Help trying to make current probe to fit scope.

[Ricky Spartacus]

I`m making a Miniature Current Probe for my oscilloscope as a learning
experience. Some of the things I`m trying to measure is a signal from
a fuel injector. I'm told that a current probe should be used to
measure the injectors.

I`ve heard of a current sensor in automobiles. I`d never found one.
Rick

 

[mike]

I`m making a Miniature Current Probe for my oscilloscope as a learning
experience. Some of the things I`m trying to measure is a signal from
a fuel injector. I'm told that a current probe should be used to
measure the injectors.

I`ve heard of a current sensor in automobiles. I`d never found one.
Rick

Was there a question?
There are different types of current sensors with different pros/cons.
Inductive sensors are a transformer with a one turn primary.
The laws of transformers apply.
mike

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
4in/400Wout ham linear amp.
Honda CB-125S
400cc Dirt Bike 2003 miles $550
Color LCD overhead projector
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[Ricky Spartacus]

I`m making a Miniature Current Probe for my oscilloscope as a learning

Was there a question?
There are different types of current sensors with different pros/cons.
Inductive sensors are a transformer with a one turn primary.
The laws of transformers apply.
mike

Forgot to include a question, but you answered it. Are you saying that
I could use a small transformer as a current sensor? If I hook-up a
fuel injector in parallel to a probe using a tiny transformer, I could
get a current reading?

My intention is to see the difference in current when my Honda runs
rich and when it runs lean. I`d thought maybe I could see a higher
voltage produced when it runs rich.

 

[Big John]

Hi Ricky,

For a low voltage DC system most folks sense current by placing a low
value, high wattage resistor in series with the load (fuel injector) and
then sense the voltage across the resistor. Say the fuel injector draws 1/2
amp peak. Using a 0.1 ohm resistor (non-inductive) the peak voltage across
the resistor would be 0.1 ohm X 0.5 Amps = 50 millivolts. OK for most
scopes and you won't be losing enough voltage across the resistor to be a
problem. A power rating of 1/2 or 1 watt should also be OK for 0.5 Amps.
This method is cheap and easy but it doesn't isolate the equipment from the
scope, but should be OK for a 6 or 12 volt electrical system. By the way
there are special resistors used to accurately measure higher DC currents.
They are called "shunts".

Current is also sensed with current transformers, but these are used for
AC currents. They won't work for your application because the narrow
current pulses you are looking at will have both a DC bias and high
frequency components.

FYI, for wide bandwidth current measurements down to DC with isolation
hall effect current probes are often used.

Hope this helps,
Big John



Ricky Spartacus wrote in message ...

 

[mike]

Forgot to include a question, but you answered it. Are you saying that
I could use a small transformer as a current sensor? If I hook-up a
fuel injector in parallel to a probe using a tiny transformer, I could
get a current reading?

My intention is to see the difference in current when my Honda runs
rich and when it runs lean. I`d thought maybe I could see a higher
voltage produced when it runs rich.

The first thing to do is decide WHAT you want to measure.
I don't know how fuel injectors work, but they MIGHT work by modulating
the WIDTH of the driving pulse rather than its amplitude. In that case,
you don't need to measure current at all. In fact, the average current
will be influenced by factors other than the amount of fuel dispensed
and lead you to wrong conclusions.

Measure the width of the voltage pulse. Use a voltage probe and a
scope. Heck, you might even be able to use an old dwell meter, but that
will be rpm influenced.

If you really need a current measurement and it is pulsed, you can get
the AC component with a transformer. Start with a split-core ferrite
with some turns as a secondary and clamp it on the injector wire.
that should give you a relative indication of the peak current. If you
want the shape, you need a lot more "equalization circuitry"
Shunt the secondary with the smallest resistor that will let you get
a reading on your scope. Lower resistance -> better low frequency response.

If the clamp-on probe from your timing light unplugs, you can also use that.

Again, making calibrated measurements and looking at AC current
waveforms is hard. DC is lots harder.
Making relative peak measurements on AC currents is not so hard.

Not measuring current at all...priceless...
mike

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
4in/400Wout ham linear amp.
Honda CB-125S
400cc Dirt Bike 2003 miles $550
Color LCD overhead projector
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/