Shop OBEX P1 Docs P2 Docs Learn Events
variable voltage regulator — Parallax Forums

variable voltage regulator

damiondamion Posts: 39
edited 2006-07-03 16:31 in General Discussion
Hello,
I'm using a variable voltage regulator to take·5v down to 3.3v. Every thing is hooked up properly and the voltage adjusts appropriately but when I hook the regulator up to·the transciever that it is powering my voltage drops to 1.24v and the regulator is no longer adjustable. As soon as·I unhook·it from the transciever it becomes adjustable again. Any advice?
·

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2006-06-26 20:15
    You may have a short circuit and the regulator is protecting itself by reducing the voltage.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-07-03 14:49
    Firstly, a 7805 have a dropout at 2 volts above the regulated voltage.
    So you just cannot use it to get a stable 3.3volts.

    I suppose an LM350 or LM317 have a similar dropout limitation regardless of their being adjustible. Try 6 volts and see if you can get a stable output.

    You would have to be using a low dropout regulator such as an LM2940-3.3 that has a dropout of 0.5volts above the regulated voltage. This would make a whole lot more sense.

    Secondly, the third wire on the regulator senses the differences around a set voltage of 1.25volts [noparse][[/noparse]maybe you read 1.24, but that is certainly in that ballpark]. Seems to me that it is likely that nothing is wrong with the regulator, just the design.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    "If you want more fiber, eat the package.· Not enough?· Eat the manual."········
    ···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • jb1311jb1311 Posts: 20
    edited 2006-07-03 16:31
    If you are using an LM317 in the TO-220 package. The pin outs are not the same as the 78XX series.
Sign In or Register to comment.