Oscilloscope preamp

[RKovach]

I have a P-C based scope that will only go down to .1 volt per division. I
would like to use a clamp-on amp probe I have, but this probe puts out .001
volt per amp, making it hard to see a good pattern for anything less than
100 amps. I was told I could use a preamp to boost the signal. I would like
to know if I could make one using a 741 op-amp, or would this distort the
pattern. I would only be using this to check alternator output on cars, so I
don't need to be concerned with high bandwidth.

 

[Gareth]

RKovach wrote:

I have a P-C based scope that will only go down to .1 volt per division. I
would like to use a clamp-on amp probe I have, but this probe puts out .001
volt per amp, making it hard to see a good pattern for anything less than
100 amps. I was told I could use a preamp to boost the signal. I would like
to know if I could make one using a 741 op-amp, or would this distort the
pattern. I would only be using this to check alternator output on cars, so I
don't need to be concerned with high bandwidth.

A simple 741 (or just about any op amp) circuit should work fine.

Gareth.
--
-----------------------------------------------------------------------
To reply to me directly:

Replace privacy.net with: totalise DOT co DOT uk and replace me with
gareth.harris

 

[Robert Monsen]

"Gareth" <me@privacy.net> wrote in message
news:bqipl1$22dd39$1@ID-211380.news.uni-berlin.de...

RKovach wrote:

I have a P-C based scope that will only go down to .1 volt per division.
I
would like to use a clamp-on amp probe I have, but this probe puts out
..001
volt per amp, making it hard to see a good pattern for anything less
than
100 amps. I was told I could use a preamp to boost the signal. I would
like
to know if I could make one using a 741 op-amp, or would this distort
the
pattern. I would only be using this to check alternator output on cars,
so I
don't need to be concerned with high bandwidth.


A simple 741 (or just about any op amp) circuit should work fine.

Gareth.

Here is a circuit you could use (view with courier font):


+---------+VCC
| |
.-. |
| | |
| |22k |
'-' |
| |
1uF 10k | | 1MEG
|| ___ | | ___
IN -||------|___|--------+----|-|___|------+ OUT
|| | | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | |\| 741 |
| +--|-\ |
| | >-----------+
+-------|+/
| |/|
.-. |
| | |
| |22k |
'-' |
| |
+---------+GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

It'll amplify the signal 100x. The amplification is set by the ratio of the
1MEG/10k resistor, so you can change it if you need to. Note that if you
change the resistors, the total resistance seen by the inputs of the opamp
should be near each other to deal with leakage. In this case, they are both
near 10k. Thus, scale it by changing the 1MEG resistor rather than the 10k
resistor to keep the resistance seen by the -input around 10k. A 1MEG pot in
series with a 10k resistor would be a good way to do this. All the way on
gives you 101, all the way off gives you 1.

The output will center around Vcc/2. Also, the output falls off to 1/2 (3db
point) at about 17kHz with a 100k load, so keep the frequency you measure
below that.

Regards,
Bob Monsen

 

[Rich Grise]

Actually, you'd be better off just getting a new current probe.
1 mV/Amp is way down there in the noise - if you amplify a 1mV
signal to a useful level, you're also amplifying the noise and
error(s). If you're dealing with only a few amps, a 0.1 ohm
resistor in series with the circuit under test will give you
100 mV/Amp, 0.01 ohm gives 10mV/A, etc.

O a current probe might be pretty simple - carefully cut a
toroid in half, polish the cut ends so they'll mate well, and
wind 10 or 100 or 1000 turns on the one half, and clamp it on.

If you do this, you _MUST_ provide a current path because the
volts out of a current transformer will try to go high enough
to force the current through the secondary.

Good Luck!
Rich

RKovach wrote:

I have a P-C based scope that will only go down to .1 volt per division. I
would like to use a clamp-on amp probe I have, but this probe puts out .001
volt per amp, making it hard to see a good pattern for anything less than
100 amps. I was told I could use a preamp to boost the signal. I would like
to know if I could make one using a 741 op-amp, or would this distort the
pattern. I would only be using this to check alternator output on cars, so I
don't need to be concerned with high bandwidth.

