Re: any comments on this circuit?
From: John Popelish (jpopelish_at_rica.net)
Date: 07/22/04
- Next message: species8350: "pc diagnostic software"
- Previous message: species8350: "pc circuit boards"
- In reply to: John Popelish: "Re: any comments on this circuit?"
- Next in thread: andy: "Re: any comments on this circuit?"
- Messages sorted by: [ date ] [ thread ]
Date: Thu, 22 Jul 2004 17:47:15 -0400
John Popelish wrote:
>
> andy wrote:
> >
> > On Thu, 22 Jul 2004 18:41:48 +0100, andy wrote:
> >
> > > It's meant to send a 1 second pulse through an electromagnet at the end of
> > > every day, to start a plant watering system, as i said in an earlier post.
> > > I've tested it on breadboard, but with only one transistor and a single
> > > coil winding. I'm trying splitting the coil into 3 parts to get more
> > > current through it. It would be good to know what people think of this
> > > circuit before i build the final version - it's only the second circuit
> > > i've built.
> > >
> > > http://www.niftybits.ukfsn.org/electronics/daily-water.png
> > > http://www.niftybits.ukfsn.org/electronics/daily-water.gif
> > >
> > > andy.
> >
> > I've been through it again, and worked out the component values for the
> > second half of the circuit again. The new version is at:
> >
> > http://www.niftybits.ukfsn.org/electronics/daily-water2.png
> > http://www.niftybits.ukfsn.org/electronics/daily-water2.gif
> >
> > If there's something wrong with the basic idea of how i've laid it out,
> > i'd rather be told why than just have it written off.
> >
> > --
> > http://www.niftybits.ukfsn.org/
> >
> > remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
> > bin unless notified with [html] or [attachment] in the subject line.
>
> Some basic comments:
>
> Observe the orientations of the 3 output transistors.
>
> The output transistors are NPN types, but a PNP symbol is used.
>
> If you use NPN output transistors, they are operating as emitter
> followers, so the emitter voltage will always be at least .6 volts
> more negative then the base voltage.
>
> The base current for the output transistors comes entirely from the
> base pull up resistor, which provides no current unless there is
> voltage across it. This uses up some more of the 12 volt supply.
>
> The BD139 must sink lots more current through the output drive
> resistor than that resistor ever supplies to the output transistors.
>
> There is little reason to expect that what output drive current there
> is will divide into three equal parts where it meets 3 bases.
Whoops. I forgot that there is a 2.7 ohm coil in each emitter so
there is something that helps force the base drive to split 3 ways.
Sorry.
> I don't want to ruin all your fun by designing the circuit for you,
> but think about these comments and show me what you come up with as a
> result.
> --
> John Popelish
-- John Popelish
- Next message: species8350: "pc diagnostic software"
- Previous message: species8350: "pc circuit boards"
- In reply to: John Popelish: "Re: any comments on this circuit?"
- Next in thread: andy: "Re: any comments on this circuit?"
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
