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**** > > ** ** >
