[dorkbotsea-blabber] Re: dorkbotsea-blabber Digest, Vol 19, Issue 24

charles redmond credmond at mac.com
Mon Apr 25 23:27:18 EDT 2005


On Apr 25, 2005, at 8:05 PM, dorkbotsea-blabber-request at dorkbot.org 
wrote:

> To overly simplify things tremendously: 300ohms, 200V max and 100v
> zener diodes. 100V/300ohms = 0.3A. 100V x 0.3A = 30W, but each
> Zener takes a half cycle so call it 15W. Are your Zener diodes
> rated at 15W?
>
No, the max I could find which wasn't special order were 5 watt items - 
but I've got 5 (minimum order) coming so strapping 3 together should 
allow for a 15 watt dump, eh?

> However: transformer impedance is not a simple thing. If you start
> clipping it, funny things can happen. So the real power dumped into
> the zeners may be higher or lower than that.
>
True but short of real analytical gear and a sweep generator I'm sorta 
stuck with trial-and-error and a VOM as well as the online wait for 
parts.

> Seems like it'd be simpler and surer to do all your limiting on the
> low voltage side.

Maybe it's my gut instinct and it's totally in error.  The low-voltage 
side is coming off the amp directly (OTL solid state, 70 watt into 8 
ohm RMS).  The step-up transformer I'm using is a scavenged 70 volt 
line transformer from (70.7 technically) from an old audio installation 
which is facing the amp with its eight ohm winding.  It's beefy - has 
lots of iron and big diameter windings.  With inductance and the size 
of this xformer my experience has been that the "high" side of the low 
voltage end (power amp output) needs to fluctuate between 2 and 8 volts 
for there to be measurable brightness on the EL wire (barely measurable 
to really bright).  On the "high" side of the xformer this is between 
30 and 150 volts.  It's only the last 20 volts which is giving me the 
problem which is why I wanted to strap it down to something like 130 or 
slightly lower.  On the low side limiting the upper limit to something 
between 2 and maybe 5 or 6 would probably do it.  That may be easier 
but am I putting the output devices on the amp at more risk?  Xformers 
and EL wire are a lot cheaper than 70 watt power amps.  Also, am I 
limiting the amount of xformer saturation by clamping the low voltage 
side and therefore unwittingly lowering the overall high side voltage 
and EL wire brightness?  Like I said, these are my gut feelings and 
they could be completely off base.

Wish I had a 'scope and a full lab's worth of test gear.  That would be 
nice.  But I only do these things every so often so it's not been a 
prudent investment to make.

chas

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