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NPN sensor to TTL input

I got a ST L294 solenoid driver and I am stuck on the enable input
(TTL) currently using a sunx cx-24 @ 5v supply. The output is NPN. I've
hooked everything up except the photo sensor (cx-24) and powered up
using 20 v. When I hooked up the cx-24 I blew the L294.

I believe it is the input( or rather the output of the cx-24 ).

I've seen suggestion of an optocoupler, or should I look at something
else.

The L294 specs on enable are L -0.3 - 0.8 v -100 uA
H 2.4 - ?? v 100 uA
 
J

John Popelish

Jan 1, 1970
0
I got a ST L294 solenoid driver and I am stuck on the enable input
(TTL) currently using a sunx cx-24 @ 5v supply. The output is NPN. I've
hooked everything up except the photo sensor (cx-24) and powered up
using 20 v. When I hooked up the cx-24 I blew the L294.

I believe it is the input( or rather the output of the cx-24 ).

I've seen suggestion of an optocoupler, or should I look at something
else.

The L294 specs on enable are L -0.3 - 0.8 v -100 uA
H 2.4 - ?? v 100 uA

This means that the enable signal has to be able to pull down to below
..3 volts while pulling 100 uA from the enable pin to perform the
enable function (it is active when it is a logic low), and must be
able to pull high to at least 2.4 volts while supplying 100uA to the
enable pin to disable it. Voltages in between .3 and 2.4 are not
guaranteed to act either as an enable or a disable.

This signal can be provided from any 5 volt powered logic family, or
from an active pull down device, like an NPN transistor, whose emitter
is grounded, with a pull up resistor connected to a 5 volt supply.
The absolute maximum voltage that can be safety applied to the enable
pin is 7 volts.
 
J

John Popelish

Jan 1, 1970
0
thanks for the reply, can you give me a little bit more direction, as I
am not a pro.
Not without learning a lot more about exactly what you are hooking
together. For instance, I have no idea what "sunx cx-24 @ 5v supply"
means. I gather that you are somehow connecting an L294 with a 20
volt supply and some sort of photo sensor and there might also be a 5
volt supply involved. If you email me a schematic of what you have, I
will get back to you tonight.
 
J

John Fields

Jan 1, 1970
0
thanks for the reply, can you give me a little bit more direction, as I
am not a pro.

---
The SUNX site doesn't seem to have a wiring diagram, right off the
bat, for the CX24, but from the information there, if it has a 4
wire connection, it should be something like this: (View in Courier)


+V
| +----
+-+----|-+
| C |
| |
| E |
+-+----|-+
| +----
GND

Where +V is the supply voltage, GND is the supply ground, and C and
E are the uncommitted collector and emitter of the output NPN
transducer.

If it only has three wires out, though, it's probably like this:

+V
| +----
+-+----|-+
| C |
| |
| E |
+------|-+
|
GND

Where GND is the common ground for the sensor supply and the emitter
of the output transducer.


If the first case is true and you want the 20V supply to be isolated
from the 5V supply, you should wire your circuit like this:



+5 +5
| |
[1k2] +---+-...
+20 | |_
| +---------O|E L294
+-+----|-+ |
| C | +---+-...
|CX24 | |
| E | |
+-+----|-+ |
| | |
+20GND +--------------+
|
+5GND


Or, if you want the 20V supply and the 5V supply to share a common
ground, like this:


+5 +5
| |
[1k2] +---+-...
+20 | |_
| +---------O|E L294
+-+----|-+ |
| C | +---+-...
|CX24 | |
| E | |
+-+----|-+ |
| | |
+----+--------------+
| |
+20GND +5GND


If the CX24 only has three wires coming out, you'll need to hook it
up like this :

+5 +5
| |
[1k2] +---+-...
+20 | |_
| +---------O|E L294
+-+----|-+ |
| C | +---+-...
|CX24 | |
| E | |
+------|-+ |
| |
+--------------+
| |
+20GND +5GND

Note also that the input to the sensor needs to be _modulated_ IR
 
thanks for the reply the cx-24 has 3 outputs +, 0, out. Although I am
not sure, the 20v is to drive the solenoid. The 5v has a dual purpose,
it powers the cx-24 as well as providing a reference voltage to the
L294.
http://www.st.com/stonline/books/pdf/docs/4032.pdf is the the reference
with Fig. 7a as the test application. I believe that the cx-24
(connects to pin9) should be isolated from the circuit, as pin 9 is
supposed to be TTL.
 
J

John Fields

Jan 1, 1970
0
thanks for the reply the cx-24 has 3 outputs +, 0, out. Although I am
not sure, the 20v is to drive the solenoid. The 5v has a dual purpose,
it powers the cx-24 as well as providing a reference voltage to the
L294.

---
No. As shown on the data sheet for the CX-24, its supply voltage
must be from 12 to 24VDC. Also, as shown on the data sheet for the
L294, its supply voltage is +5V. In order to make the circuits
operate properly you need to connect them like this:



+5 +5 +Vs
| | |
CX-24 [1K2] +----+-----+--+
+---------+ | | Vss Vs |
+20>----|+ OUT|---+---O|E | +5
|CX24 | | GND Vi | |
| 0 | +----+-----+--+ [R1]
+----+----+ | | |
| | +------+
+--------+------------------+ |
| | | [R2]
+5GND +20GND | |
+------------+

+Vs is the voltage used to drive your solenoid (It could be the
+20V if that's what you want to use), and Vi is the reference
voltage input. The voltage developed at the junction of R1 and R2
will determine how much current is allowed to flow into the
solenoid. Read the data sheet.
http://www.st.com/stonline/books/pdf/docs/4032.pdf is the the reference
with Fig. 7a as the test application. I believe that the cx-24
(connects to pin9) should be isolated from the circuit, as pin 9 is
supposed to be TTL.

