Hi Gene! On 03/14/2014 09:23 PM, Gene Heskett wrote: > Greetings again; > > After putting in a diode, that part seemed to work, but this C41 seems to > have gone south again. (and again (and again)), and its looking like a 4th > one has failed. Symptoms seem to be an output impedance in the 100k > region, making 375 rpm maximum, and dropping nearly 100 rpm when I put a 10 > megohm digital meter on its output to measure it, and getting a hair over 5 > volts for my trouble, and a fresh LM358 in the output voltage follower > stage acts exactly like the one I pulled out thinking it was bad. > > So it looks like I need another, but it seems to me there ought to be > another choice besides this non-linear POS, and the PMDX-106. Or, do I > fire up eagle & design my own? > > The PMDX-106 works great, but its physically too big for the area it needs > to fit in by about an inch both ways. > > Anybody else got any suggestions? > > Thanks guys.
Do you talk about this unit? http://cnc4pc.com/Tech_Docs/C41R2_USER_MANUAL.pdf I think it should be fairly easy to reverse-engineer the schematic of this little thingy, as there are only through-hole components and (as it looks like) just a single copper layer. But your comment about the LM358 in conjunction with the problem that you cannot go below ~200mV output voltage caught my attention: The LM358's output stage does not really sink current down to GND, so if the input of your following circuit does not have a pull down, it is likely that this amplifier is the cause of the problem. If so, you could simply replace it by a RRIO (rail-to-rail input and output) type, most standard opamps are pin compatible. If 16V max. Vcc are enough, the TLV2372 could be a suitable replacement. There should be many more to suit your needs, a quick parametric search yields at least three parts which should fit as well: http://www.ti.com/lsds/ti/amplifiers-linear/operational-amplifier-op-amp-products.page#p78=In;Out&p1261max=32;36V;36&p480=2;2&p10max=0.02;1000000&p2954=PDIP Cheers, Philipp
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