Your circuit shows a voltage divider in the MOSFET gate circuit. With a high of 3.3V from the Propeller, the gate will see only 1.67V — not enough to turn it fully on. If the purpose of R1 is to limit inrush current to charge the gate capacitance, 100 ohms will suffice.
But even 3.3V won't be high enough for some "logic level" MOSFETs, since many were designed for 5V logic. Be sure to check the specs to make sure it will saturate with 3.3V on the gate.
-Phil
Thanks Phil, I will reduce it to 100ohms. I have seen some applications which have used 22ohms even.
The ITL530 which mike suggested has a gate threshold of 1 to 2V (Vgs).
Good call Phil, the original intention of the gate resistor was to include protection in case the mosfet shorted out. The 100k became the solution, so back to the 200ohm, if there is a short on the mosfet, the pin on the Prop will "possibly" be destroyed if it sees 10V with no current limiting. An opto is ideal to solve both problems, protection and providing more than enough Vgate to saturate.
Post Edited (originator) : 3/19/2007 11:10:32 PM GMT
I think that I have established that I'm not engineer. Your question was extremely useful to me... because I have a similar triggering issue having to do with a camera and a huge flash...
Not to push the conversation backwards or anything but I absolutely loved the circuit by originator... would it work for your app? Did anyone see a real problem with it?
The original idea I posted will not work with the 2nd opto, but use a pin for continuity and there is no problem with it. I use that similar circuit on many things.
If you have wires going off the main Propeller circuit board, where they "may" get shorted to voltages above above 3.3V, then there should be a method of protection where the wires are frequently getting conneted/disconnected, and the opto does this protection very well. Other options are based on opto, like DC or AC solid state relays, LED based. You should have something like a 200ohm on a pin that is going off board, in case of dead short to GND if the Props outputs is high, the pin will survive most likely.
In the case as Phil pointed out, the gate voltage has to meet some threshold to turn on, so the highter the series R, the lower the voltage to the gate(assuming there is a pulldown forming the divider, the pull down is very useful for safety).
A side benefit of the opto is level translation, all that needs to happen to turn on the opto is for the led to turn on(turn on enough, but 200ohm off a Prop will suffice). The output voltage of the opto IC is set by the volage applied. Some cases require fast speed, in which a suitable "fast" opto should be chosen. To launch a rocket, speed is not an issue.
If you socket the opto, and if it dies because of a large voltage applied to it from off he board, then no big deal. If your Prop is SMT, then it is a bigger deal to replace the Prop than a socketed opto.
Post Edited (originator) : 3/19/2007 11:57:11 PM GMT
I think that I have established that I'm not engineer. Your question was extremely useful to me... because I have a similar triggering issue having to do with a camera and a huge flash...
Not to push the conversation backwards or anything but I absolutely loved the circuit by originator... would it work for your app? Did anyone see a real problem with it?
Rich
Nor am I an engineer...yet
I have attached a modified version of orig's diagram with a few modifications of how I interpreted the changes that need to be made. (Lets hope I finally got it right!)
Also, attached is a non-opto version with the series resistor in place. (Again, lets hope this ones right!)
Your new circuit looks good. The opto will unquestionably ensure sufficient gate drive. In fact, you might even double check VGSMAX for this device to make sure 12V isn't too much. If it is, you can always revert to the IRF version.
That is a good one. One last thing, if you are using a logic level mosfet, make sure it can handle the 10V at the gate. If you have a batch of logic level mosfets already on hand that you prefer to use, just divide the gate input off the opto as you have already learned how to do.
An IRF540 is one mosfet that will be fine with your circuit.
Comments
Lol, I will have to get a rocket first
My last rocket had its final flight not long ago, the fins just couldn't take it
I am going to get this controller running then look for a rocket to test it in, im leaning towards this one, a 2 stage.
Thanks Phil, I will reduce it to 100ohms. I have seen some applications which have used 22ohms even.
The ITL530 which mike suggested has a gate threshold of 1 to 2V (Vgs).
Post Edited (originator) : 3/19/2007 11:10:32 PM GMT
I think that I have established that I'm not engineer. Your question was extremely useful to me... because I have a similar triggering issue having to do with a camera and a huge flash...
Not to push the conversation backwards or anything but I absolutely loved the circuit by originator... would it work for your app? Did anyone see a real problem with it?
Rich
If you have wires going off the main Propeller circuit board, where they "may" get shorted to voltages above above 3.3V, then there should be a method of protection where the wires are frequently getting conneted/disconnected, and the opto does this protection very well. Other options are based on opto, like DC or AC solid state relays, LED based. You should have something like a 200ohm on a pin that is going off board, in case of dead short to GND if the Props outputs is high, the pin will survive most likely.
In the case as Phil pointed out, the gate voltage has to meet some threshold to turn on, so the highter the series R, the lower the voltage to the gate(assuming there is a pulldown forming the divider, the pull down is very useful for safety).
A side benefit of the opto is level translation, all that needs to happen to turn on the opto is for the led to turn on(turn on enough, but 200ohm off a Prop will suffice). The output voltage of the opto IC is set by the volage applied. Some cases require fast speed, in which a suitable "fast" opto should be chosen. To launch a rocket, speed is not an issue.
If you socket the opto, and if it dies because of a large voltage applied to it from off he board, then no big deal. If your Prop is SMT, then it is a bigger deal to replace the Prop than a socketed opto.
Post Edited (originator) : 3/19/2007 11:57:11 PM GMT
Nor am I an engineer...yet
I have attached a modified version of orig's diagram with a few modifications of how I interpreted the changes that need to be made. (Lets hope I finally got it right!)
Also, attached is a non-opto version with the series resistor in place. (Again, lets hope this ones right!)
So like this version
Yeah, ive had it in mind for a while. There's a bit more to it than firing the pyro's, will post when I have more finished (or need more help lol)
Your new circuit looks good. The opto will unquestionably ensure sufficient gate drive. In fact, you might even double check VGSMAX for this device to make sure 12V isn't too much. If it is, you can always revert to the IRF version.
-Phil
An IRF540 is one mosfet that will be fine with your circuit.
Next step, accellerometer