damon henry wrote: > I have been around and using E-meters for many years now and this is the > first time I have ever heard of using a zener diode as a method to create > an isolated power supply for one. Is it really that simple?
Yes, but it does not provide an ~isolated~ supply. [...] > If I understand this method I would simply connect the emeter power > through a zener diode off my traction pack and that would provide both the > required isolation and enough of a voltage drop to keep from burning the > meter out. That sounds easy :) Do you just wire the zener into one of the > power leads from the traction pack to the emeter? Does it matter whether > it is on the positive or negative lead? Are there other components that > could/should be added like a fuse or current limiting resistor? Correct. Place the zener in series with the positive power wire from the pack, oriented such that it drops the pack voltage by the zener voltage (if you wire it backwards, it will drop only an ordinary forward diode drop of 0.7V and you will probably fry the meter). You cannot place the zener in series with the negative supply wire since the E-meter supply ground is common with the traction pack voltage sense ground. Normally, we need to provide a small isolated DC/DC to power the E-meter, but this isolation requirement is because the E-Meter connects the traction pack negative (from its voltage sense) to the supply ground and we don't want our 12V house battery/supply to be referenced to the traction pack. For a 48V pack, you may be less concerned with ensuring the traction pack is isolated from the chassis, and if only the E-meter is powered from the pack via the zener, then it remains isolated from the 12V system anyway. What are you using on your bike to power the lights, horn, etc.? If you have a 12V battery or DC/DC for this anyway, then you could just power the E-meter from this 12V source. Doing so will result in the E-meter connecting traction pack negative to the 12V ground (chassis), defeating any isolation your present DC/DC offers, however, at traction pack voltages of 48V or less it is not uncommon for non-isolated DC/DCs or voltage taps off the traction pack to be used to power 12V accessories. Include the same fusing in the power lines to the E-meter as you normally do (or as shown in the manual if you've been neglecting fusing ;^) Cheers, Roger. _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
