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Capacitor, Resistor and MOSFET ... a newby question

animals

Sep 1, 2010
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I'm still trying to get my head round the simple stuff.

This circuit: http://archilochus.netfirms.com/images/q_timer.jpg am I understanding it correctly?

The current runs through the diode and, as the path through the capacitor gives less resistance (and as the transistor is 'open'), through the capacitor which charges to its nominal value.

This is where I'm stuck. What causes the capacitor to drain? As it's fully charged has it become more resistant than the resistor?

I'll have to build this later. But first, I think I need someone to state the plain obvious so that I can understand.
 

Mitchekj

Jan 24, 2010
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It may be helpful to step back a level and concentrate on the parts' functions. You may be thinking purely theoretically, while these parts in question are serving in more of a 'real world' problem solving application.

The cap is there to filter unwanted noise off of the signal. It's most likely a small value (nano to pico farads perhaps) to get rid of any undesireable high frequency interference which could hinder the FET switching how you want it to.

The resistor in this application is called a "pull-down" resistor, as it's making sure that the gate is 'low' when there is no signal coming from your IC, thus the FET will be off. Getting deeper into the theory of things, it has a couple of roles: when the signal from your IC goes low (or off) it's acting as a path to drain the charge from the FET's gate capacitance, perhaps to shorten the turn-off time, as well as holding the voltage at ground potential to keep the FET from turning back on until the IC tells it to again. The value should be relatively high (10k, 100k, sometimes 1M all depending on a number of factors) in order to not shunt the gate drive current while the signal from your IC is high. In actuality, you may be losing a few microamps (plus or minus, depending) through the pull-down when driving the FET on, but it is of no real concern in this circuit.
 

animals

Sep 1, 2010
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Sep 1, 2010
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I've totally misunderstood the circuit (and its function) then.

Not the on/off/on/off flashing as I expected.
 
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Mitchekj

Jan 24, 2010
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The part that you posted looks like it's sole purpose in life is to take a signal from a timer, and trigger a camera?

They're using the FET since the IC cannot source enough current to trigger the relay coil, which is a common problem. Think of the FET as a switch. The Gate is the control lever... you apply voltage (say 5 to 20Vdc depending on the type) and it closes the switch (low impedance from drain to source.) This will allow current to flow through your relay coil by introducing ground to the other side of the coil, closing the relay's contacts and triggering your camera. When you take the voltage away, the FET opens (high impedance from source to drain.) FETs happen to be finicky with their gates, which is why you usually see these caps and resistors used as they did in the schematic you posted.

Now that's a high-level explanation of a FET, mind you... there's much more in-depth doctorate level work going on inside of them, which I don't mind not understanding. :)

Edit: Ah, the on/off/on/off flashing will be controlled by the IC (not shown on your schematic.) If the IC sends a square wave (hi/low/hi/low) then your camera will flash at the same rate. That's where the cap and resistor come into play... to help facilitate these changing states, and it's all becuase of the physical properties of the FET and the rest of the circuit, namely gate charge/capacitance, noise, etc.
 
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