RE: 9VAC from 5VDC?

With enough inductance you can use a square wave and the inductance will
round it out.  I typically use a center  tap transformer on the primary
with VCC to the center, then grounding one side then the other gets the
polarity reversal. I have used TIP120s (Darlington) and 2N3055s for the
transistors.

-----Original Message-----
From: owner-cbm-hackers@musoftware.de
[mailto:owner-cbm-hackers@musoftware.de] On Behalf Of Jim Brain
Sent: Friday, September 13, 2013 11:55 PM
To: cbm-hackers@musoftware.de
Subject: Re: 9VAC from 5VDC?

On 9/13/2013 9:57 PM, Bil Herd wrote:
> Do you want to create AC from DC or from AC? (thought is was 5 VDC as
> input)
AC from DC.

A uC will run a 50Hz/60Hz DSS routine, either outputting to a PWM channel
or R2R DAC.
The output will drive a power transistor to the first pin of the Xformer
primary winding.
The second pin will be at ground.

The secondary winding will then have an isolated AC voltage.

Since my AC is rusty, I realize now that 9VAC is 9VAC RMS, which is, as
john notes, 12.73Vpeak, or ~25Peak to peak.  So, the secondary winding
needs to give ~25V.  Since the primary will probably have 4.7V PtP, that's
1:5 (gives 23.5V, assuming no loss, which is not realistic, so say 22VPtP,
which is 11V peak, or 7.7VAC RMS).  Maybe 1:6 would be better.

If such a transformer cannot be found, I could build a voltage quintupler
(5 power diodes and 5 power caps, I assume heavy caps), but I'd rather let
the transformer bump up the voltage.  Either way, the transistor need to
support 5A of power.

If 1:3 can be found, I could create two half wave PWMs/R2R channels.
With 4 transistors, I could drive the 5V through one way for half a cycle,
then the other way for half a cycle, but that means I'm only drive 4.3V (2
PN junction drops).

If I use the PWM into some FETS and do the filtering on the output of the
FETs, I might be able to do 5V swing, but I am much less familiar with FET
operation, so I would have to breadboard it.  It also would then require
larger capacitors and higher wattage resistors.

Jim

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