10

I am just getting into soldering, developing prototypes on perf boards and microcontrollers. The current for some can be as high as 2 A. I am trying to build a drone and the BEC for one ESC is 5 V and 3 A. This may sound stupid, but how do people not get electrocuted when soldering connections that carry this current. And I don't see insulation put directly on the soldered connections or on the circuit board either.

Especially when you look at prototyping in general, there are wires hanging out everywhere and likely more than 2 A coming out of the power source. At those soldered connections, shouldn't it be practically lethal to go near it? The wires insulate the paths, but the connections should be dangerous to touch, right?

Peter Mortensen
  • 1,676
  • 3
  • 17
  • 23
Ahmed Anwer
  • 297
  • 2
  • 9
  • 36
    It's 5 V. You might feel a small tingle if you placed your tongue on it. But it's otherwise safe. (Well, safe unless you are operating the thing while putting your tongue on it. ;) – jonk May 07 '21 at 20:43
  • 46
    First of all, no one is soldering *powered* circuits. Second, there is an Ohms law, governing that there is a very little current going trough high-resistance human body when 5V are applied to it. – Eugene Sh. May 07 '21 at 20:43
  • 12
    anything less than 50 volts is reckoned to be touch safe. Dry skin just doesn't allow enough current to flow through the body below that voltage. The currents possible on the board are irrelevant. – Neil_UK May 07 '21 at 20:44
  • 27
    @EugeneSh. No one is *intentionally* soldering powered circuits (mostly). I have to admit that after 40 years in the industry I've unintentionally soldered some powered circuits, LOL! – John D May 07 '21 at 20:46
  • 4
    Realize that the danger in soldering circuits that are powered (from batteries or a safe power supply) lies more in **making a short circuit by accident** (a drop of solder could cause that) than in being electrocuted / shocked. So power a circuit **off** before doing soldering. The mains sockets carry a **dangerous voltage** but that danger is removed when using a power supply. A power supply provides **isolation** preventing too much current from flowing through humans. – Bimpelrekkie May 07 '21 at 20:51
  • 5
    Even 2000 amps will not shock you, at 5V. But a red hot heater might burn you, even at 1V. – Indraneel May 07 '21 at 21:43
  • 25
    @JohnD _"No one is intentionally soldering powered circuits (mostly)"_ - In my job as a telephone technician we regularly soldered phone line circuits with the power on (50VDC with possible 120VAC ringing). But the soldering iron tips were not grounded so they didn't short out the circuit. Getting shocks was part of the job back in those days! – Bruce Abbott May 07 '21 at 22:16
  • 6
    The far, far bigger danger is accidentally touching your finger with the hot soldering iron... It won't kill you, of course, but it'll make you quite angry with yourself. – Cody Gray - on strike May 08 '21 at 06:20
  • 4
    Yes. Be careful you don't accidentally short across the battery. You will possibly get big sparks and little pieces of debris shooting off of the spark area. I suppose it would be a good idea to wear glasses so that if something does happen at least you don't get junk in your eye. – user57037 May 08 '21 at 07:04
  • 4
    You wouldn't change your car's tires while driving, would you? So why would you solder around on a board under power?? – Aganju May 08 '21 at 14:50
  • 3
    @Aganju You avoid it where possible. You'd only do it (intentionally) when you had no real choice: if the circuit is doing something that can't be switched off. And then you have to be careful, as Bruce says above: ungrounded iron tip, very steady hand, focus, know exactly what you're doing and do it. Easy to mess up. Unintentionally, otoh, none of those things apply, and then it's "Oh Crap!" time! – SusanW May 08 '21 at 17:23
  • 3
    I lent my Antex soldering iron to a good friend. I got it back, cleaned the bit with a wooden handled wire brush, and applied solder. I got an almighty jolt and it was only luck that the hot iron didn't burn me or anything else. I found my "friend" had (I guessed) borrowed the plug for something else. He had reattached it to the soldering iron lead and used a pointed head screw to secure the Earth wire to the plug pin. The screw had severed the wire which had wandered about and touched the live end of the fuse, making the exposed metal of the iron live at 230v. Thanks, pal. – Michael Harvey May 09 '21 at 11:58
  • 4
    The soldering part is like asking how people build car prototypes without getting hit / run over by them. The most basic of safety precautions - don't power them while working. – Peter Cordes May 09 '21 at 17:50
  • @MichaelHarvey Wow. That's BS – DKNguyen May 09 '21 at 21:01
  • 2
    @DKNguyen - Yes! I didn't make it clear the plug was a UK 13A 3-pin plug. – Michael Harvey May 09 '21 at 21:42
  • Also, some soldering irons have grounded tips (others have ceramic shafts). If you have a high current supply and you try soldering a hot circuit, there will be sparks, tripped breakers, blown components, etc. Generally, unless you are probing, power off when fiddling is a good idea. – Rich May 11 '21 at 02:06

