You should use a higher resistance value. I use 0.39 Ohm to measure 0 - 4A range. Which lead to 1.6 V at resistance when a current of 4A is passing through it. Then it becomes easier to protect over current. The resistance must be able to handle a power of 7W. There is no need here for a precision resistance, but a constant resistance value over temperature. Measure the resistance value with an accurate/precision multimeter to convert volt into ampere.
Arnaud _____ From: Jeff Berkowitz [mailto:[email protected]] Sent: dimanche 28 octobre 2012 16:24 To: [email protected] Subject: Re: [Vo]:Home made data logging ammeter Thanks Arnaud. The goal of the diodes is to protect the board - it cost more than the resistors did. But yeah, I know. Your point is especially true because those 1N5400s are odd - they list a forward voltage around 1.2v, rather than 0.6v as you would expect for a typical silicon diode. I guess they have two junctions in series? Otoh, they are spec'd to carry up to 200 amps forward for half a cycle (8ms) You can't protect semiconductors with MOVs or fuses either ... too slow. Just a question of curiosity, but is there a good answer? Jeff On Sun, Oct 28, 2012 at 3:44 AM, Arnaud Kodeck <[email protected]> wrote: Nice job ! But let me explain that your protection diodes aren't effective for current overrun on sensing resistances. As you said, the voltage drop on forward current of the diode is around 1V maybe a bit less . let's say 500mV. At 500mV, the resistance will dissipate (U^2/R) (0.5)^2/0.05 = 5 W. At 1V, the power dissipated at resistance is 20W! So the resistance will burn even if the diodes are closed, because maximum power rating of the resistance is 1W. To have 500mV at the resistance, current flow will be 10A. Diodes are only 3A . The protection will be effective only for non repetitive small spikes, but rather more to protect your phidgets. Arnaud _____ From: Jeff Berkowitz [mailto:[email protected]] Sent: dimanche 28 octobre 2012 02:42 To: [email protected] Subject: [Vo]:Home made data logging ammeter Hi all, I built this USB-connected ammeter so we could log current flow accurately and rapidly while doing electrolysis and also electroplating. http://pdxlenr.blogspot.com/2012/10/a-pretty-good-data-logging-ammeter.html Jeff
