On 22.01.16 12:22, andy pugh wrote:
> On 22 January 2016 at 11:51, Erik Christiansen <dva...@internode.on.net>
> wrote:
> >> I also don't think that it is particularly well suited to being driven
> >> by the 7i84 outputs.
> >
> > If the 7i84 won't take 24v, then a transistor and two resistors, per
> > output, fixes that.
>
> The 7i84 outputs are 5 to 28V sourcing drivers, it isn't the voltage
> that is the problem, it's the common positive on the control side of
> that relay module.
Ah, the SSR is upside-down both ways. OK, if you have no pull-down
outputs on any I/O board, then it's simple to make some from the
sourcing drivers:
--######--- To an SSR input
/ 1K
|/
10K |
24V sourcing driver --######------| NPN
| |<
# \
# |
4K7 # |
# |
| |
GND ----------------------------------------
The 1K will drive the SSR with round about 22 mA @ 24v.
The 10K will drive the transistor with a bit less than 2.4 mA @ 24v.
The 4K7 will help turn it off.
It would be hard to find a transistor without the current gain to
saturate (as desired) here. Something like a BC337 will handle 45v
and 800 mA of collector current, even though it's in the common TO92
package.
That'll still require the upside-down power supply on the SSR output.
How expensive are three discrete DC SSRs, compared to the extra PS?
Erik
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