Lee, If the meter is powered from the pack (not from the 12V aux battery) then why would you need an isolated supply? I would take a simple buck down converter (48V to 12V nominal) without worrying about insulating the power supply, as long as pack and meter are floating and there is double-insulation between user and "hot" (pack) voltage although at only 48V that may not even be required (one of the reasons that PoE is using 48V to power network equipment over Ethernet cabling)
Cor van de Water Chief Scientist Proxim Wireless office +1 408 383 7626 Skype: cor_van_de_water XoIP +31 87 784 1130 private: cvandewater.info www.proxim.com This email message (including any attachments) contains confidential and proprietary information of Proxim Wireless Corporation. If you received this message in error, please delete it and notify the sender. Any unauthorized use, disclosure, distribution, or copying of any part of this message is prohibited. -----Original Message----- From: EV [mailto:[email protected]] On Behalf Of Lee Hart via EV Sent: Monday, July 27, 2015 3:42 PM To: Electric Vehicle Discussion List Subject: Re: [EVDL] 48V Energy Meter (Lead-Acid)... EVDL Administrator via EV wrote: Robert Bruninga via EV wrote: >> But remember it is still dropping all that voltage as HEAT. So say >> your meter device needs 500 mA at 12v, and your pack is 48v. Then >> the zener is dropping 36v at 500 mA and will be disipating over 18W of heat. The E-meter/Link-10/LinkPro family of meters have switching power supplies; so their supply current is inversely proportional to supply voltage. The current also changes, depending on what the meter is displaying, and ambient brightness (the LEDs/backlight draw more currrent in bright light, such as if sunlight is striking the face). That makes them hard to power with a simple zener or series resistor. The manual claims 9.5-40v, but they only work right from about 10.5v to 36v. At 10.5v, supply current with all segments lit and sunlight on the face is around 200ma. At 36v under the same conditions, supply current is only about 75ma. The LinkPro with its LCD display has a lower maximum of around 50ma. When in "sleep" mode (display blanked) and in the dark, current is more like 20ma at 10.5v, and 10ma at 36v. The wide range of currents means you can't use a simple dropping resistor. Its drop would change drastically depending on voltage and what the meter was doing. Even a simple zener is problematic. A 48v nominal pack could easily vary from 60v (on charge) to 40v (under load when almost dead). If you use a 27v zener, then 40-60v at the battery is 13-33v at the meter. That keeps it within proper operating range. But that zener will have to dissipate up to 27v x 200ma = 5.4 watts. That's well beyond what you can do with any common readily available zener. > But perhaps it's all moot since off the shelf low-power DC:DC > isolators have gotten quite cheap, thanks to Chinese sweatshop manufacturing. I agree. However, 99% of the cheap DC/DC converters have really lousy isolation voltages and reliability. They often *advertise* some high isolation voltage (like 1000 volts), but the fine print on the data sheet says this is only a ONE-time test, for 1 second maximum, and only spot tested and not guaranteed. When you take them apart, you find the isolation transformer is wound with nothing but ordinary magnet wire for its primary and secondary, wound directly on top of each other. This gives at best a 100v continuous isolating voltage rating. -- Pollution is nothing but the resources we are not harvesting. We allow them to disperse because we've been ignorant of their value. -- R. Buckminster Fuller -- Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com _______________________________________________ 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) _______________________________________________ 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)
