Re: Very thin Wire?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Steve Greenfield by way of Terry Fritz <twftesla-at-qwest-dot-net>" <alienrelics-at-yahoo-dot-com>

Mike, I'm putting this back on the list. There are
many who've done a -lot- more than I've even thought
about with solid state Tesla Coils.

--- Hollmike-at-aol-dot-com wrote:
> In a message dated 11/6/01 12:51:36 PM Mountain
> Standard Time, 
> alienrelics-at-yahoo-dot-com writes:

> > What do you mean by "optical relay"? Do you mean
> an
> > optocoupler? Is it possible that the optocoupler
> is
> > the part that doesn't have enough output? Power
> > MOSFETs have a very high gate capacitance and so
> > require pretty low impedance drive.
> > 
> 
> I suppose it could be called an optocoupler, but it
> has a 'diagram' printed 
> right on the side of the unit that is schematically
> identical to a relay.  I 
> have used a similar circuit to operate a fuel
> injector, but rather than using 
> a second 555 timer for pulse width, I used a 74121
> one shot timer.  I may 
> revert to that method since I know it worked.  The

Sounds good to me. I haven't used 74121's so check
their max speed. Keep in mind that a rating of 1MHz
doesn't mean a chip will be useable at that clock
rate. Because you are looking to use a shorter period
for PWM purposes, so you can easily push outside a
1MHz rating even running the frequency at only 100KHz.

> optical relay should not 
> have problems at 20kHz(at least that is my hope
> actually I am hoping to run 
> it at 4-5kHz if the transformer can deal with it). 
> The problem I was having 
> is that the PWM chip''s output was only a few
> millivolts, so it would not 
> operate the relay, which requires at least 3V.  The

I'm officially confused. What PWM chip puts out only a
few mV? It should be putting out a square wave at the
supply voltage.

> relay itself can   The 
> relay output is rated for 60V and 5A, so I think
> it's impedance is low enough 
> (wouldn't it be 12 ohms?).  Well, I won't bug you to
> draw up the schematic.  
> I just hoped you might have had it and thought I'd
> ask.  I have the 74121 
> chip available anyway.  I was just hoping maybe I
> could fix my mistake rather 
> than redoing the whole circuit.  Lazy, I guess.

OK, so you are trying to use a solid state relay,
then. Are you trying to use it in place of a power
MOSFET?

If so, 60V is a bit low. The 60V rating means that it
will withstand 60V across it when it is -off-. The 5A
rating means it will withstand that current when it is
-on-.

I wouldn't expect a solid state relay to switch at
5KHz, much less 20KHz. DC SSR's are designed to just
turn a DC load on or off, not cycle quickly. 60V is
-really- low if this is supposed to drive the primary coil.

=====
Steve Greenfield               // Digital photo scanning, retouching,
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