amplifier with no transformer

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I have contemplated such a design for many years ,But every time I have tried to discuss this it gets thrown in my face as how dangerous it is ,But back in the old days lots of stuff ran directly off of the line and there are many new designs in solid state format these days that are off line supplies that are not isolated or use power transformers.

jer
 
geraldfryjr said:
But back in the old days lots of stuff ran directly off of the line and there are many new designs in solid state format these days that are off line supplies that are not isolated or use power transformers.
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There were "hot box" radios. Now days you must have two layers of insulation to have a hot box. Your volume knob will need two layers of plastic. A input transformer might work but but they were never designed for power line isolation. The output can not be touched by a person! If the speaker was in the same box....OK....but you can not have a connector a person can touch.

As to "solid state format these days that are off line supplies that are not isolated" show me some! I deal with UL, VDE, CSA, etc.
 
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gary:

The coil (the L) in the speaker was part of the LC filter of the power supply. I have one of those large speakers, about 12" dia in a 1950's AM/SW radio at home.
 
Here you go.

**broken link removed**

I am not trying to start any flames here.
I'm just saying that it can be done providing one does it while abiding within the rules of safety is all.

If you have ever messed around amplifiers in the the 1kw or more range it would be common sense not to touch the output leads while it is operating.
I did once by accident and I got a pretty good jolt.
Fuse's are a great thing and are a must in all stages of the power flow.

jer
 
geraldfryjr said:
Here you go.
**broken link removed**
Click to expand...

I produced a night light that came directly off the power line. Some thing like what you pointed to. We had to double insulate the cover. The clear lens for the LEDs was a great problem to design. We had to pass the drop ball test (bowling ball drop on product). There could not be any hole in the case. There could not be a "input or output jack" like an audio amplifier. It is very hard to design a non isolated product. Even a on/off switch needs to be double insulated. I use a plastic rod to reach the switch, in other designs. It took us a year longer than it should to get through UL.

The safety of opening up the case....is not the problem. The safety issue is just touching the output jack. You need to prove to an UL engineer that the jack is at ground (green wire ground) even under a worst case fault. (if not at ground with in 42 volts)
 
Yes,I agree it is not something that I would advise to do especialy for a commercial product.

To my analogy has to why you should use a transformer especialy in high power equipment is that it does offer a small form of current limiting factor.

If something happened and should an arc start from the power supply line there will be 10's of 1,000's of amps available and nothing is going to stop that arc once it gets going and lots of damage will occur before any thought of a safety device or the power company could put a stop to it.

With a transformer the only current that is available is what the core will allow untill the windings open up due to overload at which point you may (might) trip the 15 or 20 amp breaker or fuse.

Jer
 
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I don't think current limit is the problem. It is isolation.
I have seen (in the US) where the hot wire and the neutral wire are reversed. This puts 110VAC on the case of the amplifier. (on the "ground" of the input jack) 110vac on the speaker wires.

Years ago one person in the lab brought in a old radio to listen to. The rest of us did not want to hear that so he drilled a hole in the box and added a head phone jack. For a week he had headphones on. Then he bumped the headphones into the ground of the metal bench and popped the breaker. All week he had 110VAC connected to his head. This is what we are talking about. Killing someone with "ground" that is really 110V or 220Vac.
 
Yeowie!!! That is why they put polarized plugs on things now days.

Unfortunately transformers are costly,But they are what makes things a bit safer.

jer
 
Using a switching supply is what brings down the size and the cost of a power transformer.
And it doesn't have to be PWM in order to make it work although it is not to hard to do for regulation purposes.

jer
 
In my hand is a 100-240Vac input switcher 120 watts. 2" x 4" x 1" Runs at 250khz. It will take a very large audio amp to use up 120 watts. It will get you off the power line with current limit and provide 2000 volts of isolation.
 

Hi Ron

Did you by any chance use a Mains rated cap of the correct value to limit the current through your circuit. Like an X2 maybe??. Just curious.

Thanks

TV Tech
 
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If you place the cap across the power line they use a X2 cap. Often there is a fuse-resistor with the cap (in this case of using a cap to limit current). This way the cap will not see large currents form the power line. It is my belief that high current caps are not needed. Most of the power line voltage is across the cap. 10 to 20 ohms of resistor changes the current on the cap (when there is high frequency noise on the line)

UL may say that a 2X cap is needed but I counter with the idea that current is around 20mA.
 
ronsimpson said:
There were "hot box" radios.
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(Meaning radios whose power supplies--tube filaments and B+--were connected directly to the "hot" side of the power line, with the chassis connected directly to the other side.)

I remember those well; we had a Bakelite-cased Zenith (AM/FM) like that well into the 1960s. I was always amazed at how unsafe they were once I learned a little about electricity.

But ironically, there was a much safer variation I just learned about, I think used primarily in the UK (could be wrong about that), where there was only 1 power line connector (a single wire) that went to the "hot" side. The other side was connected to a convenient ground (e.g., water pipe). So if the power was plugged in incorrectly (connected to neutral instead of hot), nothing would happen.

Has anyone actually seen one of these antiques?
 
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I have not seen.......Sounds good. (better)
Let me get this right. You ran the power line through the radio to grandmas toilet? I got in trouble for that stuff.
 
ronsimpson said:
I have not seen.......Sounds good. (better)
Let me get this right. You ran the power line through the radio to grandmas toilet? I got in trouble for that stuff.
Click to expand...

Right. And you put the radio on a table next to the bathtub, so you can turn it on while taking a bath ...

(Yikes!)
 
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