Solid State switch for PS - Page 1
Forum section: electronics / projects

author: Zeebit
author_rank: Newbie
timestamp: December 08, 2013, 11:07:04 am
content: Hello everyone,I am a beginner trying to make a bench power supply. Go easy with the technical terms if you can.The PS I am making is taken right from the LM317 datasheet from National. Here is the circuit. The author of the page is probably a member here too.http://quan-diy.com/projects/benchps/benchps.htmA few modifications will be done on the circuit to suit my needs.-------------------------------------------------------I have read that it is not good to use mechanical switches on the output of PS so I thought I'd make use of a solid state switch instead. I did a bit of googling and I came across transistors being used as switches.Will using an n-channel power mosfet work as an output switch for the PS I'm making?

author: Skimask
author_rank: Super Contributor
timestamp: December 08, 2013, 11:32:01 am
content: No.

author: Zeebit
author_rank: Newbie
timestamp: December 08, 2013, 11:48:08 am
content: Why?What are my options?

author: Skimask
author_rank: Super Contributor
timestamp: December 08, 2013, 01:02:04 pm
content: Time to open up your books and figure out why an N channel MOSFET won't do what you want it to do...Or I (or somebody else) could spoon feed you.How would you like to be known?

author: Zeebit
author_rank: Newbie
timestamp: December 08, 2013, 01:09:37 pm
content: Electronics is not my career. Its just something I spend my free time on. I don;t have any useful reference on hand.If you could spoon feed me that'd be great.

author: madires
author_rank: Super Contributor
timestamp: December 08, 2013, 02:13:31 pm
content: Please have a spoon full ofthis.

author: Zeebit
author_rank: Newbie
timestamp: December 08, 2013, 03:56:50 pm
content: Hmm. If I read that correctly, it won't work because the gate voltage needs to be higher that the drain voltage.What are other options for switching the output on and off?

author: M. András
author_rank: Super Contributor
timestamp: December 08, 2013, 05:53:49 pm
content: Quote from: Zeebit on December 08, 2013, 03:56:50 pmrelay/double pole switch

author: minibutmany
author_rank: Regular Contributor
timestamp: December 08, 2013, 06:49:11 pm
content: For a simple PS project a mechanical switch isn't going to be a problem. If you don't like the clunking noise you can use a solid state relay.

author: Jon86
author_rank: Frequent Contributor
timestamp: December 08, 2013, 09:45:43 pm
content: No, and N-Channel mosfet should work just fine. You're going to have some drop in your regulator, so if you get the gate voltage straight off your transformer tap or some higher voltage, you should be fine. Otherwise, just use a switch or a relay. Electronic switching really isn't necessary, and you're just going to increase your power losses.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 09, 2013, 12:14:51 am
content: By "output switch", you mean a switch that turns the output of the supply on and off while the input power to the PSU is on, right? I would not use a N-channel MOSFET because no matter if you can get the gate voltage high enough, it will drop SOMETHING across the source and drain in order to turn on. I suppose it depends on the RDS,on of your transistor, but I don't see the need to add another source of error, here ...I would use a relay, and if you want, put a small ceramic/film capacitor across the output terminals of the relay to deal with any high voltage/frequency bullshit that might occur when you turn it on and off. (someone please verify if this will actually be helpful)I should go see how my most recent power supply purchase accomplishes this function.

author: Zeebit
author_rank: Newbie
timestamp: December 09, 2013, 01:15:35 am
content: Okay then. I'll ditch this and just use a mechanical switch.Thanks everyone!