 

[Tibur Waltson]

Hi Rich,
I'd been trying to build a current probe for my analog scope to measure
everything, but today I need it to test my alternator's output making sure it
puts out 30A under load. Looks like you've explain it well to Mr. RKovach.
But what does it mean to clamp it on? And if I do this, I _MUST_ provide a
current path because the volts out of what current transformer will try to go
high enough to force the current through which secondary? Sorry for asking,
I'd never build a current probe before?
TIA
Tibur

"Rich Grise wrote in message

Actually, you'd be better off just getting a new current probe.
1 mV/Amp is way down there in the noise - if you amplify a 1mV
signal to a useful level, you're also amplifying the noise and
error(s). If you're dealing with only a few amps, a 0.1 ohm
resistor in series with the circuit under test will give you
100 mV/Amp, 0.01 ohm gives 10mV/A, etc.

O a current probe might be pretty simple - carefully cut a
toroid in half, polish the cut ends so they'll mate well, and
wind 10 or 100 or 1000 turns on the one half, and clamp it on.

If you do this, you _MUST_ provide a current path because the
volts out of a current transformer will try to go high enough
to force the current through the secondary.

Good Luck!
Rich

RKovach wrote:

I have a P-C based scope that will only go down to .1 volt per division.
I
would like to use a clamp-on amp probe I have, but this probe puts out
..001
volt per amp, making it hard to see a good pattern for anything less than
100 amps. I was told I could use a preamp to boost the signal. I would
like
to know if I could make one using a 741 op-amp, or would this distort the
pattern. I would only be using this to check alternator output on cars,
so I
don't need to be concerned with high bandwidth.

 

[Rich Grise]

Well, I'll assume you know how a transformer works in general.
There's the primary, the secondary, and the core, which is
magnetic material. If you use a toroid - there's an example
of a fairly large one here -
http://www.alltronics.com/transformers.htm
The toroid on top,
The windings are the secondary, the toroid core is the, well,
core, and the primary consists of one "turn" of wire, which
is the lead passed through the center hole of the toroid.
(not shown on that page above) For the formulas, it acts
exactly like one turn. So if you've got, say, 100 turns of
wire for the secondary, and the one "turn" (it actually
doesn't do any turning - if you did, and looped it through
the hole twice, that'd be two turns.) for the primary. And
the voltage ratio in a transformer is the turns ratio, and
the current ratio is the inverse of the turns ratio. So
with 30 A through a 100-turn-secondary current transformer
would give you a current of 30/100 A, or 30 mA. Stick a 100
ohm resistor across the secondary leads, and it will develop
100 * .03 = 3 volts. A 1K ohm would develop 30 volts and so on.

So in that case, you'd unbolt your alternator output lead,
thread it through the toroid, and bolt it back into place.

And you can probably find a suitable toroid at surplus
somewhere.

Good Luck!
Rich

Here's just one sampling of |"current transformer" schematic|
on google:
http://www.alltronics.com/transformers.htm

"Tibur Waltson" <Toush@hi.com> wrote in message news:<4a51e983a9b49657af6e168d545e90b8@news.bubbanews.com>...

Hi Rich,
I'd been trying to build a current probe for my analog scope to measure
everything, but today I need it to test my alternator's output making sure it
puts out 30A under load. Looks like you've explain it well to Mr. RKovach.
But what does it mean to clamp it on? And if I do this, I _MUST_ provide a
current path because the volts out of what current transformer will try to go
high enough to force the current through which secondary? Sorry for asking,
I'd never build a current probe before?
TIA
Tibur

"Rich Grise wrote in message
Actually, you'd be better off just getting a new current probe.
1 mV/Amp is way down there in the noise - if you amplify a 1mV
signal to a useful level, you're also amplifying the noise and
error(s). If you're dealing with only a few amps, a 0.1 ohm
resistor in series with the circuit under test will give you
100 mV/Amp, 0.01 ohm gives 10mV/A, etc.

O a current probe might be pretty simple - carefully cut a
toroid in half, polish the cut ends so they'll mate well, and
wind 10 or 100 or 1000 turns on the one half, and clamp it on.

If you do this, you _MUST_ provide a current path because the
volts out of a current transformer will try to go high enough
to force the current through the secondary.

Good Luck!
Rich

RKovach wrote:

I have a P-C based scope that will only go down to .1 volt per division.
I
would like to use a clamp-on amp probe I have, but this probe puts out
.001
volt per amp, making it hard to see a good pattern for anything less than
100 amps. I was told I could use a preamp to boost the signal. I would
like
to know if I could make one using a 741 op-amp, or would this distort the
pattern. I would only be using this to check alternator output on cars,
so I
don't need to be concerned with high bandwidth.