I am assuming that by "alt code" you mean Windows/IBM compatible Alt codes for character entry.
Because you did not include a character in your post, it is difficult to say which character you want. I do not believe that exact character exists in Unicode, and consequently you'll need a custom character set and font to use it. I'll list a couple that exist in Unicode that might come in handy, along with the Unicode code point and name:
⎓: U+2393 DIRECT CURRENT SYMBOL FORM TWO
⏦: U+23E6 AC CURRENT
∿: U+223F SINE WAVE
You can type Unicode code points by holding down the Alt key, pressing "+" and entering the hex value of the code point.