[Milsurplus] Hello Smart People Again:

[Jim Whartenby]

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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