Re: telequipment s54a trace too low on y

Leon Robinson
 

But it will still hurt like h**l.


Sent from K5JLR

-------- Original message --------
From: Chuck Harris <cfharris@...>
Date: 09/16/2018 8:08 AM (GMT-06:00)
To: TekScopes@groups.io
Subject: Re: [TekScopes] telequipment s54a trace too low on y

A rule of thumb that was around during the days when
hams and TV repairmen were often exposed to shocks from
the gear they were working on was it takes more than
1 Joule of energy stored in a capacitor to kill you.

The threshold case was stated as a fully charged 10uf, 450V
capacitor.

J = 1/2 C x V x V

In the case of anode supplies on CRT's, the V is high, on
the order of 12KV, which leads to capacitance needed to
reach one Joule of energy as greater than:

C = 2 J/(V x V) or

C = 2/(12000 x 12000) = 1.39E-8F, or 0.0139uf @ 12KV

Which is about 2500 times the capacitance of a CRT's internal
structure (I measured 6pf for a 2465 CRT), and way more than
the capacitors used in the power supply's voltage multiplier
circuit, which is by design about 0.0005uf total capacitance
at 12KV for a 545 type scope, and similar for a 2465.

The above assumes you are a normal healthy adult, with dry
skin (as opposed to wet), and no weeping wounds at the contact
point.

These values were designed into the circuitry to protect the
accident prone technician.

The point is, you are at nearly zero probability of killing
yourself with the CRT's anode supply.  It just doesn't have
the Joules necessary.... it isn't even close.

Far more deadly is the lower voltage supplies found in scopes.

When you go around assessing danger, remember: 10uf @ 450V

-Chuck Harris



Adrian wrote:
Hi James,

Yes it does make sense and always assuming lethal voltages will stay there just long
enough to kill you is a good survival rule!

Just short out the HT/EHT once things are unplugged from the wall, it's easy enough
to make up an insulated stick with a grounding lead to do the job. Depending on where
you can get to the EHT (on the Telequipment D43 it's easy to get to the output cap of
the voltage multiplier) you can form the 'hot' end of the probe into a hook and leave
it hanging there for safety in case some idiot plugs the wrong power-cord back in the
wall when you are not looking....don't ask... the other end can be an alligator clip
to chassis.

Adrian

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