G-08 fire prevention...

From: Scott Swazey <sswazey_at_qualcomm.com>
Date: Fri Apr 11 2008 - 20:42:10 EDT

I've been looking at the schematics of the G-08. I'm trying to "harden"
the G-08 so it won't catch fire.

I think I have figured out the problem, it's the 3A slo-blo fuse called out
in the schematic. It protects the Yoke from a Transisor Vce short. I
believe that a Slo-blo is the wrong type of fuse to use.

Here's why. The most common failure on a G-08 is a Vce Short in one of the
power transistors (e.g. 2N6254) and this happens when large currents are
flowing through the yoke. This means that, at the point of failure, there
is +63 volts across the series connection of the yoke, the fuse, and a
1.5ohm 10Watt resistor to ground.

First Simplification: Assume resistance of the yoke and the fuse is
zero. At the point of failure, the current in the resistor is 63V/1.5 or
about 40amps. The 10W resistor will have to dissipate 40*40*1.5 =
2400Watts! until the fuse blows. From the littlefuse 3A Slo-Blo
<http://www.littelfuse.com/data/en/Data_Sheets/313_315.pdf>datasheet at
40amps, the 3A fuse will blow in about 60ms.

Realistic Simplification: assume the resistance of the yoke to be about 1
ohm. This drops the current in the resistor to 63/2.5 = 25amps. The
resistor will have to dissipate 25*25*1.5 = 780Watts! until the fuse
blows. From the littlefuse 3A Slo-Blo
<http://www.littelfuse.com/data/en/Data_Sheets/313_315.pdf>datasheet at
25amps, the 3A fuse will blow in about 200ms. 200ms at 780W is plenty of
time to smoke 10W 1.5 ohm resistor.

Therefore, I believe that the 3A slo-blo is the wrong type of
fuse. Amplifone monitors use a 3A fast blow in the output stage. So I'm
thinking a 5A or 7.5A fast blow fuse could work. In the circuit, the fuse
is primarily protecting the yoke windings as the transistors are the ones
that are going to fail.

Thoughts... Comments...



PS I've significantly simplified this analysis to keep it brief. I can go
into much greater detail if necessary (e.g SOA vs fuse size, effect of yoke
inductance on peak current and back EMF voltage, etc.)

At 03:00 AM 10/20/1999, Al Kossow wrote:
>Date: Wed, 27 Aug 1997 15:53:15 -0700 (PDT)
>If anyone is curious, I measured the inductance and DC resistance
>of [the G-08] XY deflection yoke today. Here's the measured
>type wire color L DC R notes
>----------- ---------- ----- -------- ---------
>Triad-Utrad blue .41mH .43 ohms
>21-143-01 red
>17-8121 green .52mH 1.0 ohms
>Electrohome yellow
>G08-003 Color

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Received on Fri Apr 11 20:42:32 2008

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