choosing a bleed resistor

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Eh... well when nothing is connected it takes forever and a day to get rid of the lethal voltage levels...

So.. you can check voltage level vs time after switching off.

I would say 10 seconds to 10 ish volts is fine...

If it takes more time then what you are comfortable with put a resistor in.

50VAC 50hz / 120VDC was once considered the critical level for safety under normal conditions.

I would say all the metal near raises risks a bit.


So... then the energies stored in the caps is the next thing to worry about.

Caps discharge at high amp rate on shortcircuit... like a welding torch...


So big caps are always dangerous.

Nonsense.

Large capacitors in combination with coils (like inside a camera flash) are dangerous, but NOT always. Large capacitors alone, charged to low voltage are NEVER dangerous, but yes, they can be used to create visual effects, sparks, burns and so on. Can be spectacular when shown to people who are easily impressed, scared or don't know better.

The above is just my opinion.
More safe is always better.

So never work on live systems, always check your measuring device for good to go before use.
Then check for 0 voltage and then check your measuring device is still in good working order after that.

https://youtu.be/EoWMF3VkI6U

This guy explains it well

https://youtu.be/sI5Ftm1-jik

He is more fun though

Sorry, but this is what happens when you don't understand the facts, just find something on the internet which suits your purpose. Your videos just creates confusion and fear. Both are attention grabbing nonsense. None of those videos show anything of value and DEFINITELY not prove the motive for bleed resistors. Even the second one only barely touches the subject, not really explaining anything.

The first one shows that a large capacitor can hold a lot of energy when fully charged, which is true, but... that does not make it dangerous. The capacitor the guy is playing with is 2.5V and that's NOT enough to cause harm. It is enough to create sparks, burn wires or copper tracks and "weld" but will NOT kill you, not even feel it. The voltage is simply too low. What kills you is the current through your heart. The current is proportional to the voltage and the RESISTANCE of what is put in between the + / - terminals. When you have bare a wire between 2.5V (the voltage of that capacitor when fully charged) the resistance is basically zero (extremely low but lets assume 0.001 Ohm) so based on Ohms law ( U / R = I) the current will be almost infinitive ( 2500A with the example of 0.001 Ohm ) and because of that very high current the power will be very high ( 2500 * 2,5V = 6250 W ) so the copper wire is burned and sparks are generated. This is theory, in real calculation you will need to add the internal resistance of the capacitor also, so the current will be lower, but still very high.

Now, what kills you is as I said, the current through your heart, and you only need about 20 mA is needed to kill you. But it is not that simple, unless you connect the electrodes directly to the wet heart. Normal human body has a resistance of over 2 M Ohm (measure with wet fingers) so with the above example this gives 0.00000125A which is EXTREMELY far from any danger. That is one reason why you can operate even an electric welder without risk of dying. Those are DC with very high current also, yet they are safe to operate. Also, if stored energy in a large capacitor would be a killer factor you would not be able to handle a battery before discharging it. Do you connect a bleed resistor to any battery? No? Why not if you are so safety concerned? Isn't it obvious that they are NOT dangerous to touch? They contain also a lot of energy and they can also burn wires, create sparks and even cause fire if the terminals are short circuited, but don't worry, you won't get electrocuted if you touch the terminals...

The second video is a little nearer to the truth but unfortunately the guy is spending more time in his acting and playing stupid than explaining facts. Of course, if he was not playing stupid he wouldn't have had so many views, so he is interested in making money, not delivering facts...

He mentioned that AC is more dangerous than DC and he also mentioned that the capacitors should be discharged before touching, but he did NOT mention that it is because his capacitors are charged to 170V, not 2.5V as in the first video. This makes a HUGE difference because using the above example it will result in 0.000085A through your heart which is 68 times more than in the first video, but still VERY far from dangerous. Of course, since current flows the shortest route, the way he touches the circuit at the end may result in larger current because he may short out the circuit between two tracks, separated by only a few mm, so his skin resistance may be pretty low. Anyway, in the second case, discharging the capacitor is a good safety measure but connecting a bleed resistor is still waste of energy and not necessary at all.

This thread is about a power supply which is generating 50VDC after the rectifier. The 100V capacitors will NEVER be charged to higher voltage than 50V. 50V is NOT dangerous, but without doubt can create sparks, weld or even cause fire if the current is high enough. Using bleed resistors is pointless in this case. If you absolutely want an indication of voltage it is a better idea to add a simple LED circuit. It will at least have a function and at the same time act as a bleed circuit.