Is this what you described? +Ve ------F---,------,------- | | Z | | a '---g-SCR | c R | | | Gnd ----------'------'------- So the SCR draws the current to short the fuse so that the zener does not have to have a high current rating. My circuit has a surge requirement of 2A, so the fuse is pretty big. This seems to make it so that a small zener can crowbar a large current. I am guessing that you would size the resistor so that at the maximun supported overvoltage, the zener would see its maximum rated current (Ve-5.1)/R = Izmax. What happens when a reverse polarity is applied? I am not up on SCR's. Would it turn on in this configuration and allow a reverse current to flow through it? Since the zener would be forward biased, almost the whole -Ve would drop across the resistor so R would have to be bigger to keep the zener from blowing up. But if the SCR does not conduct in this direction, it all for not because the fuse would not blow. Maybe you would add the mosfet from the previous circuit to protect against reverse polarity making... +Ve ---,--F---,------,------- | | | | Z | | | a | '---g-SCR | | c | R | | | | Gnd ---M------'------'------- Does this make sense? --BobG -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Steve Smith Sent: Monday, May 19, 2003 2:15 PM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [EE]: Overvoltage protection and Resetable fuses? The most conventional form of this circuit is the crowbar (cant do ascii art but heres a discription) the power +ve enters the circuit via a fuse to the cathode of a small 5v1 zener and the anode of an scr. The gate of the scr is connected to the anode of the zener and a low value resistor. The cathode of the scr and the other end of the resistor is the gnd end. It works the same and will rupture the fuse wen the scr is biased on (about 5.6v) it dependeds on the gate senisitivity of the scr but around 0.5v higher than the zener voltage (BT152) it then shorts out the logic line and blows the fuse there are no circuit losses except the watts loss in the fuse and the SCR will not require a heatsink. The only requirement is that the AsqS of the SCR is greater than the prearcing AsqS of the fuse. Regards Steve... -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Robert E. Griffith Sent: 19 May 2003 16:57 To: PICLIST@MITVMA.MIT.EDU Subject: [EE]: Overvoltage protection and Resetable fuses? I am considering the following circuit to protect against the wrong PS being connected to my circuit as follows (it expects regulated +5V) . Vin -----------.------------- Circuit Vcc (+5v) | R | Fuse | ,--, | | Z | | | Gnd --------'--M------------- Circuit Gnd R = resistor (limits inrush to charge the mosfet gate) F = fuse (lowest low trip currnet available) Z = zener diode (around 5.3+V) M = N-channel mosfet (FDN337N) The mosfet protects against reverse polarity (an idea given to me on this group a few months ago). The idea with the zener, fuse, and resistor is that if Vin exceeds the zener voltage, current will flow through the fuse, tripping it. If Vin is the specified +5v, no current flows through the zener so the only current through the fuse is the inrush to charge the mosfet's gate. The resistor is there to limit this inrush so that it does not trip the fuse. Also, I am thinking resistor will help protect the zener from over current in the time before the fuse blows (not sure if that is needed). That is why the resistor in on the common branch and not on the mosfet lead, parallel to the zener. So, Question 1), is this a reasonable circuit? Question 2), Would it be a good idea to add a 2nd zener across the Circuit Vcc to clamp the voltage in the time before the fuse blows? Question 3), Can I use a PTC resettable fuse such as Bourns MF-SM050-2? The datasheets for PTC resettable fuses list a trip current and a hold current. I do not understand the hold current. I am thinking that when a fuse blows, it's a short and there is no current. But resetable fuses must be more complicated than this - how does the fuse know it can reset if it does not sample the situation to see if the overcurrent still exists? Maybe the PTC fuse would just limit the current to hold current which would defeat the circuit by clamping the mosfet's gate to the zener voltage which would keep the mosfet turned on. Thanks for your help. --BobG -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.