Re: dorkbotsea-blabber Digest, Vol 19, Issue 24
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
> 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
> 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.
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