You are right. I was not paying attention. If the zener is only droping from pack voltage down to the 35v maximum input voltage fo the device and it only draws 150 mA, then you'd probably use two 12v zeners to drop from the fully charged pack voltage of 56 down to around 32v and so th zeners would need to be rated at 0.15A * 24v or 3.6W and since there are two of them, then a pair of 2W zeners would do it. Bob
-----Original Message----- From: EV [mailto:[email protected]] On Behalf Of EVDL Administrator via EV Sent: Saturday, July 25, 2015 11:59 PM To: Electric Vehicle Discussion List Subject: Re: [EVDL] 48V Energy Meter (Lead-Acid)... On 25 Jul 2015 at 17:38, 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 droping 36v at 500 mA and will be disipating over 18W of heat. I'm no expert, but I just can't see it being this bad. The manual I have as previously referenced http://www.evdl.org/docs/ce-emeter.pdf -- admittedly an old one, but I would hope they don't use MORE current now -- says 50ma to 150ma depending on backlight intensity, and sleep at 28ma. You also don't need to drop 36v. The meter will be happy at anything between 12 and 30 volts. So with a 48v pack you're dropping perhaps 20-24v at 150ma, or 3.6 Watts maximum. That still requires a fairly beefy zener, but not 18 Watts' worth. Sure, it's waste, but it's negligible in a golf car that's typically using between 1 and 3.5 kW just moseying along. Cor's suggestion of an extra zener clamp across the meter is an EXCELLENT idea. But perhaps it's all moot since off the shelf low-power DC:DC isolators have gotten quite cheap, thanks to Chinese sweatshop manufacturing. Disclaimer: I have not tried any of this with an E-meter, though a long time ago I did something similar (IIRC using a power transistor and a zener) with an even older Cruising Equipment amp-hour counter. David Roden - Akron, Ohio, USA EVDL Administrator = = = = = = = = = = = = = = = = = = = = = = = = = = = = = EVDL Information: http://www.evdl.org/help/ = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Note: mail sent to "evpost" and "etpost" addresses will not reach me. To send a private message, please obtain my email address from the webpage http://www.evdl.org/help/ . = = = = = = = = = = = = = = = = = = = = = = = = = = = = = _______________________________________________ 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)
