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Safely box-in transformer so that I can play with it without risking shock

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king.oslo

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Hello there,

I am building a power supply.

I would like to breadboard-build the circuit before I get a PCB manufactured. I don't have a power-supply at present, so I have no way of supplying the 30V 16A required apart from if I could safely use the transformer to power the breadboard-circuit.

I have no experience with mains current. I only have a good head, and common sense.

I reckon perhaps I could box the transformer into plastic whilst playing with it on the desk. I could drill one hole for input 230VAC and one fused (16A) output 30VAC.

Which things should I pay attention to so that I do not risk getting shocked?

Also, I have been told it is safe to work with sub 50VDC potential, how about AC? Is it correct to assume that the 30VAC output cannot shock me? I have been told that AC is more dangerous than DC.

What about cooling of the transformer? Does it get hot? Most of the time it will deliver more than perhaps 1A, apart for the brief moments I test the 16A capability.

Thanks.

Kind regards,
Marius
 
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30V AC or DC is pretty safe. I would keep my hands dry!
The transformer will get warm with no load or 1A load. For 16A you will need good airflow. 16A for a minute will be fine.
Use a fuse. Under full load you will pull about 2A form the power line. Use 3A fuse. Maybe 4A. You could use 1A at first knowing it will not hold under full load but it will open sooner if you wired things wrong.
 
Thanks.

My mains voltage is 230V. That is why I would like to box in the transformer. Is this sufficient to prevent me getting shocked if I have to move it?

At 0VA - 30VA output, as you say, the transformer will get warm, but will the box melt, or will it stay at comfortable temperatures?
 
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The box will be fine of you keep the current down. Transformers are often in plastic poxes. If you use the transformer hard you may want to drill air holes in the bottom of the box around the transformer and another set on the top or the side, neat the top.
 
People want digital statements like "you're safe if the voltage is less than X volts". Unfortunately, the real world doesn't work that way. You can quote industrial or government standards that state that a particular voltage level is safe, but I can find cases where people were injured with lower voltage levels. My canonical example is a person working on the 12 volt system of a car. Everyone thinks this is safe -- until I give them the example of a relative who was working on a car and got his metal ring caught between a + battery terminal and ground -- and got a hell of a burn.

So the evaluation of risk also involves knowing how much energy the circuit can deliver, regardless of the voltage.

I like ronsimpson's statement: "30V AC or DC is pretty safe. I would keep my hands dry!" He's right on -- but you could still be injured if e.g. a sharp edge of a conductor penetrated the skin and greatly reduced the resistance. Many people poo-poo these corner situations -- except perhaps the person who is actually injured in such a case. In my case, I work around my DC power supplies with pretty casual regard up to around 20 volts; my little "inner voice" starts talking in the 30-40 volt range, and I'm pretty cautious at the 40-50 volt level. Above that, I take the same precautions I use when working on line-powered stuff or higher voltages. The main technique is to connect test equipment with the power off, stand back, and turn the power on.

Another thought: even at "low" voltages (say, less than 40-50 volts), you can get a shock. A shock may not harm you directly, but your instinctive reflex action can cause you to e.g. jerk back. A friend once told me of how a friend of his got a pretty nasty cut requiring a bunch of stitches from a sharp piece of sheet metal in such a situation.

Sorry if that sounds like I'm a safety Nazi -- I'm not. But you also don't reach geezerhood like me if you don't pay at least some attention to the hazards... ;)
 
Nah you don't sound like a safety nazi. You sound cautious. I am cautious too, that is why I made this thread! So thanks you and Ron Simpson! :)

Are there any specific rating my plastic box should have, or can I use any old plastic box in my local electronics store?

Thanks!M
 
squishy,

Everyone thinks this is safe -- until I give them the example of a relative who was working on a car and got his metal ring caught between a + battery terminal and ground -- and got a hell of a burn.

Yes, and many years ago I heard of a guy who had to have his hand amputated because he was wearing a metal watch band when working with a car battery. And that was back in the days of 6 volt car batteries. As everyone should know, a car battery is really a high energy source.

And don't forget that sometimes, although rarely, transformers short out between the secondary and primary, thereby exposing folks to line power.

Ratch
 
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Hi king.oslo,

you might call me a nazi, but I don't care. I'm German and I was born in the middle of WWII in East Prussia and had to flee from the Russians. (a 1 year old baby can neither hold a gun nor fire it)

There is something I'm missing in the entire thread:

How the hell will you work with currents of 20A using a breadboard?

Breadboards are made for low currents - I guess about 2A maximum to be on the safe side and not burn off any contact rows within the board.

If you intend to avoid Indian (magic) smoke signals you better use wires of sufficient cross section to connect the high current parts.

Using the right transformer for your project and connect protective earth (PE) to the iron core you won't get into any trouble. Make sure that PE has the longest wire to be the last being ripped off when applying mechanical stress to the cable.

Boncuk
 
Hello Boncuk! :)

I am not calling anyone a nazi.

No high current will go to the breadboard.

In regards to your advice about PE, what will this protect against? Is this to protect against shorting between the windings or something else? And is PE the grounding cable that is yellow and green in Europe?

If I ground the chassis to PE, and transformer core to PE, will I not get a shock from the case if the windings fail? In essence, they will be in parallel?

Thanks! :) M
 
Maybe the O.P. should use an actual, old-fashioned breadboard--that is, a slab of wood with screws or nails for connectors. After all, that's where the term came from.

I agree; a typical electronics solderless breadboard has nowhere near the current capacity, if the O.P. is actually going to be drawing 16A @ 30V (that's almost 500 watts).

What exactly are you doing that draws that much current, if you don't mind my asking?
 
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So you're just using a 16A transformer because that's what you have? No problemo if so.

Just make sure you don't short anything across the power rails on your breadboard, or you'll probably fry it. Otherwise, you should be fine.

I wouldn't worry about transformer windings failing (ASS-U-MING you don't short-circuit it). Think of all the electric/electronic devices we use every day that depend on transformer winding isolation. It is possible for them to fail, but it's such a remote possibility that I wouldn't lose any sleep over it.
 
I am using a buck controller to switch 2N3772 transistors. I will have current limiting and a LED display.
 
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Buy an appropriately sized isolation transformer:

https://en.wikipedia.org/wiki/Isolation_transformer

I took a power electronics lab course, once upon a time. The first step required for every session was to connect the line power source to the isolation transformer. This procedure probably saved several lives or injuries over a period of time. ... More expense for sure, but there should be no doubt that this item is necessary for any sort of experimentation.

... appropriate transformer size is the maximum anticipated Volts*Amps product ... plus a safety factor
 
I would tend to put something like this:-

https://uk.farnell.com/1688794

in series with the primary of the transformer. That will limit the inrush current, which can be surprisingly large, and take out fuses that are rated at a current much larger than the full load current.

I know that 500 W is on the small side for using inrush limiting, but it can't hurt.
 
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