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I have created a schematic for a circuit that I wish to create. I believe the underlying circuit is pretty solid; however, I am having trouble picking parts with the right specs. I have attached the schematic of the circuit I wish to create. My question has to do with the transistor. How much voltage do I need to provide the following transistor to make it fully on. I know the data sheet says it needs 2 volts as its threshold; however, that only allows a small amount of curent to flow through the transistor. How do I know how much voltage I need to make it fully on. Also, how do I determine what type of resistor to use from the 5V GPIO pin to the gate of the transistor.

Here is my circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

My other question is what do I need to look at to make sure the diode is strong enough to handle the EMF kickback. The diode I am looking at is from Jameco with model number 1000 VOLT 1 AMP SILICON RECTIFIER DIODE.

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    \$\begingroup\$ VERY IMPORTANT: You connected a transformer directly to 7805 regulator! There is a good chance that such setup will destroy everything to the right side of the transformer. You need a rectifier, a bulk capacitor and capacitors recommended in the 7805's datasheet for this to work correctly. If the transformer is there to represent wall-wart which outputs DC voltage, than it's safe, but capacitors small capacitors for 7805 will still be needed for good operation. \$\endgroup\$
    – AndrejaKo
    Mar 8, 2013 at 19:00
  • \$\begingroup\$ @jippie: There are a lot of things to discuss with this circuit, so I'd rather not split the question into lots of pieces. That will make is harder to see the whole picture. Think of this more as a design review question than specifically about the FET and a diode. \$\endgroup\$ Mar 8, 2013 at 19:17
  • \$\begingroup\$ @OlinLathrop you are probably right, I didn't notice the missing rectifier stage when I copied the circuit. \$\endgroup\$
    – jippie
    Mar 8, 2013 at 19:19

2 Answers 2

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The gate threshold voltage of a FET isn't all that useful in a design. What you care about is how on the FET becomes when your gate voltage is applied. This is the Rdson spec, which may be listed at multiple gate voltages.

You are using a 2N7000, which is not a particularly impressive FET. At 4.5 V gate voltage, it gets down to 5.3 Ω. That may be OK if the relay coil is significantly more than that, which it probably is. Let's say the relay coil is 80 Ω. In that case the total resistance will be 85.3 Ω, the current at 5 V will be 59 mA, the drop accross the FET 311 mV, leaving 4.7 V accross the relay. Not great, but should be good enough. However, note that this was a example. Do the math with whatever resistance your relay coil really has.

To drive a FET like this, there is no need for a resistor between the digital output and the FET gate. Just connect them directly. The flyback diode needs to be rated for whatever maximum current will go thru the relay coil. A little less will be OK since the diode will only be conducting the current for a short time. Unless you have a "large" relay (in which case the 2N7000 probably won't be good enough), the 1N4148 you show should be good enough to catch the kickback current.

A few problems with your circuit:

  1. You need to rectify the 12 V out of the transformer before running it into the 7805 regulator. Four diodes in a full wave bridge configuration would do nicely. Use something like 1N4001, or a integrated full wave bridge module. Put a cap after the bridge to hold the DC level high enough for the regulator between the line cycles.

  2. The peak voltage of a 12 V sine will be 17 V. Minus about 1.4 V drop for the full wave bridge, you are left with 15.6 V at the peaks. When unloaded, it could be a few volts more. This is well within the voltage spec of a 7805, but will cause it to dissipate a lot of power. If you can find a "6V" transformer, that would be a better choice.

  3. The relay can most likely work from AC, but the FET can't switch AC. The top of the relay must also go to the output of the full wave bridge instead of the transformer directly. Keep in mind that the relay will then see the higher unregulated voltage, which could be 15-20 V with this transformer. That might be OK for a "12v" relay, but you might need to put a resistor in series with it to stay within the maximum allowed coil current spec.

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  • \$\begingroup\$ This is the best answer I have ever heard on this. Thank you so much. I know about the rectifier, I just forgot to add it. One follow up question, where do I find the resistance between the gate and drain. I trust you that there is no need to put a resistor between the GPIO and gate, but how did you know that the current would be less than the 20mA limit on my microcomtroller? Do you have any good way to simulate these things before I actually build it ? \$\endgroup\$
    – Ryan F
    Mar 8, 2013 at 22:05
  • \$\begingroup\$ @Ryan: The gate of a MOSFET is insulated from the rest of the FET. That's what the MOS in MOSFET refers to. The resistance is very high. It is basically infinite except for leakage. Mostly the gate looks like a capacitive load. The digital output will be fine with that. \$\endgroup\$ Mar 8, 2013 at 23:04
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To the first question : look at the Rds(on) specification : there are two (at 75ma and at 500ma). In each case Rds(on) is given for a specific gate voltage. Pick the more appropriate current and use the given gate voltage (4.5V for 75ma, 10V for 500ma)

In a less ideal world, that voltage may be unachievable (say you need 500ma and don't have 10V). In that case use Fig 7 to show you the Rds(on) you are likely to achieve.

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