[ARC5] [Milsurplus] Hello Smart People Again:

[J. Forster]

Hi Dave,

OK, I've had a closer look at the circuit. Simply put the resistor values
are way off.

Assume it's redesigned to work properly. That means that the collectors of
the two transistors are alternately near 0 or +48 V. The +48 comes from
the auto-transformer action of the center tapped primary.

I don't know the load power you need but I will assume it's about 56
Watts. That says the current from the 28 V supply will be about 2 Amps.
That 2 Amps flows through each transistor alternately.

The gain (hFE) of the 2N3055s is 20 - 70 at 4 Amps. Worst case is 20. Soi
the max Base drive needed is 0.1 Amps. 48 Volts @ 100 okm = 0.48 A. The
base is being way overdriven.

Try getting rid of the 15 Ohm resistors and changing the Base resistors to
about 400 to 500 Ohms.

If you use 4800 Ohms, the dissipation requiured would be 48*48/480 = 4.8
Watts.

One more point. For this kind of circuit, you should not try to power it
by bringing up the input voltage slowly. They sometimes don't start and
just sit and cook. They start better if you just switch on the supply.

FWIW,

-John

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






> 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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