ACS712 module

[FrankT]

Rather than hijacking this thread (https://www.eevblog.com/forum/beginners/fix-current-sensor/) I was wondering whether the ACS712 is suitable for monitoring 240v 15amp ac?

There is an x30 version that can handle 30 amps, but how do you get 30A continuous into a small smd part?

And in particular, the board from ebay, (http://www.ebay.com/itm/400770304649) doesn't look like PCB tracks are...
1) heavy duty enough for 15A (they claim 30A), and
2) there is enough isolation for safe 240v operation (look at the 4th picture of the underside of the board).

[FrankT]

2) there is enough isolation for safe 240v operation (look at the 4th picture of the underside of the board).

I can answer my own 2nd question - isolation isn't an issue because it is a current sense so the voltage difference between the connections will be small.

[Aodhan145]

They should be ok. If you would like I can take out my 20A ones and try them on a 15A mains load and measure the temp to make sure the traces don't get incredibly hot. I can just stick my fingers on the trace to feel how hot it gets. What I don't get is how 30A mains can go through a soic chip. How does it not explode

[mariush]

It does not explode because the internal resistance is extremely small , about 1.2 mOhm at 25c according to the datasheet  (see page 4, primary conductor resistance ) : http://www.allegromicro.com/~/media/Files/Datasheets/ACS712-Datasheet.ashx

Power dissipated in the chip from just this current would be  P = IxIxR = 20A x 20A x 0.0012 ohm = 0.48 watts.  This is a substantial for a plain SOIC chip, but not really a killer.

With some wide pads on the pcb and some space around the chip, the chip will radiate the heat away and stay cool and everything would work fine. Of course, as it heats the measurement will drift a bit, the resistance itself may deviate a bit from that exact 1.2 mohm, basically just because the IC says it can measure up to 20A, it doesn't mean it's fine for you to use it to measure currents up to 20A for long periods of time.

Alternatively, you could always go for a small heatsink on the IC itself, though it wouldn't do much cooling.

[FrankT]

Thanks for the replies.  They are pretty cheap, so I've ordered a couple to play with.

[Aodhan145]

The PCB traces are very short as well so they are only a few m? so the same applies. the width of the trace is  It has traces on both sides but what I don't like is only 2 vias on each trace They could only carry like 1A each. I measured the trace width with my callipers it is around 175 mil on each side which means the traces temperature would probably rise between 20-30 degrees C  to carry 15A.

[Jeroen3]

Those via's don't really matter if the terminal block is properly soldered.

And this note:

2 The Allegro evaluation board has 1500 mm2 of 2 oz. copper on each side, connected to pins 1 and 2, and to pins 3 and 4, with thermal vias connecting
the layers. Performance values include the power consumed by the PCB.

[H.O]

Hi,
The 3000 qty price for the ACS712-30 is $1.80 at Digikey.
The EBAY module is using a terminal block with either 0.2" or 5mm pitch. The highest current rating of such a terminal block (again at Digikey) I can find is 24A and the cheapest one of those cost $0.53 @1200pcs.

This complete module is priced at $1.70, including shipping, and they're making a profit at that.

1) Is it reasonable to think they are using a genuine Allegro chip? Perhaps but I wouldn't bet on it. A quick Google search for fake Allegro current sensor doesn't seem to turn up anything though.

2) Is it reasonable to think the terminal block they are using is rated for the current they're claiming the modul can handle? I'm pretty sure the answer is no. I certainly wouldn't want to run anything close to 30A thru that thing on a continous basis.

With that said, it's cheap enough to just get a couple and try it out.