On Oct 24, 8:30*pm, "Dave Plowman (News)"
wrote:
In article ,
* *The Medway Handyman wrote:
Frank Erskine wrote:
On Fri, 24 Oct 2008 15:37:46 GMT, "The Medway Handyman"
wrote:
I have a digital multi meter from TLC
http://www.tlc-direct.co.uk/Products/TMM3800.html
I had to change some 12v 35w halogen lamps today & one wouldn't
work. *I set the meter to 20v DC & tried to measure the voltage at
the porcelain cable connector. *Nothing.
Tried to measure the voltage on the one next to it, which was on at
the time & still got nothing.
Is my multi meter broken or is a digital meter no good for this sort
of thing? *If the latter, what do I need to buy?
Try switching it to AC... :-)
Doh!
What a pillock I am. *Automatically assumed 12v would be DC.
You may not get an accurate reading with some DVMs on the AC range either
as SMPS run at a much higher frequency than mains. And may not give any
output at all in the absence of a load.
Just for guidance SMPS are quite light and usually shaped so they'll fit
through the downlighter mounting hole. A true transformer will be much
heavier - and likely larger too. Most these days are SMPS.
--
*He who dies with the most toys is, nonetheless, dead. *
* * Dave Plowman * * * * * * * * London SW
* * * * * * * * * To e-mail, change noise into sound.- Hide quoted text -
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Not only switching power supplies!!!!!
A year or two ago we repaired a car battery charger (can't remember
what the problem was). To check we then hooked up a DMM to the output
switched on and got some real funny voltage readings. Seemingly ow
voltage IIRC.
Oops what's going on? Until we suggested that what the digital meter
was seeing from the battery charger output was completely unsmoothed
and unfiltered rectified voltages varying between zero and some sort
of rectified peak AC voltage. We would have been better off using an
analog meter!
Sure enough when we hooked up a spare car battery charging current
flowed into the battery and the voltages made sense.
A suitable charging voltage for lead acid batteries is around 2.3
volts per cell (maybe a little higher). So for the so called "12 volt
car battery" 6 cells at 2.3 volts each = 13.8 volts. That's very close
to the 14 volts often mentioned.
Commercial outfits often float their batteries (just maintaining them
at full charge) at either 2.15 or 2.17 volts per cell. Doesn't sound
like much of a difference but avoids boiling the life out of a fully
charged battery.
So 6 x 2.15 = 12.90 (British Telcomm.) and 6 x 2.17 = 13.02 (Old AT&T
and Bell System Spec.) Except these were usually for telephone
exchange battery strings for a nominal 48 volts. i.e. 24 x 2 = 48
volts.