So I'm busy with an automotive project whereby a PIC12F675 monitors the vehicle battery voltage. When the voltage increases to above 13.2V (indicating that the vehicle has been started), it delays for 10 seconds and then has to switch on a solenoid. The solenoid in question is a Cole Hersee 85A (http://www.colehersee.com/home/item/cat/174/24059/). Its coil seems to draw around 700 mA when powered. When the battery voltage drops below 13.2V it disengages the solenoid. I am using an IRF9Z34N MOSFET to drive the solenoid coil.
Everything worked great until I fried the MOSFET. At first I could successfully switch the solenoid on and off for a number of times. At that stage I was only testing my circuit, so only the solenoid coil was connected while the solenoid contacts were not connected to anything. When that worked, I then connected the solenoid contacts (one end to the vehicle battery and the other end to a load). On the first test the solenoid switched on and wouldn't switch off again. The MOSFET now gets hot and won't switch off. I'm not sure if it was the switching on or the switching off that killed it...
I have attached a schematic showing my connections from the PIC to the MOSFET. I have also attached a graph of the rise time of Vgs on the MOSFET (2V and 50us per div). I was worried that the rise-time was too slow and that consequently the MOSFET didn't turn on fully quickly enough, but that is evidently not the case. This measurement was done WITHOUT any load connected to the MOSFET. Don't know if that will make a difference.
One assumption I made is that because this MOSFET has a built-in diode, I would not be needing an additional diode across the solenoid coil. So I did not connect a diode across the solenoid coil. Was my assumption wrong here?
Any ideas as to why I am breaking my MOSFET? I only have one left so I would prefer not destroying another one...
Everything worked great until I fried the MOSFET. At first I could successfully switch the solenoid on and off for a number of times. At that stage I was only testing my circuit, so only the solenoid coil was connected while the solenoid contacts were not connected to anything. When that worked, I then connected the solenoid contacts (one end to the vehicle battery and the other end to a load). On the first test the solenoid switched on and wouldn't switch off again. The MOSFET now gets hot and won't switch off. I'm not sure if it was the switching on or the switching off that killed it...
I have attached a schematic showing my connections from the PIC to the MOSFET. I have also attached a graph of the rise time of Vgs on the MOSFET (2V and 50us per div). I was worried that the rise-time was too slow and that consequently the MOSFET didn't turn on fully quickly enough, but that is evidently not the case. This measurement was done WITHOUT any load connected to the MOSFET. Don't know if that will make a difference.
One assumption I made is that because this MOSFET has a built-in diode, I would not be needing an additional diode across the solenoid coil. So I did not connect a diode across the solenoid coil. Was my assumption wrong here?
Any ideas as to why I am breaking my MOSFET? I only have one left so I would prefer not destroying another one...