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  1. #11
    Quote Originally Posted by the great waldo View Post
    Hi Bert

    Do you think it's not worth bothering with the bleed resistor ?
    cheers

    Andrew
    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.

    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


    Grtz Bert


    Verstuurd vanaf mijn SM-A320FL met Tapatalk
    Last edited by driftspin; 10-07-2018 at 09:16 PM.

  2. #12
    Ok cheers Bert

    I'll check that out. I had a few jumps years ago with my old Marshall valve amp ,and that was after it was turned off for a week. I became fairly wary of capacitors after that.

    Cheers

    Andrew

  3. #13
    Quote Originally Posted by driftspin View Post
    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.

    Quote Originally Posted by driftspin View Post
    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.

  4. #14
    Quote Originally Posted by the great waldo View Post
    Ok cheers Bert

    I'll check that out. I had a few jumps years ago with my old Marshall valve amp ,and that was after it was turned off for a week. I became fairly wary of capacitors after that.

    Cheers

    Andrew
    It's a good idea to be careful, but unless you know what is what and why, it is easy to fall for nonsense on YouTube. Valve amplifiers use high voltage in the whole circuit, very far and different from what you will use. Also, if you absolutely must handle the capacitor you can always touch the terminals with a resistor first so that they discharge through the resistor, or measure the voltage across the terminals before touching them. The other thing is that you can add a LED on the DC side which will indicate if there is enough voltage, and act as a bleeding circuit. Just get a LED, connect a 5k resistor in series and that's it. It will be your power on indicator, and your capacitor will be discharged after a few seconds to so low level that you don't need to worry at all.

    A slow starter on the primary side is more important than bleeding resistor. The bleeding is fixed by the circuitry which will be connected to the rectifier. The drivers work well as a "bleed resistor", they will quickly discharge your capacitors when you power off, so there is nothing to worry about on the secondary side. Take care of the primary side because toroidal transformers have a large inrush current, even without anything connected to the secondary side, so large transformers should have a slow starter circuit.

  5. #15
    Quote Originally Posted by the great waldo View Post
    Hi A camera

    I thought that might be a problem and bought one of these
    https://www.conrad.at/de/renkforce-6...20-622412.html

    I assume that should do the job, although I would like to build a power surge reducer into the case to be a bit more self contained, if anyone has a straight forward circuit diagram for such a device i would like to see it. Thanks in advance.
    Andrew
    Hi,

    Here is a simple circuit which will limit the current during about 1.5s which should be enough.

    http://electronics-diy.com/soft-star...wer-supply.php

    Even the one from Conrad which you linked will work, but it is limiting between 0.4-1s only. The circuit needed is pretty simple, but of course, if it is a commercial product then it has a price. Never the less, well worth investing in it. I have one built in my PSU, but an external one works just as well.
    Last edited by A_Camera; 11-07-2018 at 10:03 AM.

  6. #16
    Hi A_Camera

    Thank you for the link that looks like just what I need.

    Cheers

    Andrew

  7. #17
    Quote Originally Posted by the great waldo View Post
    Hi A_Camera

    Thank you for the link that looks like just what I need.

    Cheers

    Andrew
    You're welcome, but be careful if you plan to build one. As opposed to the 50VDC, the 230AC is very dangerous and should be handled accordingly and taken seriously.

  8. #18
    Thanks
    I'm always wary of mains power. Just out.of interest what is the amp rating of the rectifier in the circuit ? The round housing limits the options a bit. There seem to be different ones listed on the net. My electronics knowledge is self learnt. Am I right in thinking that the resistor capacitor network is the timer for the circuit. The article is a bit vague in the description of the circuit. I assume the rectifier is only to supply a dc to the relay so does not need to be high amps ? If I'm thinking wrong I'm happy to be corrected!!

    Cheers
    Andrew
    Last edited by the great waldo; 11-07-2018 at 12:32 PM.

  9. #19
    Quote Originally Posted by A_Camera View Post
    Hi,

    Here is a simple circuit which will limit the current during about 1.5s which should be enough.

    http://electronics-diy.com/soft-star...wer-supply.php

    Even the one from Conrad which you linked will work, but it is limiting between 0.4-1s only. The circuit needed is pretty simple, but of course, if it is a commercial product then it has a price. Never the less, well worth investing in it. I have one built in my PSU, but an external one works just as well.
    If you put in a DPCO relay you could use it to drop in a bleed resistor when the power is removed, after all this is what this thread started out as

  10. #20
    Hi Cropwell.
    That seems to be simple/sensible solution. Kill 2 birds with one stone. I'm surprised no one makes a pcb or kit like that.
    Cheers
    Andrew

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