Connecting two open collector devices

Z

Zul

Guest
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
 
On Mon, 11 Aug 2008 14:08:12 +0200, "Falko Rudolph"
<falkorudolph@gmx.de> wrote:

"Zul" <zulkafli@gmail.com> schrieb im Newsbeitrag
news:b0148a88-9f63-43b0-89c9-0889356b8dbd@x35g2000hsb.googlegroups.com...
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.
Click to expand...
---
74HC238, CD74HC238

JF
 
Falko Rudolph wrote:
Use PNP-Darlingtons instead of NPNand switch Supply with it instead of GND.
It's the easiest way.

regards
Falko Rudolph


"Zul" <zulkafli@gmail.com> schrieb im Newsbeitrag
news:b0148a88-9f63-43b0-89c9-0889356b8dbd@x35g2000hsb.googlegroups.com...
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


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

--

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
 
Zul wrote:
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
Click to expand...
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
 
John Fields wrote:
On Mon, 11 Aug 2008 14:08:12 +0200, "Falko Rudolph"
falkorudolph@gmx.de> wrote:

"Zul" <zulkafli@gmail.com> schrieb im Newsbeitrag
news:b0148a88-9f63-43b0-89c9-0889356b8dbd@x35g2000hsb.googlegroups.com...
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.

---
74HC238, CD74HC238

JF
Click to expand...
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
 
On Mon, 11 Aug 2008 11:39:01 -0700, Tim Wescott <tim@seemywebsite.com>
wrote:

John Fields wrote:
On Mon, 11 Aug 2008 14:08:12 +0200, "Falko Rudolph"
falkorudolph@gmx.de> wrote:

"Zul" <zulkafli@gmail.com> schrieb im Newsbeitrag
news:b0148a88-9f63-43b0-89c9-0889356b8dbd@x35g2000hsb.googlegroups.com...
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.

---
74HC238, CD74HC238

JF

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.
Click to expand...
---
Here's a trick:

For something like a 2N4401,

http://www.fairchildsemi.com/ds/2N%2F2N4401.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
 
On Aug 12, 8:22 am, John Fields <jfie...@austininstruments.com> wrote:
On Mon, 11 Aug 2008 11:39:01 -0700, Tim Wescott <t...@seemywebsite.com
wrote:



John Fields wrote:
On Mon, 11 Aug 2008 14:08:12 +0200, "Falko Rudolph"
falkorudo...@gmx.de> wrote:

"Zul" <zulka...@gmail.com> schrieb im Newsbeitrag
news:b0148a88-9f63-43b0-89c9-0889356b8dbd@x35g2000hsb.googlegroups.com...
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.

---
74HC238, CD74HC238

JF

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.

---
Here's a trick:

For something like a 2N4401,

http://www.fairchildsemi.com/ds/2N%2F2N4401.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
Click to expand...
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
 
On Tue, 12 Aug 2008 08:58:24 +0200, "Falko Rudolph"
<falkorudolph@gmx.de> wrote:

(snip)
Use PNP-Darlingtons instead of NPNand switch Supply with it instead of
GND. It's the easiest way.

(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?
Click to expand...
---
The emitter of the PNP driver will be connected to the different V+.


JF
 
On Wed, 13 Aug 2008 08:29:36 +0200, "Falko Rudolph"
<falkorudolph@gmx.de> wrote:

"John Fields" <jfields@austininstruments.com> schrieb im Newsbeitrag
news:j3t2a4ti8bv3oi8qbn3kv90ho56vaols92@4ax.com...
On Tue, 12 Aug 2008 08:58:24 +0200, "Falko Rudolph"
falkorudolph@gmx.de> wrote:

(snip)
Use PNP-Darlingtons instead of NPNand switch Supply with it instead of
GND. It's the easiest way.

(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?

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

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