SCR Arragment for power supply output - Page 1
Forum section: electronics / projects

author: GoatZero
author_rank: Regular Contributor
timestamp: March 17, 2014, 12:52:44 am
content: School project time, this circuit its supposed to go in the output of my 10A power supply, The transistor activates the varistor in order for it to make a short circuit and activate the fuse, this way the current flows though the load and makes the led turn on, which indicates the fuse needs to be replaced, however this might work for a 3A power supply, but im making and 10A one, so in order for it to work i would need to use a way to big wattage resistor,So i was told that i need to replace it with an SCR diode, that what i was told, i tried google with little luck, since i did not understand much about the topic and i cant find anything at newark that allows me to put 1 and 1 together,does anyone know how can i use a SCR diode to accomplish the same as i described at the start of my post?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2014, 01:22:57 am
content: Look for "SCR crowbar".

author: GoatZero
author_rank: Regular Contributor
timestamp: March 17, 2014, 04:43:19 am
content: thanks

author: GoatZero
author_rank: Regular Contributor
timestamp: March 17, 2014, 05:07:34 am
content: i found a note that my professor did for me, and i just made the schematic for it, pic related820 its the total impedance of my circuit, reason of why i will use it as R4, however im still trying to understand the SCR diodes and how exactly would this circuit work, apparently the push button normally open helps me reset it, however i still dont know how can i calculate R3 and what the inductor doing there, has anyone done this kind of circuit before?mostly i would like to try to understand this circuit in order for me to calculate it and then implement it

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2014, 07:10:04 am
content: Hmm, a 2-20V crowbar; anyone want to guess when it fires?Sad to say your professor doesn't know much, if that's what he drew...I suppose that should be to your advantage, as presumably he won't know very deep questions to ask.Tim

author: marabut
author_rank: Contributor
timestamp: March 17, 2014, 02:40:05 pm
content: I'm not sure what exactly you want to achieve but none of presented circuits will work.Hint: draw the way current flows before and after triggering the "crowbar" circuit. And at least select input and output terminals (for now, I see only multiple outputs there).

author: GoatZero
author_rank: Regular Contributor
timestamp: March 17, 2014, 05:01:34 pm
content: mmm i have been digging a bit more the drawing, it was not an inductor as i though, it was a relay and it turns out the circuit looks more like this hopefully someone will recognize it nowMy bad, the drawing was so fast and so convoluted that i didnt notice until now

author: marabut
author_rank: Contributor
timestamp: March 17, 2014, 05:38:38 pm
content: Looks better but... still it's a good example of bad engineering.Think of the moment the circuit will be triggered. At which exactly voltage SCR will be switched on? Can you figure out the value?Read datasheet of one of popular SCR, for example TIC106. In your circuit it will be triggered when gate current will be grater than Igt (gate trigger current). The variation of this parameter is from 5uA (typical) to 200uA (max). And additional error you get from temperature drift of gate trigger voltage (20% when temperature rises from 20 to 75 deg.C).Why not to start from verified circuit like this:http://www.next.gr/other-circuits/switch-circuits/resettable-shutdown-circuits-circuit-l15008.htmland then make necessary modifications?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2014, 06:18:57 pm
content: Note that a relay takes tens of miliseconds to operate, so this will not protect against overvoltage surges.In contrast, a properly rated SCR crowbar will protect the load within a microsecond. Depends what you are trying to achieve.Tim