Like this, it is simple with minimum of equipment. :) :)

Peter van Hoof wrote:
> In 99 % of the cases red and red goes on one and black and black on the other pole.
>
> If you want to make absolutely sure the primarys can be connected black to black
> and red to red put the two reds on line of 110volt ac.
>
> Put one of the blacks on neutral (leave the secondary open for now)
>
> Measure voltage between the two blacks. if your voltage is near 0 volt ac polarity is correct
> and line belongs on the w=two reds, neutral on the two blacks (or reversed)
>
> If the voltage is near 220 volt one of the two coils needs to be reversed. (black of one coil
> and red of the other on phase, and the other two on neutral.)
>
> Peter van Hoof
>
>
>
> ----- Original Message ----
>   
>> From: Harold Hallikainen <harold@hallikainen.org>
>> To: Microcontroller discussion list - Public. <piclist@mit.edu>
>> Sent: Monday, July 13, 2009 6:15:09 PM
>> Subject: Re: [EE] Transformer wiring help
>>
>>
>>     
>>> I understand the center tap output with my choices for +12, -12  or 24
>>> VAC, Am thinking for 220 VAC, I want to connect the center red and black
>>> together, and the outer red and black go to the 220 VAC. For 110 volt,
>>> do I connect the reds together to say the hot 110, and the blacks
>>> together to the neutral 110?
>>>       
>> Yes, that's right. For 240VAC, connect the two primaries in series. For
>> 120VAC, connect the two primaries in parallel. This is equivalent to a
>> single primary with double the wire size (cross-sectional area).
>>
>> Harold
>>
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