Light bulb (or LED) to drain Caps?

Discussion in 'The Workbench' started by HumanJHawkins, Apr 20, 2012.

  1. HumanJHawkins

    HumanJHawkins Active Member

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    Hey all,

    Has anyone here used something like an LED with 10 to 20 Ohms resistance as a resistor in a conductor to drain current from filter caps? I'd like to get something that glows or otherwise shows that something is happening, so I know when it's done. (Though I suspect 500 volts would blow most LEDs?)

    Also, do you just drain each cap to ground, or from the positive to the negative side? Or, is there a typical place one can connect to drain them all at once?

    Thanks,
    Jeff
     
  2. Alabama Thunderpussy

    Alabama Thunderpussy Well-Known Member

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    If you want to make it simple, just use some alligator leads and watch the voltage reading on your multimeter drop as you monitor the source.

    That being said, make sure you already have your cap-bleeding jumper.

    500v(to be fair, you'd likely not be draining off that much voltage) would blow all but the most robust and expensive of LED's ,that i'm not sure even exist. The highest rated LED i know of is rated for only 200v, and is likely very expensive. Now, even if your current limiting resistor were calculated so that a voltage drop was sufficient enough to stay within the LED's voltage limits, the current limit of the LED would be then breached many times over. The 200v LED's only operate at around 20-30mA max.
     
  3. Marshallhead

    Marshallhead Well-Known Member

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    If you have a meter with Hz range, set it to that and put one lead on the hot connection of the filter cap, one to ground and the stored charge will bleed away. Once the Hz reading gets to zero, switch to volts and the charge should be next to nothing.
     
  4. Alabama Thunderpussy

    Alabama Thunderpussy Well-Known Member

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    How exactly does that work since DC voltage has no duty cycle to count.:confused:
     
  5. HumanJHawkins

    HumanJHawkins Active Member

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    The multimeter thing is probably wisest, but since we're problem solving... Maybe a small 25 watt incandescent bulb with a resistor to balance the voltage? Or a long string of LEDs.

    lol. Nah... Multimeter it is. Thanks all.
     
  6. Marshallhead

    Marshallhead Well-Known Member

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    It doesn't matter as you're not using the meter to measure hertz, you're just using it as a convenient bleeder resistor. It does however give a reading as the voltage dissipates, although I never pay any attention to it and never worried about why.

    Try it next time you have an amp out of it's box.
     
  7. MKB

    MKB Well-Known Member

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    IMHO using an LED like this is not a good idea. LED's can be damaged by high forward currents, and even more quickly damaged by too great a reverse voltage (measured well under a volt). The energy (voltage and short term current) stored in a tube amp power supply filter cap would quickly blow most any standard LED made. So if you really want to use an LED, you could get it to work if you limit the current and voltage across the LED in both directions, which would take about 3 to 4 additional protection diodes (and these would be different types), along with a resistor or two. All the LED would do is perhaps flash quickly during cap discharge, and is that really useful?

    It would be much simpler to just use a resistor.
     
  8. Ken

    Ken Well-Known Member

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    Anyone draining caps should have a multi-meter. I'm with MKB: use a resistor and check with the meter after a minute.

    Ken
     
  9. HAMPAMP TUBE AMP SERVICES

    HAMPAMP TUBE AMP SERVICES Member

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    Use a 10k 10watt resistor to bleed current from the filter caps, just connect a couple of jumpers to it and find the first set of filter caps right after the rectifier (normally 2 caps in series) attach one lead to ground and the other to the rectifier output (if you have a standby switch, this is a good place to connect the lead. turn the standby on of course). Most of the time, just shorting the caps + lead to ground will work but if you ever have a situation where the amp has been drawing heavy current due to a short circuit or failing parts, then you run the risk of welding the grounded device to the chassis. I've done it, it happens. I have several screwdrivers with craters in the shaft that had to be pryed off the chassis, I was lucky the caps didn't blow up in my face. Be safe, do it right. A resistor will bleed the current at a slower rate (give it about 30 seconds or so). Any value resistor between 5k and 500k will work but use a 5 or 10 watt resistor. Check the voltage with a meter to make sure it is down to a safe level. If you drain the main filter caps, most of the dangerous charge will be gone, but you can check all of the caps to make sure. The bias circuit has smaller cap in it which should be discharged before working in that circuit but you have to connect from ground to the negative lead of that/those cap(s).
     
  10. HumanJHawkins

    HumanJHawkins Active Member

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    Thanks much. Excellent to have the full story in one place like this.
     
  11. clel miller

    clel miller Member

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    I do not think I have ever seen an amp, with the stand by off, that did not drain down to 15 volts (or there abouts) all by itself.
    I always check, but I have never seen a cap, in a turned off amp, that was over 20 volts.
    Best
     
  12. Alabama Thunderpussy

    Alabama Thunderpussy Well-Known Member

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    Nor have i, but they are out there. An amp that has had HT on the caps with no tubes in it can hold a very high charge on the caps for quite a while after its turned off.
     
  13. clel miller

    clel miller Member

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    There you go....I can see that scenario happening.
    But as far as a "working" amp, that is in use by a guitar player, I always see low voltage when the amp is turned off.
    Best
     
  14. HAMPAMP TUBE AMP SERVICES

    HAMPAMP TUBE AMP SERVICES Member

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    As long as the filter caps have bleeder resistors parallel with them and they do their job the voltage will discharge, but not every amp has bleeders and resistors do open. Just about all modern amps have bleeders but I will always have my "clip lead bleeder" on standby.
     
  15. Csquare4

    Csquare4 New Member

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    I just usually short pin 1 of V1 to ground with a jumper wire and turn the standby on. The dropping resistors to V1 work fine for bleeding off the caps and all you need is a simple jumper wire with two alligator clips. Just make sure the Mains switch is off and/or better yet, the mains power is unplugged.
     

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