Function of resistor in specific circuit?

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DaveP

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The 470 ohm resistor is a current limiting resistor for the LED circuit. If you exclude the 470 ohm resistor, the LED will glow very bright for a while and then burn out.

The 10k resistor is a pull up resistor that is used to put the idle voltage to the MOSFET gate in the range where it will conduct properly. When you press the "tactile switch" this applies a positive voltage from the battery directly to the gate on the MOSFET and the MOSFET conducts voltage to the coil.

When you release the switch, the gate voltage goes back to whatever the 10k resistor is supplying when the ecig is off. The resistor assures that the idle voltage is in a range that does not allow the gate to conduct until you press the switch. Without the 10k resistor, the gate would be in an indeterminate state and might trigger unexpectedly.

EDIT: Big Blue was posting at the same time I was! We agree on the theory of the circuit. BTW, I worked for Big X for 36 years! Big Blue guys will remember the competition of years ago between our companies if your Big Blue was a large computer company.
 
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AttyPops

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Bigblue30 is back! Hey blue!

Allyn..... Blue explained it well. Another explanation is:

Without that resistor it would just be a "dead-short" to the ground side of the battery when the switch is closed. With the resistor, the logic level of that gate is High when the switched is closed, and low (ground state) when the switch is open, but it's limited by the resistor and not a short-circuit. It's a pull-down resistor.

Edit: lol. I saw BB's post but not yours, DaveP, until now. lol. Everyone's posting.
 
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Allyn

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Ok, I think I understand now. It took several readings of the answers and re-tracing the wiring diagram a few times, but I think I got it. With the master switch in the on position and the tactile switch in the off position, voltage to terminal 1 (gate) is negative, but with relatively little current. I'm guessing just enough current to firmly keep the mosfet switched to off. When the tactile switch is closed positive voltage is applied with enough current to reverse the flow of electrons through the 10k resistor. Most of the positive current continues on to terminal 1 switching the mosfet to on while a small amount bleeds through the 10k resistor back to the negative post of the battery. Efectively, the 10k resistor is acting sort of like a spring in a relay. Remove the spring and the relay may fire even though there has been no signal to fire it. However, replace the 10k resistor with a piece of wire and when the tactile switch is pushed there is a dead short to the negative side of the battery and no current flows to terminal 1.

Is that about right or am I still missing something? Also, how do you determine what value of resistor to use if say you wanted to use a different MOSFET?
 

DaveP

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The function of the resistor is to proved a path to ground for the gate. This stabilizes the gate so the no current flow will occur and the voltage will be essentially zero. When you press the switch, a positive voltage is applied to the gate. The resistor now isolates the positive voltage at the gate from being taken to ground potential, as it would if the gate were simply tied to ground to provide a ground with no signal applied.

You could supply a standby low to the gate by just tying it to ground, but then when you pressed the switch, the positive battery terminal would be sent directly to ground through that hard link. The resistor prevents that from happening. The 10k value isolates the gate but still can provide a high impedance ground reference when no gate voltage is supplied. It limits the amount of current that can flow to ground, so when you press the switch, you get little conduction to ground in terms of milliamps, to the pulse can remain high enough to trigger the gate when you push the button. Sample circuits are usually supplied with the device's specification sheet. The spec sheet should list the recommended value.
 
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bigblue30

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EDIT: Big Blue was posting at the same time I was! We agree on the theory of the circuit. BTW, I worked for Big X for 36 years! Big Blue guys will remember the competition of years ago between our companies if your Big Blue was a large computer company.

Yep. I pick the Big Blue 30 when I had 30 years with them. (that was a few years back....lol). I retired 2 years ago. What group were you with? Send a PM if you do not want to say here.
 

WillyB

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The 10K is there to make sure the Mosfet does not "fire" until the switch is pushed. The 10k "ties" the gate to ground (minus side of the battery.) Without it the Mosfet might turn on when you do not want it to. It also makes sure that the Mosfet turns off when the switch is opened.
And doesn't it also limit the amount of current the switch needs to handle? The same way a resistor and LED work together?
 

kjj

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Not really, no. Current doesn't flow through a MOSFET gate like it does in a BJT base.

In a NPN BJT, the current flows from the base to the emitter, and each little bit of current can pick up a much larger bit of current from the collector, allowing the whole thing to act like an amplifier. It gets tricky to keep right in your head if you think of electrons as the bits of current, since those flow in through the collector and out through the base and emitter, but as far as the conventional positive current model goes, it mostly works.

In a N-channel MOSFET, however, there is virtually no flow of current from the gate to the source, at least not until right before the magic smoke gets out. Instead, the gate is like a capacitor, collecting a charge imbalance relative to the source. When that imbalance is high (aka when the capacitor is full), current can flow from the drain to the source. When the imbalance is low, or negative, current can't flow.

PNP and P-channel are similar, but backwards.

Think of the base current as smugglers sneaking through the gate, while the gate charge is thugs scaring the guards away, without actually crossing themselves.
 

kjj

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Ahh, I was looking at the MOSFET and the resistor, not the switch and the resistor.

While you don't need to worry about gate current, you are right that you do need to watch out for the switch. The good news is that even with small switches, you are pretty safe picking a resistor at random from your pile.

On the low end, 500 ohms, for example, would be just fine on a 50ma switch. I think you could go down to about 85 ohms on a single 3.7 volt battery (assuming a full charge around 4.25 volts), but it'll be burning close to a quarter watt. With 1/8 watt resistors, I wouldn't go much under 150 ohms. With two batteries, the minimum is probably more like 300 ohms.

On the high end, 50M would also work. Of course, the MOSFET would take about a quarter second to gradually turn off when you released the switch. You probably wouldn't notice.

10k is used in most examples for three good reasons. One, the current on the switch is then tiny enough to disregard almost all of the time. Two, it supports switching speeds up to like 20 or 40 kHz in case someone blindly copies the design into an active device (like a PIC doing PWM). Three, it is a big round number and you can hardly get around in a Radio Shack without tripping on one.
 
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