8 Answers8

64

It’s a common misconception with beginners that a 2A power supply will force 2A through anything that touches it. This isn’t the case. Most power supplies are (more or less) constant voltage, so the resistance of the load determines what current flows. The human body has a relatively high resistance and so low voltage circuits can be handled with only tiny currents flowing. In my experience I can’t feel any current below about 1mA and for me that happens at about 100V (don’t try this at home). Even for low-voltage circuits it’s good practice not to handle circuitry when it’s powered, as it’s easy to cause shorts or disrupt sensitive signals.

Frog
  • 6,686
  • 1
  • 8
  • 12
  • 15
    Basically the OP needs to understand Ohm's Law - and remember that they're basically just a resistor electrically. :) – Graham May 08 '21 at 21:48
  • I've had this same problem when I was just starting electronics. I've never understood that 2A rating... until I've learned about inner workings of a switch mode power supply and It's feedback system. – John Cortex May 10 '21 at 11:38
  • @Graham: Ohm's Law basically says that everything's a resistor, and the OP falls into the category of "everything". – Michael Seifert May 10 '21 at 12:08
  • As I learnt in aerospace apprenticeship: "It's the volts what jolts, and the amps what cramps". – RedSonja May 10 '21 at 12:48
  • @RedSonja I learnt the latter part as "the mills that kills", since it does only take milliamps. – Graham May 10 '21 at 14:33
  • @Graham quite right - in my experience 1mA is just perceptible, 1.5mA is unpleasant. 2mA is rumoured (not from a reliable source) to be the worst for inducing heart problems. – Frog May 10 '21 at 19:22
  • 1
    @Frog I did an electrical safety class at uni where they talked about this. They said higher currents could actually be safer - they stop the heart and may cause burns, but the heart can be restarted (if there isn't burn damage). Lower currents cause your heart to go into fibrillation, which used to be harder to deal with because only hospitals had defibrillators. Widespread availability of defibrillators should make this less of a problem these days, so perhaps the risk levels at different currents aren't what they used to be. Still better not to need resuscitation though! :) – Graham May 11 '21 at 09:48
  • @Graham That seems to be an unsubstantiated rumor in engineering circles. How would you even measure that? Dry skin resistance varies by orders of magnitude so you can't just look at a dead electrocuted guy and figure out that he died from a 2mA shock as oppposed to a 100mA shock. In practice all you can do is write down the open circuit voltage of the wire that killed him. – Navin May 11 '21 at 19:52
  • @Navin my comments are based on measurements that I have made experimentally. While I have never knowingly executed a subject, I am aware of the conditions under which a few lethal electrocutions have occurred and could estimate the likely current within reasonable grounds. – Frog May 11 '21 at 19:57
  • @Navin It seems like you haven't actually done training on this. For dry skin resistance, for example, that's why you're trained never work with HV whilst wearing a watch or jewellery. As for measuring it, this happens in ***medical*** circles. They have 200 years of experience of putting electric shocks into the body or into extracted organs. We know from medical experiments how electricity works with the body, and that's why America runs on 110V mains to be less dangerous. – Graham May 12 '21 at 07:32
28

"How do people prototype circuits without getting shocked?" They switch off the power before soldering it up. This is not for fear of shock but for fear of damaging the components. The soldering iron tip is usually grounded for the same reason.

Only experienced engineers should ever try soldering a live circuit. They know how to avoid harming the components, for example by using a special soldering iron with the tip floating and not grounded, and also considering the potential for static buildup.

Another answer has explained why the threat is not supply current, but voltage.