---
You don't understand. The output of the CX-24 is an uncommitted
open collector, so you use that to pull down the enable input of the
L294 when you want to energize the solenoid. When you don't want to
energize the solenoid you turn off the output transistor and allow
the voltage on the enable input to rise to +5V. Now, since you only
have three connections to the CX-24, the emitter of the output
transistor is common to the 20V supply ground, so if you use a 5V
supply to run the L294 (and the enable), that power supply ground
_must_ be tied to the 20V ground. Consequently, all the grounds are
common.
 
you guys make everything sound so easy, I actually have hooked up a
different sensor, it does use 5v with NPN output, but I need to use the
cx-24, which I have on hand. I'm going to try your circuit today. Let
you know.
The 5v on the L294 is to produce a voltage reference, which translates
to the output current spike I need. There is no mention of a 5v supply
for the L294 except for diagnostic

The diodes that they are referring to are regular diodes??

thanks again
 
J

John Fields

Jan 1, 1970
0
you guys make everything sound so easy, I actually have hooked up a
different sensor, it does use 5v with NPN output, but I need to use the
cx-24, which I have on hand. I'm going to try your circuit today. Let
you know.
The 5v on the L294 is to produce a voltage reference, which translates
to the output current spike I need. There is no mention of a 5v supply
for the L294 except for diagnostic

---
Not true. Take a look at page 2 of the data sheet and you'll see,
under "ABSOLUTE MAXIMUM RATINGS", that Vss, the logic supply voltage
can be run up to 7V.

Then look at "ELECTRICAL CHARACTERISTICS" on page 3 and you'll see
that the test circuit is run with Vcc = 5V.

The reference voltage is something else again, and is used to set
the peak current into the solenoid. Look at figure 2 of the data
sheet.
 
Yes I did note the fast switch diode, that is where I started to have
trouble. I knew it wasn't zener, so I opted for a schottky, I suffer
from limited resources on short notice.

I actually built the fig. 7a found in the app notes pages. There is no
mention of the 5 volt. I have it set-up to try the led, but I didn't
hook it up. That is funny because the L295 says it doesn't require
externat 5v.

I will hook it into the test tonight.
 
J

John Popelish

Jan 1, 1970
0
Yes I did note the fast switch diode, that is where I started to have
trouble. I knew it wasn't zener, so I opted for a schottky, I suffer
from limited resources on short notice.

I actually built the fig. 7a found in the app notes pages. There is no
mention of the 5 volt. I have it set-up to try the led, but I didn't
hook it up. That is funny because the L295 says it doesn't require
externat 5v.

I will hook it into the test tonight.

Sorry for the delay in my response, but I was called away, yesterday.

Thanks for the link to the data sheet and application note:
http://www.st.com/stonline/books/pdf/docs/1331.pdf
http://www.st.com/stonline/books/pdf/docs/4032.pdf

I can find no where on the data sheet or application note that
specifically says that Vss (logic supply) is optional, but application
note 7a implies that it is. I think you need to supply Vss with 5
volts if you want the diagnostic output to function, and that output
isn't used in application note circuit, 7a.

As long as the sensor supplies only a pull down current path, then it
should work with this circuit, with the possible exception that an
external pull up resistor might be needed to get a full 5 volt swing
on the RC and transistor network going into pin 7. A 10 k resistor
between the top of the 5.1 volt zener and pin 9 should do it.

That said, I think the enable pin is not responsible for your L294
failures. I suspect a slow output diode, D2 or some other error in
your circuit.
 
Got the enable ( pin 9 ) volts working fine, cost me a few chips along
the way. Down to 3 chips left before I need to order from digi-key. Now
everything seems to ok, the diodes that I have are schottky and
standard. Would the schottky diodes work, if not please let me know
what type I should look for
 
J

John Popelish

Jan 1, 1970
0
Got the enable ( pin 9 ) volts working fine, cost me a few chips along
the way. Down to 3 chips left before I need to order from digi-key. Now
everything seems to ok, the diodes that I have are schottky and
standard. Would the schottky diodes work, if not please let me know
what type I should look for

Schottkys should work fine, as long as they have the required voltage
and current rating. One other possible problem occurs if the supply
leads are long. Then the wiring inductance can generate large voltage
spikes during switching. The ap note does not show any, but there
should be a bypass capacitor between the point where D2 ties to pin 1
and the point where Rs ties to D2. Something on the order of 470 uF
35 volt electrolytic would probably work, if it is a low inductance
type, like Panasonic type FC or FM.
 

luke

Sep 1, 2009
15
Joined
Sep 1, 2009
Messages
15
I got a ST L294 solenoid driver and I am stuck on the enable input
(TTL) currently using a sunx cx-24 @ 5v supply. The output is NPN. I've
hooked everything up except the photo sensor (cx-24) and powered up
using 20 v. When I hooked up the cx-24 I blew the L294.

I believe it is the input( or rather the output of the cx-24 ).

I've seen suggestion of an optocoupler, or should I look at something
else.

The L294 specs on enable are L -0.3 - 0.8 v -100 uA
H 2.4 - ?? v 100 uA


CX-24 is now obsolete. CX-421 is the exact match in the current sensor offerings.
http://www.clrwtr.com/SUNX.htm
 
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