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
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