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Author Topic: Safety Question ALS-600 & solid state amps  (Read 528 times)
KF6OCI
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Posts: 104




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« on: December 27, 2009, 01:38:43 PM »

Is it correct that a solid state amp like the ALS-600
is safer to work on because once you have it
unplugged for awhile you don't have to worry about
massive capacitor build up of voltage that can
kill you.

Do you have to chickenstick everything?Huh

  Thanks,

     Dan
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K0BG
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Posts: 9896


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« Reply #1 on: December 27, 2009, 02:19:52 PM »

Yes. It is ALWAYS best to make sure there is no voltage anywhere, no matter the type of amp or power supply.

Assuming an unplugged power supply...

If the power supply is a switcher, the amount of capacitance isn't very much, but you still can get a shock from it. It is, after all, about 50 volts which can cause a lot of damage to a human.

If the PS is analog, assuming no load at all, and the bleeder resistors are open, there is a chance you could get zapped somewhat harder than a switcher.

In any case, you have to be careful about RF energy, and again it doesn't matter what the amp is; RF is RF!
« Last Edit: December 27, 2009, 02:22:35 PM by K0BG » Logged

AA4PB
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Posts: 13032




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« Reply #2 on: December 27, 2009, 02:29:05 PM »

You should be careful of everything. Even 12V can be hazardous. If you make contact with a ring or watch, 12V at 20A or more could melt the thing into your finger or arm or at least cause a nasty burn. 50V can kill you under the right conditions.

It's all a matter of Ohms law (I = V/R). I is generally what causes the damage. If V is large or R is small then I can be large enough to cause damage.

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W8JI
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« Reply #3 on: December 27, 2009, 02:49:01 PM »

Is it correct that a solid state amp like the ALS-600
is safer to work on because once you have it
unplugged for awhile you don't have to worry about
massive capacitor build up of voltage that can
kill you.

Do you have to chickenstick everything?Huh

  Thanks,

     Dan

I would not chicken stick a tube amp unless the chicken stick had a proper resistor. Just dumping the B+ to ground can damage parts.

All of the Ameritron amps have suitable bleeders. If you unplug the amp and start taking screws out, by the time you have it open the HV will be down. In the rare case there is an open bleeder you should be able to see the HV stay up a bit on the HV metering. After the cover is off you can ground the HV through a suitable 50-500 ohm resistance with a metal composition or some other high joule capacity resistor. NEVER dump HV to ground with a dead short.

As for the ALS-600 specifically, it comes with two different supplies. A switcher than can have up to 300 volts dc in the switching supply, and a transformer type that only can have 50-60 volts dc after unplugged. Both reach zero volts on the output line in a few seconds so the amp itself is dead very quickly, although the switcher bleeds the 300 volts that is stored inside the power supply over several seconds.

With FET's the very last thing you want to do is go poking around with a shorting stick. That would be pretty dumb. The proper procedure would be to unplug and wait a few seconds and watch the HV disappear on the meter. If you see the HV meter go from operating voltage down to zero as you power it down while watching the meter, it will be safe.

I generally never crowbar anything that has a working HV meter. I watch the HV meter to be sure it is working, and wait for it to reach zero. Then I remove the cover and clip a 51 ohm 2 watt metal composition between HV and ground.

There can be a rare case where a bleeder is open, but it always shows on the working HV meter. The voltage will quickly sag to the voltage of one electrolytic (around 400-450 volts) or more and then  park there for a LONG time without decreasing. If you ground the HV line with a 51 ohm high surge energy rated resistor, it will quickly sink to zero volts without blowing up semiconductors. Normally the amp would have a loud "popping" noise in operation if it had an open bleeder, so it is pretty obvious

If you just use a screwdriver you can pop semiconductors. That's true in both tube and solid state amps.

Tom

 



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WX7G
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Posts: 6322




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« Reply #4 on: December 27, 2009, 04:10:14 PM »

European Union safety standards define SELV (Safe Extra Low Voltage) as being 60 VDC or less.
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KF6OCI
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Posts: 104




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« Reply #5 on: December 27, 2009, 04:52:16 PM »

Thanks everyone.

I'm going to save W8JI's for reference.

    Dan
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