Guy Inchbald
  • 1,435
  • 5
  • 10
  • 10
    And no matter how experienced you are, you shouldn't try soldering a live circuit unless you need to! – Hearth May 09 '21 at 13:33
  • 8
    Once, in 1967, I had to wire a capacitor across an intermittently operated 120V relay. That day I added another Life Goal: Don't do that! – waltinator May 09 '21 at 17:13
22

Human skin has a pretty high electrical resistance at low voltage (you can measure it with an ohmmeter, just grab the probes). If you stick your finger on low voltage like below 24V you won't feel anything. Some current will pass in your finger but it will be microamps. Last time I punctured a finger with the pointy end of a wire that had about 12V on it, I did feel a small shock, because that bypassed the skin resistance.

Wet/sweaty skin has lower resistance, and mucous membranes much lower. Back in the day we used to check if a 4.5V battery was still good by licking the terminals, I don't think anyone died from that. If you try it with a 9V battery though, you won't try it again.

But, just because current is flowing in wires doesn't mean it will also flow into you.

Above a certain voltage, skin "insulation" breaks down, then its resistance becomes low enough to let a dangerous current flow. You could think "ohm's law", so touching a wire with 230V would cause about 10x more current in your finger than at 24V, but this is not true. At 24V the skin is a good insulator, but not at 240V. So at 230V the resistance is much lower, current increases dramatically, and you get zapped. That's when you absolutely have to take precautions.

But if you only use low voltage, no problem. Safety is more about not getting burned with the soldering iron, not breathing the solder fumes, and not getting bits of exploding MOSFET in your eye.

If you solder or probe on powered circuits, you can cause a short and destroy the circuit, then you drop the soldering iron on your finger.

And if you solder or probe on a powered circuit using LiPo batteries, what you should be worried about is fire...

enter image description here sauce

bobflux
  • 70,433
  • 3
  • 83
  • 203
  • 20
    Wait, you did *not* enjoy licking the 9V battery terminals as a kid? :D – ThiefMaster May 08 '21 at 09:08
  • 9
    9V is a bit spicy lol. – bobflux May 08 '21 at 10:43
  • 9
    "If you try it with a 9V battery though, you won't try it again.". When I was a kid, that's how I always checked my 9V batteries. – user4574 May 08 '21 at 18:05
  • 5
    @user4574 That's _still_ how I check my 9V batteries. And most of the rest of my batteries. :P – Erhannis May 08 '21 at 20:57
  • 3
    @Erhannis How do you touch both ends of the batter with your tongue? Is you're tongue that long, or do you use some sort of connector? (lol) – trlkly May 09 '21 at 02:25
  • 2
    @trlkly Nah, I just use one of my lips for the far end. I've noticed, actually, that the two sides of the battery have a slightly different "taste", I think - it's a bit hard to pin down, though, and I haven't given it more than a little thought. – Erhannis May 09 '21 at 02:32
  • 2
    @trlkly there's just about 1cm between + and - in case of a 9V block battery. it's hard to lick one pole without licking the other one. – ThiefMaster May 09 '21 at 12:15
  • 4
    Taste is certainly based on chemical reactions, and positive voltage will certainly dive a different reaction than negative. Now that I _am_ thinking about it, I'd expect them to taste completely different. One acidic and the other bitter? – ShapeOfMatter May 09 '21 at 19:44
  • 4
    Electrolysis also releases natural flavors like a little bit of copper and whatever plating is on the electrodes... – bobflux May 09 '21 at 20:04
  • 1
    As a kid in the 1960's I had a nice 90v B+ (?) battery to play with. Putting fingers on the terminals would give a little jolt. – aMike May 11 '21 at 01:23
12
  1. If it's got two points of contact with the rest of the circuit, then the current vs. voltage relation is fixed. You can give it a fixed voltage and get a fixed current, or a fixed current and get a fixed voltage -- you can't independently control voltage and current.
  2. If you make two points of contact with a voltage source, you're the "it" from item 1 in this list.
  3. It's current through your heart that kills, but it's voltage that drives that current. In general, 62 volts DC or around 48 volts AC is relatively safe. Sporting maybe -- you'd feel it -- but safe.

While I'd be cautious about a circuit with 48V on it, you can pretty freely work around open terminals with 12V or less on them.

Note that for power electronics at these low voltages you do want to worry about things getting hot and burning you. Auto mechanics know that a wrench dropped across a 12V battery will get red hot -- some of them have the scars to prove it. So while worrying about catching your drone on fire (and possibly that fire catching your house on fire) is a very very valid concern*, you don't have to worry about electrocution.

* Search around for "LiPo fire" or "drone battery safety" -- you should find plenty of material explaining how to play with it all and not burn your place down.

