[Milsurplus] Hello Smart People Again:

[J. Forster]

Did you miss anything?

Yes. The frequency of the inverter is set by the supply voltage and the
saturation flux density of the transformer core. As the supply goes up, so
does the inverter frequency.

Having both transistors on at the same time during startup burns things
out. The circuit is not a good design. I'll look for a better one later.

FWIW,

-John

=======================





> You didn't mention the power ratings for the resistors.  The 100 ohm
> resistor is
> dissipating a bit over 1 watt and the 15 ohm about a tenth of that so use
> 2 watt
> and 1/4 watt types or larger.
>
> With no load on the inverter, the power transistors should be running
> cold.  If
> not, then you most likely have a both transistors on at the same time.  To
> slow
> down the turn on time of the switching transistors, put capacitors in
> parallel
> with the 15 ohm resistors.  The value is determined by T=RC.  Start with T
> =
> 1/10 to 1/20 of the transistor on period and R = 100 ohms.
>
> The last comment; if you are going to use 28 volts to power the inverter,
> you
> will saturate the power transformer since it only has a 12 volt winding.
> Did I miss anything?
> Jim
>
>
> ----- Original Message ----
> From: "arc5 at ix.netcom.com" <arc5 at ix.netcom.com>
> To: milsurplus at mailman.qth.net; ARC5 <arc5 at mailman.qth.net>
> Sent: Thu, August 5, 2010 11:29:14 AM
> Subject: [Milsurplus] Hello Smart People Again:
>
> Hello even smarter people.
>
> This simple little 60-hz cross-coupled inverter is commonly published:
>
> http://home.netcom.com/~arc5/inverter.JPG
>
> It uses a 115V to 24VCT (12-0-12) 5A fil transformer
> to produce around 250 V out, which I want to double and
> use as a dynamotor replacement.  I've built it with 2N3055s,
> which should handle the current.  I do get AC voltage out,
> for awhile....
>
> I haven't powered it above about 12 volts in.
> One of the 15-ohm resistors burns up. 
> If I remove it, the circuit still starts but heats-up the
> 100 ohm resistors with no output load.
> My scope is munged, but i'm guessing both transistors
> are on at certain times in the cycle. 
> I've swapped the transformer leads (same 15-ohmer still heats)
> and I've swapped both transistors, since they're old and
> might be leaking under load.  Unless my whole pile of
> old 2N3055s is leaking, and I suppose that's possible.
> I can see (barely) a large spike at the start of each
> switch cycle.  Is that whacking my transistors?
> What are your ideas?
>
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