Need help with current sensing circuits

P

Patrick

Guest
Hello!

I was wondering if anyone has or knows where I can find a simple, cheap
current sensing circuit that I could use to sense AC currents. I am
currently working on a power control project that will monitor 120 VAC
outlets with a total maximum current of 15 amps. We are currently
working with a current sensor that outputs an AC voltage (actually a
varying DC voltage since it does not drop below zero) from an AC input
current from 0-50A. When the input current is zero the output is about
2.5V. At the full rated input current of 50 amps the sensor output has
a peak voltage swing of about 2.5V which ranges from zero to 5V at the
full 50 amps (swings around the 2.5V q-point).

The current sensor works great, but the problem is that we need to find
out what the actual current is and this poses somewhat of a challenge
since the output of the sensor is a varying DC (sinusoidal) voltage.
We've tried some software techniques to capture the peaks, but it takes
up too much processor time and is therefore not feasible.

Anyone have any ideas or circuits I might be able to try? Any help
will be greatly appreciated.

Patrick
 
There's an opamp circuit called precision rectifier. Use the rectified
voltage to charge a capacitor to read a DC voltage instead of AC. You
will need of course a resistor across the capacitor to discharge it to
a lower DC voltage when the current being measured drops.

Patrick wrote:
Anyone have any ideas or circuits I might be able to try? Any help
will be greatly appreciated.

Patrick
Click to expand...
 
Patrick wrote:
Hello!

I was wondering if anyone has or knows where I can find a simple, cheap
current sensing circuit that I could use to sense AC currents. I am
currently working on a power control project that will monitor 120 VAC
outlets with a total maximum current of 15 amps. We are currently
working with a current sensor that outputs an AC voltage (actually a
varying DC voltage since it does not drop below zero) from an AC input
current from 0-50A. When the input current is zero the output is about
2.5V. At the full rated input current of 50 amps the sensor output has
a peak voltage swing of about 2.5V which ranges from zero to 5V at the
full 50 amps (swings around the 2.5V q-point).

The current sensor works great, but the problem is that we need to find
out what the actual current is and this poses somewhat of a challenge
since the output of the sensor is a varying DC (sinusoidal) voltage.
We've tried some software techniques to capture the peaks, but it takes
up too much processor time and is therefore not feasible.

Anyone have any ideas or circuits I might be able to try? Any help
will be greatly appreciated.

Patrick
Click to expand...
Hi, Patrick. From your description, I would guess you might have an
Allegro ACS750SCA-050, which has a zero ohm current shunt and attached
Hall effect circuit to detect current going through the shunt. When no
current is going through the shunt, its output is Vcc/2, or 2.5V. When
current is going through the shunt, a voltage is superimposed on this
DC level of +/-40 mV per amp up to +/-50 amps (which gives an amplifier
output of 0.5V to 4.5V). It has a -3dB response at AC current up to
13KHz, and a combined guaranteed accuracy of +/-2% at room temperature.

You have been very unclear on exactly what you want. You've said
you're using some kind of PC, SBC or uC to read the voltage, but the
repetitive readings are taking up too much processor time. You'd like
a simple, inexpensive means to do what you're doing now, but you
haven't been clear on just what you're doing now, how simple, and how
inexpensive. "Measuring current" can mean just measuring peak current,
measuring average current or RMS current. You haven't indicated
exactly what information you need, or in what time frame you need it.
Do you need information every half cycle, every cycle, a certain number
of times a second? Oh, and by the way, what kind of accuracy do you
need? Your error budget is already over 2%. Do you need a combined
accuracy of 2.1%, 3%, or 12%? The key to a good answer is a well-asked
question.

Your lowest cost solution for doing just what you're doing now would be
using a PIC with built-in ADC. You can read the voltage output off the
ACS750 many times for every AC cycle (generate an interrupt off the
power supply transformer secondary?), and then either figure out the
peak current (if that's your pleasure) or add 'em up to integrate total
current. There are many ways to communicate that information to
whatever you've got as a processor. And there's no law against having
two processors in a piece of electronics.

If you only need information on peak current on only half the cycle,
you're only taking a few readings a second, and you can live with
several percent accuracy, the simple and cheap answer is to use a
schottky diode feeding a cap with a bleeder resistor, like this (view
in fixed font or M$ Notepad):


.--------------.
| ACS750 |
| |
| |
| |
| |
| | 1N5817 To high Z-in uC ADC
| |\ |
| -| >---o----->|-o----o--->
| |/ | | |
| | --- .-.
| | --- | |
| | 0.1uF | | |1M
| | | '-'
'--------------' | |
| |
=== == GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

By using software compensation for the diode drop, you can achieve
accuracy within several percent. The obvious flaw here is that when
the current decreases, it will take half a second to get an accurate
reading. If that's OK with you, that's the simplest. You can juggle
settling time and decay time by changing resistors to get something
closer to what you want.

There are a lot of solutions to your problem. The Analog Devices chip
mentioned in another post being an elegant one chip solution, and while
not the least expensive, will give you the best, most accurate answer.
The precision rectifier is a good answer which solves the issue with
the voltage drop of the diode above. There are many other answers.
But you could give a little more information if you want to get the
best answer.

Good luck
Chris
 
Chris wrote:
snip

.--------------.
| ACS750 |
| |
| |
| |
| |
| | 1N5817 To high Z-in uC ADC
| |\ |
| -| >---o----->|-o----o---
| |/ | | |
| | --- .-.
| | --- | |
| | 0.1uF | | |1M
| | | '-'
'--------------' | |
| |
=== ==> GND GND
snip
Click to expand...
Small error. Try this:

`
` From ACS750 To ADC
` 1N5817
` >--->|-o-----o--------->
` | |
` --- .-. VCC
` 0.1uF--- | | +
` | | |1M |
` | '-' .-.
` === | | |1K
` GND | | |
` | '-'
` | |
` '------o 2.5V
` |
` .-.
` | |1K
` | |
` '-'
` |
` ==` GND
`
`
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Sorry. Less haste, more speed.

Chris
 
Sorry-- the first circuit is the one you should use. The multitasking
threader is defective tonight -- long week.

Chris
 
make a drawing at www.yahoo.com ( 100 dpi will do fine with a scanner)
; go to the mailbox the ID is
'schetsen' and the password is 'diverse'. send the drawing to the same
mailbox 'schetsenATyahoo.com'
and I'll see what I can do for you ( for free of course )
 
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