Measuring current

[Vmax]

I’m trying to measure current using the voltage drop across a resistor. It’s basically a battery charger. The simplified circuit is attached. “A” is common "–" between picaxe and battery. I need to measure the voltage drop across A and B, but B is more negative than A. Any idea how I can change this to a positive voltage I can measure with an ADC pin?
Any help appreciated.

 

[Dippy]

You can use an op-amp to give a decent output from a low-side current sense resistor.
Though it makes life easier if your sense res is between load and ground as opposed to gnd and battery -ve.
If you have a very low Sense-R you won't get much resolution on your ADC without some gain as obv your Sense R will be pretty low.

 

[leftyretro]

I’m trying to measure current using the voltage drop across a resistor. It’s basically a battery charger. The simplified circuit is attached. “A” is common "–" between picaxe and battery. I need to measure the voltage drop across A and B, but B is more negative than A. Any idea how I can change this to a positive voltage I can measure with an ADC pin?
Any help appreciated.

Brain fart here, never mind ;-)

Lefty

 

[Dippy]

I reckon Stan will have a bright idea!

 

[manuka]

It depends on the current levels of course, but this old "PD across a very low value series resistor" ammeter trick still makes good sense, & with your sort of setup (assuming ~100ma-1 Amp charge?) you can use READADC10 without an Op-Amp. Ohms Law ( V=IxR ) of course means 1 Amp passing x .1 Ohm = 100mV of course across it- an easy PICAXE read.

I used similar (but with a more conveniently sourced series 1 Ohm) for a modified bike computer Amp.Hour meter => http://picaxe.orconhosting.net.nz/bikeamph.jpg

 

[Dippy]

There you go Vmax. Easy.
But what sort of resolution/accuracy/current range were you looking for? This will define your method.

 

[Vmax]

Hi Dippy:

Now that you mention op-amp, I vaguely remember someone using one. I’m not very familiar with it. One new chip to learn about, I guess. Current would typically be .1 to 1 Amp but could sometimes go as high as 10 Amps.

 

[manuka]

Vmax- New chip? Op. Amps date from the 1950s valve era, & even the classic 741 IC is ~40 years old!

With your sort of generous currents you certainly can avoid using op. amps IMHO. In fact,given your near "arc welding" charging level,maybe even consider a Hall Effect Sensor (HES) such as the UGN3503U. Although a low level output (readily boosted by some non op. amp tricks) these are very PICAXEable & have of course no insertion losses. A bonus is that they can also read discharge & charge. Are you familiar with these little darlings? See Andrew's Picasa pages => http://picasaweb.google.com/picaxe/InputCircuits/photo#5135515320012399298

 

[premelec]

Take a look at www[dot]amploc[dot]com and also Allegro Microsystems units -

 

[Dippy]

Oh Premelec, you should know by now that anything over a quid/buck frightens people

A 0R1 and 10 amps. That's a lot of watts and vdrop.
Have you actually done the calcs for voltage drop over your desired current range with your specificed resistor?
And then have you correlated that against just using a 12bit ADC to identify 'best' resolution?
Is that size of Vdrop @ 10 Amps going to upset anything?

Have you looked at the previous posts about changing the position of your sense Res?

 

[Vmax]

Thanks guys:

Manuka: I didn’t mean to say that an op-amp is new, just new to me. I’m still a newbie with this stuff.

I’ve done some looking around and come up with the attached circuit. It’s an inverting amplifier (gain of 10). Not certain about the role of the cap. Can anyone tell me what to do with the null/offset pins (i.e. tie them to ground)?


Thanks

 

[premelec]

I don't want to confuse you further but looking at your original post it appears you just want to charge a battery with a generator... assuming the battery is low voltage rather than hundreds of volts you can power the op amp with a three terminal regulator to reduce the battery voltage to 5 volts or so and use the op amp to raise the _positive_ voltage across the current sense resistor x 10 or 100 or whatever.

You need to consider the common mode voltage characteristics of whatever OP Amp
you use - that's the voltage the input terminals can be at and still have the amp work right [see data sheet!]. Some don't work all the way to ground potential - the most versatile are called R to R - Rail to Rail and can be R to R input or output or both.

I suggest you also look at zetex current monitoring parts e.g. ZXCT1009 or ZXCT1010 which gives a simple schematic of what's in this $1.50 part that translates a small current sense voltage to an output current proportionate which can then be read... there are limits to how high the voltage can be with chips like this - 20v in the case of the cited part....

Anyhow this should give you a better idea of current monitoring methods... hope you are still having fun with it!

 

[manuka]

I'm a great op. amp fan, but given your heavy duty setup why bother here? As your shunt is now 0.05 Ohms, 1 Amp thru' it will have a pd of 0.05V = 50mV across. As this is readily READADC(10),at least TRY a direct PICAXE read & see how suitable it'll be!