or on A.B.S.E or by a URL link so we can properly help. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
that's what I thought Jim -- won't stop a surge but at least help.

don't trust my numbers, actually can't remember the R value off hand -- even the 25ma is an exaggeration. 

really just wanted to know the purpose of the resistor in the neutral side.

actually, it's been very reliable -- like I say, may end up using it in projects where i don't have room for a transformer.

using (4) 1000v rectifiers, and plenty of headroom with the resistor power ratings. 
On Fri, 25 Jan 2013 14:26:40 -0800 (PST), mkr5000 <mikerbgr@gmail.com>
wrote:

>that's what I thought Jim -- won't stop a surge but at least help. > >don't trust my numbers, actually can't remember the R value off hand -- even the 25ma is an exaggeration. > >really just wanted to know the purpose of the resistor in the neutral side. > >actually, it's been very reliable -- like I say, may end up using it in projects where i don't have room for a transformer. > >using (4) 1000v rectifiers, and plenty of headroom with the resistor power ratings.
I've done a number of "capacitive dropper" supplies, mostly for appliance applications, where the user has no exposure to the "hot" circuits. They work just fine. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Fri, 25 Jan 2013 10:51:35 -0800 (PST), mkr5000 <mikerbgr@gmail.com>
wrote:

>I have a simple circuit on a breadboard that has been doing fine 24/7 for months which is -- > >120vac, hot into a 200k 1/2 watt that feeds a dropping cap (.47uf, 400v) that then feeds a bridge rectifier. The output of the bridge has a zener and a filter cap which supplies my DC.
--- What's the value of the filter cap, the Zener voltage, the load resistance, and the output voltage? ---
>I breadboarded it because I really never used a transformerless design in a project and just wanted to see how reliable it was. (so far -- very reliable). > >My question is about the 100 ohm resistor (or around there) that is used in the neutral leg of the AC feeding the bridge that you often see in this circuit.
--- I've never seen it often and, in fact, I've never seen it before. Where did your information come from? ---
>I'm not using one, just tying the neutral directly to the bridge. > >So what is the purpose of this resistor? A safety precuation? Just to provide "some" isolation from ground (neutral 120vac?) Or maybe it acts as a cheap fuse?
--- Or, perhaps, a troll. ---
>Of course, you'd be stupid to not have the ground isolated from the chassis or anything that you could touch (like they did in the 50's and 60's).
--- 100 ohms is hardly "isolation" when the supply's input impedance is on the order of 200k
> >I'll have to say that it's been very reliable on the breadboard anyway -- but I'm only drawing 25ma or so with my load.
--- 120VRMS into 200k can only source 600 microamps into a short, so there's something wrong with your numbers. -- JF
On Fri, 25 Jan 2013 10:51:35 -0800 (PST), mkr5000 <mikerbgr@gmail.com>
wrote:

>I have a simple circuit on a breadboard that has been doing fine 24/7 for months which is -- > >120vac, hot into a 200k 1/2 watt that feeds a dropping cap (.47uf, 400v) that then feeds a bridge rectifier. The output of the bridge has a zener and a filter cap which supplies my DC. > >I breadboarded it because I really never used a transformerless design in a project and just wanted to see how reliable it was. (so far -- very reliable). > >My question is about the 100 ohm resistor (or around there) that is used in the neutral leg of the AC feeding the bridge that you often see in this circuit. > >I'm not using one, just tying the neutral directly to the bridge. > >So what is the purpose of this resistor? A safety precuation? Just to provide "some" isolation from ground (neutral 120vac?) Or maybe it acts as a cheap fuse? > >Of course, you'd be stupid to not have the ground isolated from the chassis or anything that you could touch (like they did in the 50's and 60's).
I had a Hallicrafters S38 SW receiver that had one side of the (unpolarized!) AC line soldered to the chassis. The chassis was isolated from the exterior metal case by a few rubber grommets that had worn through, so the case was hot. I checked it with a neon bulb (hold one lead, touch the case with the other) and reversed the plug if it lit. I wonder how many people were killed by stuff like that, like a hot-chassis radio falling into a bathtub or something. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
mkr5000 wrote:

> I have a simple circuit on a breadboard that has been doing fine 24/7 for > months which is -- > > 120vac, hot into a 200k 1/2 watt that feeds a dropping cap (.47uf, 400v) > that then feeds a bridge rectifier. The output of the bridge has a zener > and a filter cap which supplies my DC. > > I breadboarded it because I really never used a transformerless design in > a project and just wanted to see how reliable it was. (so far -- very > reliable). > > My question is about the 100 ohm resistor (or around there) that is used > in the neutral leg of the AC feeding the bridge that you often see in this > circuit. > > I'm not using one, just tying the neutral directly to the bridge.
Is it possible to reverse the polarity of the input? That is, to feed the hot directly into the bridge and have the cap and 200K resistor connected to the neutral? If so, I'd guess that the 100 ohm resistor is a current limiter, should the polarity be reversed and some point in the circuit inadvertently becomes grounded. Otherwise, 100 ohms is effectively in series with 200K, so its effect on the normal operation of the circuit would not be noticeable. -- Paul Hovnanian mailto:Paul@Hovnanian.com ------------------------------------------------------------------ All wiyht. Rho sritched mg kegtops awound?
"mkr5000" <

>I have a simple circuit on a breadboard that has been doing fine 24/7 for >months which is -- > > 120vac, hot into a 200k 1/2 watt that feeds a dropping cap (.47uf, 400v) > that then feeds a bridge rectifier. The output of the bridge has a zener > and a filter cap which supplies my DC.
** What is the 0.47 cap for when the 200k does all the dropping ?? I think you have stuffed up the schem. The 200k is meant to go across the 0.47 uF ( so it discharges when disconnected from the AC supply ) - the cap alone will pass about 20mA AC to the bridge and zener. The 100 ohm is simply a surge limiter and can be in series with the A or N conductors. ... Phil
On a sunny day (Fri, 25 Jan 2013 18:31:37 -0800) it happened "Paul Hovnanian
P.E." <paul@hovnanian.com> wrote in
<M8-dnSxfFc5rop7MnZ2dnUVZ_r2dnZ2d@giganews.com>:

>mkr5000 wrote: > >> I have a simple circuit on a breadboard that has been doing fine 24/7 for >> months which is -- >> >> 120vac, hot into a 200k 1/2 watt that feeds a dropping cap (.47uf, 400v) >> that then feeds a bridge rectifier. The output of the bridge has a zener >> and a filter cap which supplies my DC. >> >> I breadboarded it because I really never used a transformerless design in >> a project and just wanted to see how reliable it was. (so far -- very >> reliable). >> >> My question is about the 100 ohm resistor (or around there) that is used >> in the neutral leg of the AC feeding the bridge that you often see in this >> circuit. >> >> I'm not using one, just tying the neutral directly to the bridge. > >Is it possible to reverse the polarity of the input? That is, to feed the >hot directly into the bridge and have the cap and 200K resistor connected >to the neutral? > >If so, I'd guess that the 100 ohm resistor is a current limiter, should the >polarity be reversed and some point in the circuit inadvertently becomes >grounded. Otherwise, 100 ohms is effectively in series with 200K, so its >effect on the normal operation of the circuit would not be noticeable.
There was a discussion about that many years ago, and IIRC it involved switching the device on when the mains period is at maximum. Steep du/dt, makes the cap a virtual short. So you need peak current limiting, and that is why the series resistor. I have used such a circuit myself a couple of times. OK if you are 100% double insulated in EU.