Tachometer Woes
 

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.

writes:

Thanks for the responses. First, no, I do not have a bypass cap
attached (there are no ICs involved here). My external circuit is
simply a 1kOhm pullup resistor between Vcc and Vout, and I'll I'm
seeing on Vout is 24VDC with lots of noise. Yes, I am turning the
tach while looking at the output on a scope. I've tried other
resistor values with the same results. The datasheets for the tach
(Deuta Werke type DF9) say to do exactly what I'm doing.

More details for you: 3 of the 4 channels work properly, and have
worked properly for many years. These tachs are installed on a public
transit fleet in which the manufacturer never used the 4th channel.
We now need that 4th "spare" channel for a new modification that needs
a vehicle velocity input. We were going to read the tach pulses in a
micro controller and compute velocity, long story... All of the
vehicles I've looked at so far have this problem on channel 4, so it's
not that I've found one yet that works.

One thing we are worried about is that the 4th channel may not be
working because it was left floating for so long. Is that a
possibility?

Or, perhaps it really doesn't exist - the manufacturer realizing you
never used it, left if off. :) Can you trace the circuit inside the
tach to see where each channel goes?

I don't think a floating open collector output would cause any problems.

In case anyone is wondering, the 3 other channels are connected to a
PCB with the pulse detecting circuits, I have no idea what the exact
circuit is, so I can't duplicate it. Eventually I need to find out
though.

One other question, as an alternative solution we were considering
connecting our new equipment in parallel to one of the 3 other known
working channels. Is this bad? The input resistance of the new
equipment is very high, around 10 MOhm. Do you think this is a doable
solution if we can't figure out what's happening with the 4th channel?

As long as each channel sees what it expects, that should not be a problem.

10M ohm is essentially an open circuit as far as it is concerned.

--- 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.

wrote in message
...

Sam,

Yes, you might be correct about that. I have finally made a contact
with someone in Germany about this problem and I'm awaiting their
response. The device is sealed (for rough outdoor use) and I suspect
the circuits are epoxyed inside, although I should break one open at
some point.

The parallel connection idea is still an option, at least to keep
further costs down.



I originally thought this was referring to an IC, not a sealed circuit. It's
quite common for a manufacture to use a standard casing for a device and
simply not internally connect pins that are not needed. As Sam said, it's
entirely possible that this is only a 3 channel tach, and that 4 channel was
a different model in the same box.

On Sun, 12 Dec 2004 23:18:56 -0800, put finger to
keyboard and composed:

One other question, as an alternative solution we were considering
connecting our new equipment in parallel to one of the 3 other known
working channels. Is this bad? The input resistance of the new
equipment is very high, around 10 MOhm. Do you think this is a doable
solution if we can't figure out what's happening with the 4th channel?

If you need to ensure that your add-on equipment will not affect the
original installation, especially if it is powered from a different
source, then isolate it via an optocoupler in series with your pullup
resistor.


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

On Mon, 13 Dec 2004 15:58:07 -0800, put finger to
keyboard and composed:

Yes, this is a totally sealed unit with an IP rating of 67. FYI, we
mount them to the outer hub of a light rail commuter vehicle on the
center axles. I'm waiting for Deuta to respond to me with a response
to exactly the issue you pose below. The stampings on the tach lead
me to believe the 4th channel IS connected though because it is
clearly stamped "collector with R = 1k5 ch 1,2,3,4", but since I
haven't opened one up I can't say for certain.

Could the 4th channel represent some special function? For example,
could it be generating a single [hard to detect] pulse per revolution
as a timing mark? Is there any special phase relationship between the
three working channels? For example, are channels 1-3 spaced 120
degrees apart or 90 degrees, or are they in sync?


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

writes:

Well according to the datasheet I have, channel 2 leads channel 1 by
90 deg. Same with channel 4, it leads channel 3 by 90 deg. So
basically according to this timing diagram, channels 1 and 3 are in
phase and so are channels 2 and 4, but their outputs are inverted to
each other. One is a trailing edge the other is a rising edge and so
forth. Channel 4 should be just another output according to the
datasheet and there is no discussion anywhere of it being a special
output. Does this phase issue change anything from my point of view
in connecting channel 4?

Not based on the way you were testing, only how they are used.

--- 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.

..


On Tue, 14 Dec 2004 17:47:32 +1100, Franc Zabkar
wrote:

Could the 4th channel represent some special function? For example,
could it be generating a single [hard to detect] pulse per revolution
as a timing mark? Is there any special phase relationship between the
three working channels? For example, are channels 1-3 spaced 120
degrees apart or 90 degrees, or are they in sync?


- Franc Zabkar

On Tue, 14 Dec 2004 11:35:21 -0800, put finger to
keyboard and composed:


Well according to the datasheet I have, channel 2 leads channel 1 by
90 deg. Same with channel 4, it leads channel 3 by 90 deg. So
basically according to this timing diagram, channels 1 and 3 are in
phase and so are channels 2 and 4, but their outputs are inverted to
each other.

Wouldn't that make them 180 degrees out of phase?

One is a trailing edge the other is a rising edge and so
forth. Channel 4 should be just another output according to the
datasheet and there is no discussion anywhere of it being a special
output. Does this phase issue change anything from my point of view
in connecting channel 4?

The reason I asked about phase relationships was to get an idea how
channel 4 fits in with the others. If the phase relationships between
channels 1,2, and 3 had been 120 degrees, then that would have
suggested that channel 4 was a special channel. A 90 degree phase
difference suggests that it is identical to the others, ie it suggests
that there are four sensors equispaced around the circumference (I
think).

On Tue, 14 Dec 2004 17:47:32 +1100, Franc Zabkar
wrote:

Could the 4th channel represent some special function? For example,
could it be generating a single [hard to detect] pulse per revolution
as a timing mark? Is there any special phase relationship between the
three working channels? For example, are channels 1-3 spaced 120
degrees apart or 90 degrees, or are they in sync?


- Franc Zabkar


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