I just bought an analog runtime hour meter. Specs state the operating voltage should be 10-80vdc. Installation instructions state to connect positive to one terminal and negative to the other terminal, polarity not necessary since the unit is polarity insensitive.
Since the unit obviously has a motor that needs to run in one direction only to increment the time, I'm having a hard time visualizing what they've done inside the sealed unit to make polarity a non-issue. Any ideas?
KC
Bridge rectifier?
Sylvia.
diodes?
On 5/13/2009 9:25 PM Sylvia Else spake thus:
Bingo.
Agreed. That's the usual way if the circuitry following can tolerate 2 x diode drops on the voltage.
Arfa
ng
in which case, it ought to run on ac?
No. It may have minimal filtering after the bridge.
in which case, it ought to run on ac?
Usually not if it has been designed just as a universal polarity DC device, as it will have no filter cap etc. It does, however, often work the other way round. Back in the day, some cordless phones had AC power supplies. When these failed, or got lost in house moves etc, a DC supply could be used instead, provided you picked one with about 50% more DC output than the RMS value of the original AC supply. This allowed for the 2 diode drops, and the fact that the filter cap would have brought the rectified DC up to approaching the peak value of the original AC.
It doesn't always work though. Many guitar effects pedals use AC supplies, and the effect doesn't work when the owners try to substitute a DC supply. This is because the pedals often have supplementary supplies inside which rely on having an AC input, to generate negative rails for opamps etc.
Arfa
KC Inscribed thus:
I'm curious ! Why do you think it has a motor ? I've seen similar devices that are purely chemical and require DC to function. You can wind them back by reversing the polarity of the supply.
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