wrote in message
...

I posted a more detailed message in .design and haven't got a response
yet. But anyway, to keep this short and sweet. Is there any trick
that people use to test and diagnose DC tachometers? I have a 4
channel tach that is powered by 24VDC. Each channel has a VCC, Vout,
and gnd. It's supposed to be an open collector output, but when I
connect my external circuit all I get is a steady 24VDC on the Vout
with a lot of noise on top of it. Does this inherently mean
something? Are there any tricks I could attempt to find out what's
going on? Thanks.

Do you have a bypass capacitor (such as a 10nF mica) bypassing the power
pins on the chip?

On Sun, 12 Dec 2004 09:45:56 -0800, put finger to
keyboard and composed:


I posted a more detailed message in .design and haven't got a response
yet. But anyway, to keep this short and sweet. Is there any trick
that people use to test and diagnose DC tachometers? I have a 4
channel tach that is powered by 24VDC. Each channel has a VCC, Vout,
and gnd. It's supposed to be an open collector output, but when I
connect my external circuit all I get is a steady 24VDC on the Vout
with a lot of noise on top of it. Does this inherently mean
something? Are there any tricks I could attempt to find out what's
going on? Thanks.

I don't know what kind of external circuit you are using, but I would
first test the tacho with a resistor (10K?) between Vout and Vcc.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

Sounds like this 4 channel tach is set up to forward the tach info onward
via that Vout terminal. Does the tachometer function of each channel work?
If so, is your external circuit configured to supply and/or work with the
open collector Vout correctly? You can do a basic check by scoping the
input to that open collector circuit and see if it's trying to be
switched......then seeing if bias is required to be supplied by your
external circuit (not untypical).....it could be that your external circuit
is Hi-z and/or capacitively or incompatibly logic coupled. Once you see
how the output of the tach circuit is configured, you can build a simple
external circuit to see if it will switch (assuming you find valid
switching voltage at the input to the open collector circuit......for
example at the base if a simple bipolar common emitter circuit.

What kind of a unit is this....just curious.

Gord

"James Sweet" writes:

wrote in message
...

I posted a more detailed message in .design and haven't got a response
yet. But anyway, to keep this short and sweet. Is there any trick
that people use to test and diagnose DC tachometers? I have a 4
channel tach that is powered by 24VDC. Each channel has a VCC, Vout,
and gnd. It's supposed to be an open collector output, but when I
connect my external circuit all I get is a steady 24VDC on the Vout
with a lot of noise on top of it. Does this inherently mean
something? Are there any tricks I could attempt to find out what's
going on? Thanks.

Do you have a bypass capacitor (such as a 10nF mica) bypassing the power
pins on the chip?

He shouldn't see 24 V on an open collector output.

So what is your external circuit?

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header is ignored.
To contact me, please use the Feedback Form in the FAQs.

"Sam Goldwasser" wrote in message
...
"James Sweet" writes:

wrote in message
...

I posted a more detailed message in .design and haven't got a response
yet. But anyway, to keep this short and sweet. Is there any trick
that people use to test and diagnose DC tachometers? I have a 4
channel tach that is powered by 24VDC. Each channel has a VCC, Vout,
and gnd. It's supposed to be an open collector output, but when I
connect my external circuit all I get is a steady 24VDC on the Vout
with a lot of noise on top of it. Does this inherently mean
something? Are there any tricks I could attempt to find out what's
going on? Thanks.

Do you have a bypass capacitor (such as a 10nF mica) bypassing the power
pins on the chip?

He shouldn't see 24 V on an open collector output.

So what is your external circuit?



Not even if his external circuit has a pullup resistor?

"James Sweet" writes:

"Sam Goldwasser" wrote in message
...
"James Sweet" writes:

wrote in message
...

I posted a more detailed message in .design and haven't got a response
yet. But anyway, to keep this short and sweet. Is there any trick
that people use to test and diagnose DC tachometers? I have a 4
channel tach that is powered by 24VDC. Each channel has a VCC, Vout,
and gnd. It's supposed to be an open collector output, but when I
connect my external circuit all I get is a steady 24VDC on the Vout
with a lot of noise on top of it. Does this inherently mean
something? Are there any tricks I could attempt to find out what's
going on? Thanks.

Do you have a bypass capacitor (such as a 10nF mica) bypassing the power
pins on the chip?

He shouldn't see 24 V on an open collector output.

So what is your external circuit?

Not even if his external circuit has a pullup resistor?

Yes, of course of if his external circuit has a pullup to 24 V, but he didn't
say what was in his external circuit. The noise is most likely from there.

We need more info.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header is ignored.
To contact me, please use the Feedback Form in the FAQs.