Electronic switch

Discussion in 'General Electronics Chat' started by Zeebit, Dec 8, 2013.

  1. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    I am in the process of building a bench PS as a beginner project.

    I am looking for an alternative to the mechanical toggle switch for the output of the power supply. Something that will switch the output on and off without spikes and "bouncing". The ramp up duration is not of concern. I just want a clean power on.

    I know I could just google it but I don't know the proper terms for this thing.

    It should handle a maximum of 25volts @ 3A.
     
    Last edited: Dec 8, 2013
  2. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    Okay so I did a bit of searching and came across an N-Channel Mosfet as a switch.

    The power supply I'm building goes from 1.25v to about 25v, 0A-3A.

    Will an n-channel mosfet work as a switch in this application?
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
    1,105
    An NFET should be ok. It will need protecting from voltage spikes.
     
  4. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    Capacitors?
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    What kind of power supply is this?
     
  6. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
    1,105
    Part of the solution. Importantly there should be a freewheel diode to protect against back-emfs when inductive loads are switched off.
     
  8. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    Yes, I've seen that.

    I would probably need an NMOS driver for this, right?
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
    1,105
    Do you mean an NMOS FET, or a driver for one of those?
     
  10. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    What I mean is, I will need a driver for the NMOS to act as a switch for the output of the power supply
     
  11. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    How does these spikes/noise look like. Do you have any scope pictures. And how big is your filter caps. Is the power switch on the transformer primary or secondary side. Can you also say something why power cycle noise bother you
     
  12. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197

    You could use PMOS for this application. Then a gate boost driver would not be required.
     
  13. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    Unfortunately, I do not have a scope.

    Filter caps after the bridge are made up of seven 2200uf caps in parallel. Power switch is on the primary side.

    Its not really that I'm bothered with the noise, but I've read that this is not very good on the equipment that is hooked to the supply and as well as the regulators. I can't remember exactly where I read this and I can't say that this is a big issue.

    One thing that could be an issue is that I play around with laser diodes and these are very sensitive. Its a pain to have it connected to driver everytime I test one. Its good if the supply could provide clean power.

    http://www.eevblog.com/forum/projects/contact-bounce-for-psu-load-control/
     
  14. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
  15. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Almost.

    With 25 volts available, you will exceed the maximum gate-source voltage of 20 volts for a standard Mosfet. Logic Mosfets have an even lower maximum gate voltage. You can use a voltage divider or a series gate resistor with a zener to limit the gate voltage to say 10-15 volts for a 20 volt rated gate. In fact, the voltage divider setup would be good because it would provide a pullup resistor on the Pmos to be sure it's also fully turned off.

    A toggle switch will bounce when activated causing noise at activation. So if you are concerned about switching noise, you could add a capacitor connected between the gate and ground.

    And, you don't want to go too low on the gate voltage or the Mosfet won't be fully turned on.
     
    Last edited: Dec 8, 2013
  16. sheldons

    Active Member

    Oct 26, 2011
    616
    101
    orrr you can buy fixed output regs with a pin provided to switch the regs output on or off....
     
  17. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
    1,105
    You have a mechanical switch on the input of the supply. Do you want the output to turn on cleanly and automatically a short time after the mech switch is closed? Or will you have a second mech switch to control the output turn on/off?
     
  18. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Shorting the adjustment pot to ground Using an NFET would make the output of the supply go down to 1.2V. Turning off the NFET would make the output go back to whatever the voltage setting is.
     
Loading...