Maker Pro
Maker Pro

Connecting two open collector devices

Z

Zul

Jan 1, 1970
0
Hello,

Currently I building a circuitry to have 8 I/O pins being demux into
64 mutually exclusive outputs to control switching of 64 selenoid
relays. I am planning to place a Darlington array in between the
output of the demux and the ground for the relay because I believe a
demux output can't sink high current enough to operate a relay.

My problem arises as both the demux and the darlington array are open
collector devices. In this case the output of the demux are either
ground or floating. For darlington array the input must be LO (output
to floating) or HI (output to 0.6V). As I see it, I could not get a HI
input from an output of a demux with open collector configuration.

I was thinking to place an inverter between the demux and the
darlington array. I also looked for a demux with totem pole
configuration with active HI to match it with the darlington array but
could not find it. All I can find are demux with active LO.

If anybody have the opinion this, appreciate the response.

thanks,
Zul
 
F

Falko Rudolph

Jan 1, 1970
0
Use PNP-Darlingtons instead of NPNand switch Supply with it instead of GND.
It's the easiest way.

regards
Falko Rudolph
 
Z

Zul

Jan 1, 1970
0
---
Here's a trick:

For something like a 2N4401,

http://www.fairchildsemi.com/ds/2N/2N4401.pdf

which can drive a pretty good size relay, you can get quite a bit more
than 5mA out of 74HC since Vbe(sat) of the transistor it's driving
will be on the order of about a volt and the logic's totem pole output
will be letting quite a lot of charge flow through the top MOSFET
before VOH gets that low.

JF

Fields - Thanks for active HI demux options

Rudolph & Walcott - Driving 6V relay with 0.35W power consumption.
Thanks for the alternative approaches and well as the explanations.

Zul
 
F

Falko Rudolph

Jan 1, 1970
0
(snip)
(snip)

Unless you're driving the relays from a different +V.

Why? As long as they are based on the same GND you only need to adjust the
base resistor. That's what o.c. outputs are made for( and more, right).
As John suggested normal transistors may be better than darlingtons when V+
is only 6V.
What do I not see?

regards
Falko Rudolph
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
Why Don't You Use Tristate Devices Like 74245 74244 They Made For I/o Buss.
 
F

Falko Rudolph

Jan 1, 1970
0
.... and Base to controlling signal and collector to the relay. That's what I
thought of.

Falko
 
Top