protecting a circuit

Discussion in 'General Electronics Chat' started by newton, Jan 17, 2008.

  1. newton

    Thread Starter New Member

    Apr 19, 2007
    Hello everyone.

    I am trying to protect a smps input 24V from anything but 0-25/26V. I have tried just a tvs diode in parallel with a regular diode and it worked, until i put 120VAC on it. then it fried the tvs. the regular diode takes care of all negative volts, but i think that tvs diodes fry above their clamping voltage right? well, i think that the crowbar circuit i see all over the web is the way to go b/c an scr can handle a lot of current and voltage...but i don't know how to size all of the components properly. I have attached a schematic and have started by picking out the SCR.

    SCR: STMicro TN1625-1000G-TR.

    I would like to know the order of the component selection and some methodology for finding suitable parts.
  2. SgtWookie


    Jul 17, 2007
    Well, if you trigger an SCR in a DC circuit, the SCR will remain in avalanche (conducting) until either it gets fried, the current across it drops to zero, or the fuse is blown.

    Have you considered MOVs? (Metal Oxide Varistors) They're pretty good at removing "spikes", or HV transients. Of course if the spike is large enough, the MOV will get fried, but that's better than losing the rest of the circuit.
  3. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    You need to better explain the application. There are different solutions for transients (such as lightning strikes, or other momentary over-voltage events) than there is for a continuous event such as someone plugging your 24V thingy into a 120V socket.

    Specifically what types of event(s) are you interested in protecting?
  4. nanovate

    Distinguished Member

    May 7, 2007
    Actually you still want the fuse to blow first ;)
  5. SgtWookie


    Jul 17, 2007
    Well, yeah! ;)

    That's why the fuse goes between the MOV(s) and the wall, and the MOV(s) are between the fuse and the transformer ... no sense in zortching a transformer, eh? Me no likey lugging heavy expensive stuff from the store (well, OK if it's for new stuff...)