[Diy_efi] Flyback Diode (was 555 timer...)

[Mike]

At 09:37 PM 7/8/05, you wrote:
>My question:
>
>What specific item am I looking for in the datasheet to know that the
>circuit won't need a flyback diode separate from what is internal to the
>mosfet?

Safe Operating Area is one as a graph and max breakdown voltage

>I looked in the datasheet for the IRF640.  It does have a section on
>"Reverse Diode Limiting Values".  Which term is important here?

The inherent reverse diode in the channel has limited current capacity,
so good idea to provide an external *fast* one like a schottky, plus a
MOV is worth it (MOV = Metal Oxide Varistor)

>I have been using an N-Channel Power mosfet, RFP50N06, for some of my
>projects.  With this mosfet I proved I have to use the external diode.  Its
>data sheet does have a section titled "Source to Drain Diode
>Specifications".  Some of the information is similar.

The app note re operating in Safe Operating Area is worth some study,

Mosfets can switch real fast and this means v=Ldi/dt can be real high
even though inductance can be low sop use MOVs to control the flyback
as well as a diode to bypass the reverse current..

Rgds

Mike






>Thank you in advance for your help,
>
>Dan Nicoson
>
>
>> -----Original Message-----
>> From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]On
>> Behalf Of Paul Bidmead
>> Sent: Friday, July 08, 2005 12:15 AM
>> To: diy_efi at diy-efi.org
>> Subject: Re: [Diy_efi] 555 timer for injector driver
>>
>>
>> the best injector driver i foun was an IRF640 FET it includes a flyback
>> suppressor and can handle up to 200V/16A
>>
>> Kind Regards
>> Paul Bidmead
>> Design Engineer
>> BidSpec
>>
>> ----- Original Message -----
>> From: "Michael Richards" <michael at fastmail.ca>
>> To: <diy_efi at diy-efi.org>
>> Sent: Friday, July 08, 2005 12:15 AM
>> Subject: RE: [Diy_efi] 555 timer for injector driver
>>
>>
>> > Do not forget that you will need a flyback as well. Personally I find
>> > it's far easier to just buy one of those expensive national
>> > semiconductor injector driver chips and let it deal with the
>> > requirements. This way you're not limited to saturated injectors
>> > either.
>> >
>> > -Michael
>> >
>> >> I'd skip the solenoid to run your injector.  You're trying to control
>> >> a slow device (injector) with possibly an even slower device
>> >> (solenoid).
>> >>
>> >> Use either a Darlington transistor or a Mosfet as has already been
>> >> suggested.  They are not too expensive from the local Radio Shack.  I
>> >> use a good heat sink on the TIP-120 shown in the diagram.  When
>> >> running at long pulse widths, it can build some heat.  (TIP-120 was
>> >> used simply because it was in my parts bin, other Darlington's will
>> >> work.  You would want to add few other things to use the Mosfet
>> >> properly)
>> >>
>> >> I've attached a .pdf of the signal generator I've developed for use
>> >> with my projects.  Use as much or as little of this circuit as you
>> >> wish.  I'm only using a breadboard version at this point. I may make
>> >> up a PCB later this month.
>> >>
>> >> Take care,
>> >>
>> >> Dan Nicoson
>> >>
>> >>
>> >>> -----Original Message-----
>> >>> From: diy_efi-bounces at diy-efi.org
>> >>> [mailto:diy_efi-bounces at diy-efi.org]On Behalf Of jeff matthews
>> >>> Sent: Wednesday, July 06, 2005 9:39 PM
>> >>> To: diy_efi at diy-efi.org
>> >>> Subject: [Diy_efi] 555 timer for injector driver
>> >>>
>> >>>
>> >>> Hi-
>> >>> To keep my ??? simple,  I built a 555 timer to work on a little
>> >>> experiment,
>> >>> I have it energizing the solenoid on a  12v relay, why won't it open
>> >>> and close my injector? (it's just a basic saturated 550cc).  Is there
>> >>> some secret to on making an injector operate?
>> >>>
>> >>> Like I stated,  this is for an experiment, just thought I could
>> >>> handle making an injector fire on the bench...
>> >>>
>> >>> Best regards,
>> >>> JEFF
>> >>>
>> >>>
>> >>> _______________________________________________
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Regards from


Mike Massen
Perth, Western Australia
VL Commodore Fuse Rail that wont warp or melt !
http://niche.iinet.net.au