Re: 27 VAC to 32 VAC?


"Chris" <cfoley1064@xxxxxxxxx> wrote in message
news:1145067561.776671.208370@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Dan Beck wrote:
Hello all,

I am a lurker novice with a few questions. I have a transformer
(specifically out of a pinball machine) that outputs 27 VAC, and it is
fused
for 8 amps in the circuits it supplies. I would like to change the
output
to 32 VAC, for a circuit fused at 0.25 amps. Is this technically
feasible,
or is my novice understanding currently embarrassing me (without me
knowing
:-)? I have "cut and pasted" the material below from a previous post.
Is
this technology relevant to my question? If so, would I incorporate this
chip into a homemade circuit board with other devices to plug in line to
the
27 VAC output? I am capable of the tech/soldering/schematic reading, but
I
am way short on the engineering.

National Semiconductor have a handy online tool to help you use their
ICs<<

http://www.national.com/appinfo/power/<<

Thank you in advance for reading, and for any thoughts you may have.

Regards,
Dan

Hi, Dan. The National Semiconductor ICs are for converting a DC
voltage to another DC voltage. You've got AC, so your problem is
different.

If your transformer is rated for 27VAC at an 8 amp load, the voltage
will be quite a bit higher if it's unloaded or lightly loaded (1/4A is
just 3% of the transformer rated load). You may already be close
enough -- you should try it and see.

If it's not high enough, though, you'll have to do something else. The
easiest way to vary the secondary voltage in a transformer is to vary
the primary voltage. That would mean you have to bump the primary
voltage up to 119% of nominal (about 142VAC for 120VAC nominal
primary). That's too high -- the transformer isn't meant to work at
that high of a primary voltage. However, as noted above, the secondary
will almost certainly be high for your light load, and you might be
able to get away with 10% overvoltage or less. That would be
acceptable. You can do this with an autotransformer.

If this isn't acceptable, you can add the secondary voltages of two
transformer secondaries. You'd need to find a second transformer with
a secondary voltage around 5VAC, and connect the two secondaries
together to get your 32VAC, like this (view in fixed font or M$
Notepad):

|
| L1 _ 27VAC _
| o----o---o_/ \o--. ,---------o_/ \o-----o
| | FU1 )|( FU3
| L2 | )|(
| o-o--)-----------' '--.
| | | T1 | 32VAC
| | | _ |
| | '---o_/ \o--. ,--'
| | FU2 )|(
| | )|(
| '--------------' '--------------------o
| T2
| 5VAC
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Be sure to measure the output voltage. If your output voltage is more
like 22VAC than 32VAC, it's bucking instead of boosting -- reverse the
leads of the second transformer.

I hope this has been of help. Feel free to post again if you have any
other questions.

Good luck
Chris


Hello Chris,

thank you for your thoughtful and informative message. It is appreciated.
After reading it I went back to look at the schematic for the transformer I
own. As you may or may not know an electronic pinball machine transformer
has a number of secondary windings; these provide the different but
necessary voltages for a pinball to function. For the project I am
contemplating (outlined in my message to James) it turns out there will be
both a 3 and 6 VAC windings I will not need. Would it be possible for me to
"stack" one of these windings on the existing 27 VAC winding, (should it be
necessary) and hence make my life easier? If so, could you draw me one of
those cool schematics as you did before, if it would be different? I have
the first one saved as an M$ Notepad file.

Regards,
Dan


.

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