Possible downstream dangers of common 5Vdc power supplies

Question

Let's say you have a 5Vdc/2A power supply that is hooked to some kind of power transformer or DC/AC converter that generates 120V+. The example I am thinking of is a nixie clock device that is powered by 5Vdc but is able to light the USSR Nixie bulbs which require somewhere north of 120V. Is it possible to be electrocuted by a device like this? Looking at a 5Vdc/2A power supply, nobody is worried about getting shocked or electrocuted when working on an Arduino, SBC, or microcontroller, but I can say from personal experience that the nixie clock project above is capable of piercing the human skin resistance and at least shocking. This lead me to start thinking about the possible dangers of voltage amplifying components connected to seemingly harmless low voltage supplies. I know it takes ~100mA to kill a person and doing some simple math(that may be wrong) a 10W supply should only be able to send 83mA at 120V but even that seems dangerous at a high level. Can someone please explain to me any possible dangers here that I should keep in mind when working with electronics? Thanks for your time.

Answer 1

Power Rating (which is the combination of Voltage x Current) of the supply is what kills you, not current or just voltage. But usually we translate the power rating (P) into voltage (V) as a metric of electric shock because the current is the dependent variable (I = V/R). The two independent variables are the Voltage (source) and the Resistance (load). And for a fixed resistance, the higher the voltage, the higher the current. In simple terms, both high voltage and high current can kill you, but a "high current" is simply dependent from a high voltage.

However, this current (I = V/R) that is calculated from the independent voltage and resistance will be capped by the maximum power of the source. In this case a 10-Watt source with 120V can deliver up to <= 83mA. And this translates to a Resistance >= 1440 Ohms. According to online sources, the human internal resistance can range from 300~1000 Ohms, but with dry skin it can easily go beyond 1000~100k Ohms. So our body resistance is pretty varying in range and not constant so some people just rely on the actual current as measurement for pain. According to another article, the Physics Department at Ohio State lists 0.01 amps (10 mA) as the threshold for a painful shock, while currents between 100 and 200 mA are likely fatal.

So going back to your 10-Watt 120V setup, if the resistance is calculated such that: $$ I = 120V/R >= 0.010 A$$ then it can suffice to electrocute a person painfully.

Now to see if it's possible to deliver that power into a person, we calculate the worse resistance scenario:

$$ I_{1k\Omega} = 120V/1000 Ohm = 0.120 A \text{ -> capped to 0.083A} $$

So we can say that a person is possible to be electrocuted by a 10-Watt 120V supply if that person's resistance is around 1k Ohms.

But if the person's resistance is higher (e.g. >12k Ohms), then the current safely goes down to more acceptable levels:

$$ I_{12k\Omega} = 120V/12000 Ohm = 0.010 A \text{ -> acceptable} $$

Now if you factor further the total energy transferred by the supply (Power * time), the longer the exposure, the more damage it does.

On a different case, the reason why a taser doesn't kill despite the very high voltage is because it is power-limited, enough to limit the current to the range enough to inflict pain but not lethal.

Answer 2

I've built many Nixie tube clocks. The usual running voltage is around 170 VDC (depending on the model of the tube), made from an inductive switching boost supply sourced by 12 VDC. I have also shocked myself accidentally on these power supplies, so I can speak here with some experience.

It will get your attention as it does have a bit of a bite. However, the usual power supply designs for Nixies are fairly low power - just some milliamps - as Nixies don't require much power. Therefore the power supply will lose power quickly and try to recharge it, and so depending how long you are exposed, you might feel a pulsing sensation despite it being a DC power supply.

I would say that touching mains at 120 VAC is a worse experience - the voltage range is kind of the same, but your local power substation can keep the voltage coming whereas the Nixie supply will struggle and fade.

But in general, getting shocked by 5 VDC is pretty hard to accomplish, and 12 VDC is also something that I routinely work with and never feel. Typical government regulations consider voltages under 50 to be benign and not in need of special equipment/training, being in the "low voltage" range.

Answer 3

The difference between the 120V Nixie supply (more about that below- not realistic) and the 5V/2A wall-wart is the power each can deliver to the body.

Ohm's Law tells us that a higher voltage will send more current through the body than a lower voltage. And it doesn’t take that much current: 30mA or so, through the right path, could kill you, but typically will be felt as an uncomfortable shock. 100-200mA is the value most often cited as being potentially lethal.

Meanwhile, the body has resistance which serves to limit the current. How much? Whole-body resistance is as low as 300 ohm; skin contact resistance is typically in the 1k ~ 100k range. (Try it yourself with a multimeter.)

A deeper dive into all that here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2763825/

Based on these characteristics ('fatal' current and body resistance) and field data, there is some agreement in worldwide safety standards that any voltage above 60-120VDC or 50V AC needs to be considered a risk for electrical shock, while anything below those thresholds is considered ‘touch safe’, so long as it is separated from mains (that is, it's a Seperated/Safety Extra Low Voltage, or SELV.)

(What is SELV? More here:https://www.se.com/in/en/faqs/FA253330/)

Let's say the resistance is 1k(skin) + 1k(skin) + 300 (body) = 2.3k. What current results at your two examples and at SELV threshold?

• 5V DC: 2.2mA, not even detectable
• 60V DC: 65mA, uncomfortable shock
• 120V DC: 130mA, painful shock, possibly fatal

Safety standards worldwide specify that devices which use voltages above that 'touch safe' threshold require extra safety measures, such as reinforced insulation, to protect contact with the dangerous voltage. This would include devices that use electrolumenescent panels, CCFL drivers, and, yes, Nixie tubes.

And what about Nixies? These have strike voltages of 180V or more with sustain currents of about 10mA per digit, at 150V or so.

(more here: https://hackaday.io/project/163516-multiplexed-esp8266-nixie-clock-project/log/159028-nixies-current-pulses-strike-voltages)

Considering that your Nixie power supply likely has to power more than one digit (such as for a clock), the supply easily has enough current at a high enough voltage to pose a risk of fatal electric shock.

Regardless, from a safety design standpoint, since the supply and the Nixie tubes exceed SELV 'touch-safe' thresholds they are treated as hazardous voltage, even if there are current-limiting resistors. To pass muster with safety your Nixie gizmo will need appropriate insulation and stand-off distance designed in.