Current switching and monitor

[Adraen]

Hi,

So, I would like to measure the current drawn by a 5V line with a maximum draw of 750ma, the other requirement is to be able to switch on/off the line.

The original idea was to use a N-channel mosfet on the low-side for on/off, and use a shunt with an op-amp on the high-side, but i've been thinking if I could do anything smarter ....

What i'm thinking is to use a p-channel mosfet on the high-side and use the Ron resistance of the mosfet as the shunt resistor and use something like ZXCT1009 to monitor the drop accross the shunt. I'm not sure if that would work though, from your experience do you think the mosfet resistance is accurate enough to use as a shunt ?

I might do this the wrong way though there might be some IC that do switching/monitor at the same time but I wasn't able to find one.

Anyway, would you have some ideas on how to go about doing something like this ?

Thanks

 

[barry]

I think the on-resistance of the MOSFET is too variable to do this accurately, although if you're just looking to detect an overcurrent it might work fine. But if you are actually trying to measure the current, I wouldn't trust it. Then again, an accurate resistor costs next to nothing, so why not just use that?

 

[Adraen]

Cheers for the feedback, I'm trying to measure the current not only for overcurrent. The point wasn't really so save money for the price of the resistance just to see if it could be done with just the mosfet.

What might end up doing is use a n-mosfet on the low-side and a shunt + ZXC on the high side

Thanks

 

[barry]

I just remembered; look at this https://www.onsemi.com/pub_link/Collateral/NTMFS4854NS-D.PDF

Sensefets are MOSFETS with an auxilliary output proportional to current. This might be just what you want.

 

[Adraen]

That's great thanks, I had a look for the last half hour couldn't find any sense FEt or Mosfet with current mirror at a decent price or available on farnell.

anyway that's great to know that some IC do that, I will keep looking might have missed something on Farnell.