at a suggestion of a coworker, I changed the resister on the 2N2222A's
collector to 1k, which changes the gain of that transistor.  Now the fet's
Vds is 1v, looks much better.

I'll try the P-channel fet, but I don't think I have any on hand.

Thanks for the input  If my mw is to high I'll be going to a different
device . . .   maybe the P complement to an IRF510 or something . . .




On 4/21/06, Thomas C. Sefranek <tcs@cmcorp.com> wrote:
>
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of
> Roy J. Gromlich - PA
> Sent: Friday, April 21, 2006 1:38 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] JFET 2N7000 as SS Relays
>
> The Vds on the FET should be a whole lot less than 2.4 Volts at 37 mA -
> actually I would think more like 0,24 Volts.  The 2N-7000 FETs have a
> nice low ON resistance when saturated - I would have to say that yours
> is not.
>
> RJG
>
> He is still using an FET source follower, as a high side switch,
> it will never saturate.
>
> Hint for John: For a high side switch use a P-Channel FET.
> Source to V+, same gate circuit, drain to the load.
> You can get some REAL low drop switches that way.
>
> Tom
>
>   *
>   |  __O    Thomas C. Sefranek  tcs@cmcorp.com
>   |_-\<,_   Amateur Radio Operator: WA1RHP
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