Products > Test Equipment
BM235 LoZ Mode
bdunham7:
--- Quote from: EEVblog on November 23, 2024, 04:38:49 am ---As such you can expect different behaviour based on the exact type of PTC used.
--- End quote ---
The part about that seems odd to me is that there are plenty of PTCs that don't behave that way and I've never seen or heard of one that does. Of course there's a lot I haven't seen or heard of so that might not mean much. Still, given the wide availability of PTCs without this "feature" and no good reason that I can think of to deliberately include it, using such a part would seem an odd choice.
If anyone has a dead BM235 they want to send me, I'd be happy to dissect it and see what's going on.
bdunham7:
OK, I think I've figured it out from an old video posted by joeqsmith. Basically they use the same PTC for the LoZ, ohms and diode ranges, which isn't uncommon, but to minimize parts count and selector switch contacts they don't shunt the other side of the PTC to ground but rather send it through the same two-transistor protection clamp that serves the ohm/diode ranges. That two-transistor Q8/Q9 clamp activates at a voltage of the BE-zener voltage plus one diode drop or about 7-8V.
In the attached picture, you can see PTC2 (large) that is the one the LoZ/ohms/diode ranges use. That goes to the aforementioned Q8/Q9 clamp and I presume that in LoZ mode the stuff to the right is just switched open by the selector switch. The voltage reading is then taken through the upper circuit R7/PTC1/etc just like any other. This accounts entirely for the observed behavior.
Kean:
Thanks, that explains the behaviour.
A model of clamp for simulation is at https://www.eevblog.com/forum/blog/eevblog-1157-transistor-zener-clamp-circuit/msg2040238/#msg2040238
This has saved me from having to pull one of my BM235s apart tomorrow to trace the circuit. :-+
mbarszcz:
Thanks everyone for the good discussion and information.
The two transistor clamp protection clicked for me today after watching Dave's video:
Strange choice to send the LoZ mode on the BM235 through that. It wouldn't matter in a lot of other applications, but in a multimeter where you are literally trying to read the voltage, it seems like a poor design decision on Brymen's part.
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