Hi David,

One way to perform a "reality check" on situations like this is to think
about what the device "sees". In order for a device to "see" 5 volts on one
of its pins, the 5 volts must be referenced with respect to one of its
other pins. In the case you gave, the 5 volts was with respect to GND,but
none of the transistor's pins were grounded. Therefore, as far as the
device was concerned, there its gate-source voltage was not 5V but
-2V,definately not what would turn it on. As others suggested, tieing the
source to ground made the 5V GND-referenced input actually look like 5V to
the FET,too.

Sean


At 08:27 PM 12/8/99 -0500, you wrote:
>I am working on a project that uses a PIC to operate 3 logic level HEXFET's
>(IRLIZ44N & IRLD014) that is driving me crazy.  I was under the impression
>that logic level HEXFET's would (when +5V is applied to the gate) would open
>up an pass whatever voltage was on the drain to the source (within the
>limits of the device).  Ain't happening.  All I get is 6 - 7 v through the
>device, with 24vdc on the drain.  And yes, I do have the gate, drain, source
>pins correctly identified.  I have tried high side, low side and would try
>third side if I knew how.
>
>Basic circuit is
>                +24vdc
>              |
>                Drain
> +5V------Gate
>                Source
>              |
>           24 vdc relay coil
>              |
>             GND
>
>To date I have not even put the PIC in the circuit, but rather just fed 5v
>from the power supply.
>Any ideas or help would be greatly appreciated - I am beginning to doubt my
>manhood!
>Thanks
>David
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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