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Author Topic: Question for the electronics guys out there  (Read 2110 times)

Ivan Beaver

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Question for the electronics guys out there
« on: July 10, 2014, 01:27:34 PM »

OK I am stumped.

One of our guys got a nasty "bite" by touching the heat sink (individual for each transistor) on a calls D amp.

So I tried to measure the voltage between devices to give him an idea of how much potential was present.

When I took my trusty DVM as soon as I touched ONE lead (not both) to a heatsink I got a "continuous beep-" (as when checking continuity on the diode scale).

It did not matter which lead I used and did not matter whether the meter was on AC or DC voltage-even when on the highest scale (1000V).

I did not dare to touch both leads-for fear "something" might happen.

I have never seen this before.  Does anybody have an idea what would cause this?

NO-I did not try a different meter or type of meter.  This was just a "quick" thing (nothing wrong with the amp) to try to get a curious measurement.

I must admit I know very little about the way class D amps work-so maybe there is something going on that I am not aware of-but why only when ONE lead is touched?
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John Roberts {JR}

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Re: Question for the electronics guys out there
« Reply #1 on: July 10, 2014, 02:07:27 PM »

OK I am stumped.

One of our guys got a nasty "bite" by touching the heat sink (individual for each transistor) on a calls D amp.
I ASSume you mean shock?  Poking your fingers around inside gear that is powered up is never wise. If the amp makes significant power that suggests it will use healthy rail voltages.
Quote
So I tried to measure the voltage between devices to give him an idea of how much potential was present.

When I took my trusty DVM as soon as I touched ONE lead (not both) to a heatsink I got a "continuous beep-" (as when checking continuity on the diode scale).
Sounds like a personal problem with the DVM that you wouldn't have using an even trustier old simpson 260 VOM.
Quote
It did not matter which lead I used and did not matter whether the meter was on AC or DC voltage-even when on the highest scale (1000V).

I did not dare to touch both leads-for fear "something" might happen.

I have never seen this before.  Does anybody have an idea what would cause this?

NO-I did not try a different meter or type of meter.  This was just a "quick" thing (nothing wrong with the amp) to try to get a curious measurement.

I must admit I know very little about the way class D amps work-so maybe there is something going on that I am not aware of-but why only when ONE lead is touched?
OK I know more about how class D amps work than you DVM.

The short version of how class D works is they use saturated switch power devices so effectively create full PS voltage Square waves. The analog output is determined by the ratio between up time and down time of that full voltage square wave. When up time= down time output = 0V. 

Your meter may be sensitive to the very high rise time, high amplitude switching waveform but a 1000v AC scale should still give you a reading.

Your meter may also just be squirelly (low battery?)... If you really need to see what is on that heat sink consider using an oscilloscope.

JR
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TJ (Tom) Cornish

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Re: Question for the electronics guys out there
« Reply #2 on: July 10, 2014, 02:11:32 PM »

If you really need to see what is on that heat sink consider using an oscilloscope.

JR
And if you really need to do that, make sure you're using one of these to avoid blowing up your scope: http://www.tequipment.net/CalTestCT25931.html?v=0
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gary makovsky

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Re: Question for the electronics guys out there
« Reply #3 on: July 11, 2014, 10:01:11 AM »

OK I am stumped.

One of our guys got a nasty "bite" by touching the heat sink

The body of Mosfet packages will vary weather it is connected to the Gate, Drain or Source thus being energized or Hot. Insulator pads are typically not used since they will decerase the thermal transfer by up to 1 degC per watt. So its a matter of heatsink efficiency to just throw some thermal grease between the part body and the heatsink.

I am very cautious even with an unplugged amp. Certain brands with all the rows of Caps will maintain their voltages for hours.
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Frank Koenig

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Re: Question for the electronics guys out there
« Reply #4 on: July 11, 2014, 11:59:38 AM »

All good observations above. I'll just add that although the switching frequency is likely some tens of kHz (well, more than 40 kHz, we presume) there are spectral components, due to the fast rise times, well into the tens of MHz, which may drive the meter crazy. How the amp passed FCC is a question for another day -- good shielding?

A question I used to throw at junior engineers was if I have a digital system with a 10 MHz clock, what design rules must I use for the interconnects on the circuit board? Answer: the 10 MHz doesn't matter, it depends on the rise times.

--Frank
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Re: Question for the electronics guys out there
« Reply #4 on: July 11, 2014, 11:59:38 AM »


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