Dangers of DC power supplies?

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ea747
 
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Dangers of DC power supplies?

Post by ea747 »

What dangers are involved with DC power supplies? I'm trying not to hurt or kill myself. I know AC is generally more dangerous (why?).

So far I know to be careful when working with large capacitors (how large?). Also, taking off the PSU cover and touching the main leads would be bad.

What voltages and currents are generally regarded as safe for DC?
What components do I need to be extra careful with?

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PlastiBots
 
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Re: Dangers of DC power supplies?

Post by PlastiBots »

Although I am not the expert, here are some thoughts:
- I don't do AC projects. 120V / 15+A is not a game I want to play in. All my projects are low voltage (12 or less) anyway.
- Also Remember that DC power supplies still start with AC. I am comfortable working with up to 18V and approx 2-3 A. Again, most all my projects never demand this.
- Caps, having *had* the experience, I treat all caps as dangerous. I'll always discharge them by arcing each one before I play with anything (with power disconnected of course).
- For DC I think you have to watch for the high voltage + higher current ratings. I'd imagine a 24V PS with 2+A would still give you a good rise if your fingers were wet.

For me, safe is <12V and < 1A. However, keep in mind that a sizable CAP inserted into one of these circuits could still mess you up. Example. When I was a kid, I was playing with one of those camera flash circuits. It was only powered by a few AA's, but had a large 1" cap. Don't recall the rating, but I think it may have been 1000uf 20V? Although I was aware of the dangers, I accidentally touched it's leads after it was charged. It blew 2 holes in my finger and caused me to throw everything I was holding (including a Walkman - how's that for history!) across the train.

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adafruit_support_bill
 
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Re: Dangers of DC power supplies?

Post by adafruit_support_bill »

There is a lot of conflicting information out there about what constitutes a dangerous voltage or current. AC is more commonly associated with higher voltage sources, but high voltage DC can be just as lethal. Best to treat them all with respect.
When I was a kid, I was playing with one of those camera flash circuits. It was only powered by a few AA's, but had a large 1" cap. Don't recall the rating, but I think it may have been 1000uf 20V?
Photoflash caps are usually 250v or more. I shorted one with a screwdriver once and it blew off half the blade of the screwdriver.

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adafruit_support_mike
 
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Re: Dangers of DC power supplies?

Post by adafruit_support_mike »

In general, humans can feel current at 1mA for AC and 5mA for DC.

10mA is enough to make muscles contract, so a person experiencing a shock at that level is physically incapable of letting go of the current source. 30mA AC or 300mA DC is enough to interfere with heart rhythm.

AC is more dangerous than DC because it's a changing signal. Most heart attacks don't involve the heart shutting down completely. The system of nerves that controls the timing of the heart muscles, called the 'pacemaker', just falls out of sync with itself.

The pacemaker works by passing signals from one muscle to another.. a contraction in muscle A triggers a contraction in muscle B a short time later. The contraction in B triggers a contraction in C, and the contraction in C triggers a contraction in A. In most heart attacks, all the muscles start twitching rapidly at the same time. Each twitch sends new triggering information to the pacemaker, so the pattern, called 'fibrillation' is self-sustaining. Fibrillation doesn't pump blood from one heart chamber to another, so the body suffocates while the pacemaker flails around unable to re-establish the correct pattern.

A DC current flowing through the heart will make all the muscles contract at once, but when you remove the current the pacemaker can pick up its usual pattern again. An AC current causes the muscles to twitch in a pattern pretty much identical to fibrillation. The pacemaker doesn't know how to recover from that.

A defibrillator sends a single pulse of about 100mA through the heart, forcing all the muscles to contract at the same time. A healthy pacemaker knows how to return to its usual pattern after the pulse. The chest-thumping in medical shows tries to accomplish the same all-at-once contraction with a mechanical shock.

The exact conditions for dangerous shock depend on many things, especially the resitance of the human body. That's another reason why AC is more dangerous than DC.

For the average human, the resistance between one hand and another, if the skin is dry, is between 100k and 1M. If the skin is wet, that can drop to 1k. That's still enough to limit the DC current that can flow through any path long enough to be dangerous. The human body also acts like a capacitor though.. the average value is around 100 picofarads. A capacitor has infinite resistance to DC, but low resistance to AC. The body offers very little resistance to AC, so more current can reach the places where it can cause trouble.

In general, it's unusual for circuits operating below 30v and 30mA to deliver a dangerous shock. It can happen, but incidents of people being injured by a 9v battery are about as rare as incidents of people drowning in less than 1 cup of water.

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mpeg2
 
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Re: Dangers of DC power supplies?

Post by mpeg2 »

A few simple lessons learned from working with tube amps (usually 350-450V DC) - but should be good practice for lower voltages: Take off rings before sticking your hand in a circuit; if the voltages are on the high side, keep one hand in your pocket (avoid having current pass through your body if you goof) & check with a voltmeter before sticking your hands in there.

Rich

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westfw
 
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Re: Dangers of DC power supplies?

Post by westfw »

I treat all caps as dangerous. I'll always discharge them by arcing each one before I play with anything
Keep in mind that voltage isn't the only thing to worry about. DC voltages less than 30V or so are usually not considered dangerous, but a big cap charged to 30V will happily melt part of the screwdriver you're using to "arc" them and send little bits of molten metal flying into your eye. Caps should be discharged with a suitably sized resistor (for a suitable amount of time.)

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