MPapps Posted March 6, 2007 Report Share Posted March 6, 2007 Hi All,I am trying to design a circuit to do the following:Use a Momentary Push To Make Switch to toggle the input of a 7805 on/offThe 7805 is supplied by regulated 12v so i need a circuit that switches its output between 0v and 12v using a Momentary Switch.I was hoping to find a design or at least a starting point somewhere but have had no luck so far.Any Suggestions / ideas would be great.Thanks :) Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 7, 2007 Report Share Posted March 7, 2007 You could connect a Mosfet in series between the output of the 12V supply and the input to the 5V supply.It could be turned on and off with a momentary button that controls this "alternate action" circuit: Quote Link to comment Share on other sites More sharing options...
MPapps Posted March 11, 2007 Author Report Share Posted March 11, 2007 Would it work by using BJTs like this?Thanks for your help Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 11, 2007 Report Share Posted March 11, 2007 Hi MPapps,1) Your circuit doesn't need base-emitter resistors because the output from a Cmos inverter goes to the supply voltage.2) Your circuit doesn't need the NPN transistor because Cmos inverters can operate fine from the 12V supply and can drive the PNP transistor.3) The 2N3703 transistor has a very wide range of current gain. Its lowest gain could be only 30. A 2N4401 transistor has a minimum current gain much higher, 100 at a collector current of 150mA.Try it like this:Corrected Quote Link to comment Share on other sites More sharing options...
MPapps Posted March 11, 2007 Author Report Share Posted March 11, 2007 Hi,Thanks for you help, but i'm a little confused isnt a 2N4401 a NPN Transistor and you have it shown as a PNP?Thanks. Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 12, 2007 Report Share Posted March 12, 2007 Thanks for you help, but i'm a little confused isnt a 2N4401 a NPN Transistor and you have it shown as a PNP?OOPS.Yes, you are correct. It is a 2N4403 transistor. I have corrected the schematic. Quote Link to comment Share on other sites More sharing options...
MPapps Posted March 12, 2007 Author Report Share Posted March 12, 2007 Looking at the datasheet do i not need a transistor with a higher current rating as my Voltage Reg could be drawing as much a 1A?Thanks Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 12, 2007 Report Share Posted March 12, 2007 Looking at the datasheet do i not need a transistor with a higher current rating as my Voltage Reg could be drawing as much a 1A?You forgot to say how much is its load current.A 7805 voltage regulator IC has a max operating current of only 8mA. It regulates very well with a load current up to 1A but can supply up to 1.5A with a small drop in regulation. Most of them limit the current at 2.2A.The max current for a little 2N4403 transistor is 600mA but the 10k base resistor in my circuit limits it current to about 150mA. The CD4069 cannot drive a resistance lower than about 10k.Use a darlington PNP transistor for higher current. Quote Link to comment Share on other sites More sharing options...
MPapps Posted March 12, 2007 Author Report Share Posted March 12, 2007 Hi,I must admit i'm a little confused by what you said, my output of my 7805 will by required to supply current upto the 1.5A, so what circuit solution can i use for that?Thanks. Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 12, 2007 Report Share Posted March 12, 2007 I must admit i'm a little confused by what you saidI said:1) The max current for a little 2N4403 is only 600mA.2) The 10k base resistor for the 2N4403 limits its current to only about 150mA.3) The CD4069 cannot drive a resistance lower than about 10k.4) Use a darlington PNP transistor for higher current.You don't understand what?Do you know a TIP125 power darlington PNP transistor? Quote Link to comment Share on other sites More sharing options...
MPapps Posted March 13, 2007 Author Report Share Posted March 13, 2007 ok thanks, what modifications would i have to make to run off 13.4v?Thanks Again. Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 13, 2007 Report Share Posted March 13, 2007 ok thanks, what modifications would i have to make to run off 13.4v?The CD4069 IC has a max operating voltage of 18V and the TIP125 darlington transistor has a max operating voltage of 60V so they are fine with only 13.4V.If your 7805 regulator has a current of 1A then it will try to dissipate 7.7W and it will probably get too hot and shut down unless it has a pretty big heatsink. Quote Link to comment Share on other sites More sharing options...
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