PCF8574 to control relays - Doh!
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Can't find a datasheet online for these relays (DSS41a05) but I
tried an experiment. Hooked the coil to +5 on one side and to the
PCF8574 through a 470ohm resistor on the other - the relay works!. I
don't know what the impedance of the relay coil is so I'll assume
zero just for argument's sake. Thats about 10.6mA which is well
within the 20mA per pin sink capability - even the 100mA total limit
( No more than three relays can be on at a time anyway). 1k was too
much resistance and I don't have anything in-between kicking around.
So, guess I don't need transistors at all really.
P.S. I know reed relays don't have much flyback but I tossed a
1n4148 in parallel anyway.
--- In basicstamps@y..., "kvasilak" <my427v8@h...> wrote:
> 300ua??!! Oh wow, thats small, the Tip120 would need a few
milliamps,
> sorry
>
> --- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
> > I was just checking out the datasheet for that and I just don't
> > understand what I'm looking at. How can you tell what current it
> > needs to stay off? The PCF8574 can only source 300uA...
> >
> > --- In basicstamps@y..., "kvasilak" <my427v8@h...> wrote:
> > > Sounds good to me.
> > > There's a commonly used darlington, TIP120, that may be what
you
> > > need...
> > >
> > > --- In basicstamps@y..., "ghidera2000" <ghidera2000@y...>
wrote:
> > > > I was thinking of using the I/O expander to drive a set of
> relays
> > > (7
> > > > of them). Now since there is no way it can source or sink
that
> > much
> > > > current, I want to switch some transistors to turn the
relays
> on
> > > and
> > > > off.
> > > >
> > > > Since the I/O expander defaults to high on all pins at
startup
> I
> > > > can't use the normal pnp transistor setup (can't have
> everything
> > > > turned on at once during power up). I'm thinking I can go
NPN
> and
> > > > have the relays turned on when the pin is low.
> > > >
> > > > Is this the correct thinking? If so, what would be a good
> > > transistor
> > > > to use? I'm using 5v reed relays to control 12v ac 8watt
> > solenoids.
> > > > If not, whats the correct way?
tried an experiment. Hooked the coil to +5 on one side and to the
PCF8574 through a 470ohm resistor on the other - the relay works!. I
don't know what the impedance of the relay coil is so I'll assume
zero just for argument's sake. Thats about 10.6mA which is well
within the 20mA per pin sink capability - even the 100mA total limit
( No more than three relays can be on at a time anyway). 1k was too
much resistance and I don't have anything in-between kicking around.
So, guess I don't need transistors at all really.
P.S. I know reed relays don't have much flyback but I tossed a
1n4148 in parallel anyway.
--- In basicstamps@y..., "kvasilak" <my427v8@h...> wrote:
> 300ua??!! Oh wow, thats small, the Tip120 would need a few
milliamps,
> sorry
>
> --- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
> > I was just checking out the datasheet for that and I just don't
> > understand what I'm looking at. How can you tell what current it
> > needs to stay off? The PCF8574 can only source 300uA...
> >
> > --- In basicstamps@y..., "kvasilak" <my427v8@h...> wrote:
> > > Sounds good to me.
> > > There's a commonly used darlington, TIP120, that may be what
you
> > > need...
> > >
> > > --- In basicstamps@y..., "ghidera2000" <ghidera2000@y...>
wrote:
> > > > I was thinking of using the I/O expander to drive a set of
> relays
> > > (7
> > > > of them). Now since there is no way it can source or sink
that
> > much
> > > > current, I want to switch some transistors to turn the
relays
> on
> > > and
> > > > off.
> > > >
> > > > Since the I/O expander defaults to high on all pins at
startup
> I
> > > > can't use the normal pnp transistor setup (can't have
> everything
> > > > turned on at once during power up). I'm thinking I can go
NPN
> and
> > > > have the relays turned on when the pin is low.
> > > >
> > > > Is this the correct thinking? If so, what would be a good
> > > transistor
> > > > to use? I'm using 5v reed relays to control 12v ac 8watt
> > solenoids.
> > > > If not, whats the correct way?