TimWescott
  • 44,867
  • 1
  • 41
  • 104
6

The amount of amps \$I\$ you get depends on the combination of resistance \$ R\$ and voltage \$V\$ you have, this relationship is called ohms law

\$\frac{V}{R}=I\$

Your drone has a low resistance in it's circuit, so you get 3 amps circulating in it.

\$\frac{5V}{R}=3A\$

\$R=\frac{5V}{3A}=1.6\Omega\ \$

Thats a pretty low resistance.

On the other hand, your body has a much larger resistance. in the order of \$R=100.000\Omega\ \$

This means that the very same 5 volts will produce a much lower current (Amps) across your body compared to the drones circuits.

\$R=\frac{5V}{A}=100.000\Omega\ \$

\$A=0.00005Amps \$

That current is so small you will not feel it at all.

Joaquin Brandan
  • 543
  • 3
  • 10
4

Skin acts like a VBO, not a resistor.

You're expecting human skin to be ohmic: follow V=IR. Nope.

Human skin works more like a VBO (Voltage Breakover) device. It doesn't flow current until a certain voltage, and then, it does.

That voltage varies wildly by conditions (generally, wetness). But 12V is a very safe number; the only time someone was killed by that was when they sought out a Darwin Award by using needles to pierce the skin at both terminals. Generally it is touch-safe at higher voltages, but I can tell you 64V is not one of those voltages :)

Harper - Reinstate Monica
  • 21,639
  • 27
  • 68
3

In olden days when testing systems which were necessarily live, the standard approach was to stick tape over the contacts you didn't want to touch. Most people would get a few unpleasant reminders of where to put the tape until they knew the layout they were testing well enough to do it in advance.

You'd not be wanting to wear a metal watch or other adornments. That's even more true in systems where you may find higher tension (HT), for example old internal combustion engines.

With lab power systems where things are significantly more lethal gloves are mandatory and have other advantages for dealing with sensitive kit.

Andrew Morton
  • 2,389
  • 1
  • 17
  • 26
philw
  • 131
  • 1
  • 1
    Re *"higher tension"*: This is old terminology for *"higher voltage"*. Why confuse the issue? – Peter Mortensen May 09 '21 at 15:22
  • 2
    The issue with internal combustion engines and metal watch straps isn't so much the high voltage to the spark plugs (that's a whole other area of concern, but normally you're nowhere near them when the engine's running), but the current a 12v car battery can put through the low resistance of the metal strap - easily enough to melt it and cause serious injury. – peterG May 09 '21 at 16:13
  • Bill Godbout of CompuPro told a story about "three fingers brown" in the army who didn't take off his rings one time working on a power supply, as a warning to follow procedures working on live systems. – manassehkatz-Moving 2 Codidact May 09 '21 at 17:42
3

I've built many RC aircraft and on average my "small" planes (considered "micro" in RC world) can use 20A of current. By comparison 2A starts going into the "ultra-micro" realm (most ultra-micros I own actually use around 10A, building 2A planes is possible but very difficult and require specialized building skills). Most normal RC aircarft use 100A and above.

Having said all the above, I'm going to answer the question from an RC builder's point of view and ignore all the physics and equations and instead base it on rules of thumb and advice passed on to me from previous generations of builders and common sense.

How do people not get electrocuted when soldering connections that carry this current

The common sense answer to this is simple. You never solder connections that carry current. NO, JUST DON'T DO IT.

Instead you solder unpowered circuits to connectors. For smaller circuits like yours that usually means a JST or one of the micro-JST connectors. Best to use the same connector that comes with your battery.

When soldering your circuit there is zero current because there is no battery connected. Only once everything is done do you connect the battery to your circuit. NEVER START WORKING ON YOUR CIRCUIT UNTIL YOU'VE REMOVED THE POWER SOURCE (for RC models this means the battery)!

Now, once in a while you may need to solder your battery to a connector (for example if you bought a battery with the wrong type of connector). Working with batteries is a bit more "risky" but just follow the one simple rule: never let the positive wire touch the negative wire. I often tape one wire when working with the other. Also, be very careful not to transfer too much heat to your battery when soldering. Best case is you can potentially damage your battery. Worst case is an explosion.

If you are not comfortable with your soldering skill then do what most people do: don't solder your own battery. Instead buy one with the right connector or change the connector on your circuit or build an adapter (a short piece of wire with two different types of connectors).

slebetman
  • 1,660
  • 11
  • 14