Discussion in 'The Projects Forum' started by huwilerp, Jan 11, 2013.

  1. huwilerp

    Thread Starter New Member

    Jan 10, 2013
    I am looking at a data sheet for the IRF510A (Fairchild Semiconductor). I am curious where on the data sheet does it mention the amount of voltage that needs to be applied to the game to allow current to be allow to flow from the drain to the source. What else on the data sheet is important and why? I know general question but I figured I would ask the experts on here.
  2. JDT

    Well-Known Member

    Feb 12, 2009
    You mean: what voltage has to be applied to the gate to cause drain-source current to flow. This is the "Threshold Voltage" (Vth). There will be a graph in the data sheet. And might be specified but will vary a lot from device to device.

    What else do you need to know?

    • Maximum drain current - will be specified.
    • Voltage drop at various drain currents and gate voltages - there will be graphs for this.
    • Maximum drain-source voltage - will be specified.
    • Maximum power dissipation - mainly depends on the package and if you use a heatsink.
    • Safe operating area - only important if you are using in linear mode - there will be a graph.
    • Also note the gate capacitance. If you want to switch fast, this needs to be charged and discharged fast. This requires gate current. When the gate voltage is not changing, the current will be almost zero.
    Best to operate at well below the maximum ratings if you want circuit to be reliable.
    Last edited: Jan 11, 2013
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  3. DickCappels


    Aug 21, 2008
    Its not an on/off switch like an SCR; its an amplifier, so there is no particular "on" voltage.

    What would probably be most useful to you is the chart in the datasheet that shows a family of curves giving drain-to-source voltage as a function of drain current for various gate-to-source voltages.
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  4. JMac3108

    Active Member

    Aug 16, 2010
    MOSFETs are used as switches as often as as amplifiers, probably more so. Most power switching applications now use MOSFETs instead of BJTs. Switch Mode power supplies and motor control PWM are a good examples.

    The gate threshold voltage is listed as Vgs(th) in the datasheet and is listed as min=2V and max=4V. This is the gate to source voltage where the mosfet will just begin turning on (drain to source current flow).

    BUT, if you are using the mosfet as a switch, you need to turn it on much harder than this, in other words a larger Vgs. Some mosfet datasheets have a graph of Vgs (gate to source voltage) vs. Rds(on) (on resistance). The IRF510 does not have this graph, but you can see in the datasheet that Rds(on) is specified at Vgs = 10V. So if you are using the IRF510 as a switch, you should drive the gate to at least 10V to ensure that you get the lowest 'on resistance' possible. This dissipates the least power in the mosfet, and keeps the mosfet temperature as cool as possible.
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  5. wayneh


    Sep 9, 2010
    You missed Dick's point. The MOSFET is still a transistor and there is no single magic voltage - there's a region, and which curve you're on depends on current, drain to source voltage.

    He didn't imply that MOSFETs don't make fine switches, just that they must be used properly.
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  6. kubeek


    Sep 20, 2005
    But you also need to keep the voltage under the Vgs max limit, which is usually ±20V for large mosfets like this one, and around 12V for logic level mosfets. A touch above that, and the mosfet will be destroyed allmost instantly.
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  7. huwilerp

    Thread Starter New Member

    Jan 10, 2013
    I came across this site by mistake, and I really really appreciate all the answers above. Thank you.