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Connecting two open collector devices

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

Reply to
Zul
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Use PNP-Darlingtons instead of NPNand switch Supply with it instead of GND. It's the easiest way.

regards Falko Rudolph

"Zul" schrieb im Newsbeitrag news: snipped-for-privacy@x35g2000hsb.googlegroups.com...

Reply to
Falko Rudolph 

--
74HC238, CD74HC238

JF
Reply to
John Fields

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

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

It sounds like John gave you a better solution to this, but what the heck:

To drive something from an open-collector output you need a pullup resistor to supply the current from VCC. If you were only able to find open-collector output, active-low devices and you couldn't drive your relays from PNP devices, then you'd need a pullup, followed by an inverter with enough oomph to drive the base of your transistor.

But John's solution is better...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

Logic-level MOSFETs will eliminate your need for base resistor networks, and won't have the large VCE drop that a Darlington has in saturation.

So would single-stage transistors with integrated base resistors -- but they won't get you as much current.

How big of a relay are you driving? A Darlington will drop about 1V when it's on, compared to a regular NPN's 0.2V or so in saturation. A

74HC part will drive 5mA to the base of a transistor, which gives you a pretty reliable 50mA, which is enough for a modest relay. 
--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

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

Reply to
Zul

(snip)

(snip)

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

Reply to
Falko Rudolph 

--
The emitter of the PNP driver will be connected to the different V+.

 
JF
Reply to
John Fields

"John Fields" schrieb im Newsbeitrag news: snipped-for-privacy@4ax.com...

... and Base to controlling signal and collector to the relay. That's what I thought of.

Falko

Reply to
Falko Rudolph 

--
Like this? (View in Courier):


    +5V          >=6V
     |            |
     D            |
---G PCH          | 
     S            |
     |            E
     +----[R]---B
     |            C
     S            |
---G NCH          |
     D          [COIL] 
     |            |
    GND          GND

Then how did you think the relay would be de-energized ?

JF
Reply to
John Fields

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