Re: Unreliable voltage detector
- From: bud-- <remove.budnews@xxxxxxx>
- Date: Sat, 01 Aug 2009 01:16:03 -0500
blueman wrote:
bud-- <remove.budnews@xxxxxxx> writes:Digging out some old notes:
A good analog meter will have an input impedance (AC scales) of about
"5,000 ohms per volt". On a 150V scale the resistance of the meter
will be 750,000 ohms.
To read 120V, the current through the meter will be 0.00016A (160
microamps).
A digital meter will have a higher input impedance, say 10 megohms
(probably higher).
To read 120V, the current through the meter will be 0.000000012A (12
microamps).
This is a very small current.
When I was saving up presents to buy my first VOHM back when I was a
young kid in the earl-70s, all consumer versions were analog. In those
days, the more you paid the higher the input impedance -- because it
was useful for electronics (so that the measurement wouldn't affect
the reading) and since getting high input impedence was expensive back
then. I think there were some models (out of my price range) that had
megohm type input impedance using FETs but I the best I could afford
was one with about 20Kohm/volt.
Interesting that now the situation is reversed -- all the cheap
(Chinese-made) multimeters are digital with megohm input impedances
while you have to pay extra to get an "old-fashioned" analog one.
I think I recently saw my old Archer (Radio Shack) multimeter lying
around in a box at my parents' house. I probably should retrieve
it. By the way, I think I paid about $20 for it on-sale back in the
day which was a heck of a lot of money then for a 10 year old kid who
was getting 10 cents a week in allowance (and it wasn't the lowest end
one either).
The first meter I bought when I was a kid was a 20Kohns/volt Lafayette kit. I suspect a bunch of us learned a lot of electronics from kits. I don't know how kids learn electronics now.
The main alternative back then was a VTVM - thats a vacuum tube volt meter for the younger types. I probably still have one of those around.
It always amazes me that the 20Kohm/volt meter movements are 50 micoamps full scale. Hard to imagine you can make something that sensitive rugged enough for field work.
Those meter are typically 5Kohms/volt on AC scales, which I used in my calculations. If I remember right you need higher current to get the diodes in a more linear operating area.
--
bud--
.
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