Where is the loop for the current to flow!
If the person is part of that loop, there migth be some questions to answer!
I have no problems holding onto a few million volts,
tho' grounding might be a shocking experience.
If the freq is low enough to avoid being the antenna, no / little displacement
current, no problem..
And voltages are measured between points, there is a problem trying to define
a voltage without a reference.
So for any imagined pair of contacts and the loop between them that might
involve a human, what is the voltage difference between those points? SELV?
ps. Loops are defined as going all the way around back to the beginning, not
just the one way trip to the other point.
Bill
--- On Wed, 12/15/10, Joe Randolph <j...@randolph-telecom.com> wrote:
From: Joe Randolph <j...@randolph-telecom.com>
Subject: Re: Need help with safety compliance for a low power, 150 volt
circuit inside a cell phone
To: ri...@ieee.org
Cc: "Emc-Pstc" <emc-p...@ieee.org>
Date: Wednesday, December 15, 2010, 11:45 PM
Rich, Gary, Brian:
Thanks for your comments and questions. This is very helpful.
Just to clarify, I'm not concerned about the isolation barrier between
the
high voltage circuit and the surface of the LCD screen. I'm concerned
about having to provide isolation between the high voltage circuit and
the
SELV circuit to which it is connected.
If the cell phone were a complete, stand-alone device, it would be
possible
to simply insulate the entire housing, so that none of the internal
circuits are accessible. However, this cell phone (like most others)
has a
USB port and a headset port. I think this pretty much requires that
the internal circuitry to which these ports connect must be treated as
a
SELV circuit. Using this interpretation, the situation can be
summarized
as follows:
1) The high voltage circuit is directly connected, without isolation,
to a
SELV circuit.
2) The high voltage circuit does not appear to meet the criteria for a
limited current circuit.
I appreciate Rich's insightful comments about whether item #2 is
really
true, and I will revisit that question with some more analysis and
testing. However, if both #1 and #2 remain true, I need to determine
how
to proceed.
The consensus view seems to be that I should apply single-fault testing
per
clause 2.2.4. Rich's question about what the potential current path
would
be has bothered me as well. I'm not sure what sort of scenarios to
test
for, but there may be some sort of scenario involving the USB port
and/or
headset port that could create a path through the body. I think I will
need to show that under any single-fault condition, the voltages that
appear on the USB and headset ports remain within SELV limits.
Does this sound correct?
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
http://www.randolph-telecom.com
>Hi Joe:
>
>
>What you describe is very similar to the flash tube
>circuits in a digital camera.
>
>Since you have measured the 150 V and the 300 Hz and
>determined they are not limited current circuits, I
>will offer comments based on these circuits being
>hazardous circuits. (You did not specify the voltage
>for the 300 Hz.)
>
>First comment: Can the circuit be altered such that
>the maximum current from the 150 V DC is less than
>2 mA? If so, both the 150 V DC and the 300 Hz will be
>limited current circuits, and no safeguards are
>necessary. (If one circuit is deemed a limited current
>circuit, then all circuits derived from that circuit
>are taken as limited current circuits.)
>
>First question: In a cell phone that is fully isolated
>from earth and everything else, what is the current path
>through the body? There must be a "goesinta" the body
>and a "goesouta" the body electrical connections.
>Safeguards would be applied between the circuits and
>these two points.
>
>Second question: What is the insulation between the
>300 Hz (voltage?) and the outside or the screen that is
>touched? This may not be physically discrete insulation,
>but obviously it does comprise insulation. Since the
>150 V DC and 300 Hz circuits are derived from a
>battery, the electric strength of the insulation between
>the circuits and the accessible parts of the screen need
>not include consideration of transient overvoltages.
>Table 5B does not apply. (I suggest an insulation
>resistance test as part of your evaluation.)
>
>Second comment: A single fault between the 3.7 V and
>the 150 V DC is likely to shut down the inverter. This
>proves that no isolation is required between 3.7 V and
>150 V. This is application of 2.2.4.
>
>Third comment: Study both the circuit and the
>construction to determine a current path through the
>body, for both normal operating conditions, and single-
>fault conditions.
>
>If you want to talk further about this, give me a
>call.
>
>
>Best wishes for a Merry Christmas,
>Rich
>
>858-592-2620
>
>
>On 12/14/2010 18:18, Joe Randolph wrote:
>>Hello All:
>>
>>I have been asked to suggest a UL/EN 60950 compliance method for a low
>>power, high voltage circuit that resides inside a cell phone. The
>>circuit activates a piezo-electric transducer that physically vibrates
>>the LCD display at about 300 Hz in 30 mS bursts to provide the
sensation
>>of a key "click" when the user touches a virtual key on the touch
>>screen. Note that what the user contacts is the physical vibration,
not
>>the actual electrical signal that activates the piezo transducer.
>>Following are some general characteristics of the driver circuit for
the
>>piezo transducer:
>>
>>* A non-isolated DC/DC converter powered from the 3.7V cell phone
>>battery generates a 150 VDC driver supply that will deliver about 5 mA
>>into a 2K ohm load.
>>* The 300 Hz AC output of the driver will deliver about 4 mA RMS into
a
>>2K ohm load.
>>* Both the 150 VDC supply and the 300 Hz AC output share the same
>>circuit reference node with the rest of the phone circuits.
>>* This circuit reference node is normally floating with respect to
earth
>>ground, but it can become grounded through the USB port.
>>
>>
>>I'm pretty familiar with 60950 and the standard compliance methods for
>>the various circuits that appear in ITE equipment. However, this
>>particular circuit does not fit neatly into any of the standard
categories.
>>
>>A brute-force compliance analysis would classify this circuit as a
>>hazardous circuit, and would require an isolation barrier between this
>>circuit and any SELV circuits, such as the USB and headset ports on
the
>>phone. The voltage is too high to meet the definition of TNV-2, and
the
>>current output, while very small, is too high to qualify as a Limited
>>Current Circuit.
>>
>>My sense is that it may be possible to show compliance by using
>>single-fault testing to demonstrate that under fault conditions, all
>>user-accessible points remain within SELV limits. In particular, I'm
>>looking at the wording of clause 2.2.4, "Connection of SELV Circuits
to
>>Other Circuits."
>>
>>I have never attempted to apply the method of 2.2.4 to a product, so
I'm
>>looking for feedback on whether the method is appropriate for this
>>application. Any other suggestions or insights would be most welcome.
>>
>>It seems to me that this type of situation may come up in other
>>applications that use high voltage at very low currents, such as CCFL
>>backlights, EL backlights, and possibly camera flashes.
>>
>>
>>
>>Thanks,
>>
>>Joe Randolph
>>Telecom Design Consultant
>>Randolph Telecom, Inc.
>>781-721-2848 (USA)
>>j...@randolph-telecom.com
>>http://www.randolph-telecom.com <http://www.randolph-telecom.com/>
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