Re: OT: sort of electronics info

You'll probably get an answer to this, but to measure very low
resistance, you need a 4 wire meter or you need to roll your own test
setup.

Ohmmeters work by injecting a known current and measuring the voltage
that develops. When the device under test has resistance low enough to
be in the ballpark of contact resistance or lead resistance, you need
two sets on leads -- one to inject the current, and another to read
back the voltage. Normally this is done inside the meter.

If you don't have a good 4-wire ohmmeter, you need two meters -- a
current meter and a volt meter. Energize the coils by whatver is handy
with the current meter in-line, then measure the voltage across the
DUT. Then divide to get your answer. You can make it easy if you have
a bench supply with a constant current mode.

You still won't know the impedance of the motors, though, just the
resistance. To measure low inductance coils, energize them with a
series resistor and measure the voltage transient as the circuit is
switched on. Figure the time constant of the transient from the
waveform (it's all on the web) and then solve for "L".

Re: large capacitors, it's impossible to tell you where to look unless
I know how large they need to be. Here's a photo of a 10F (yes, ten
farads) cap fit into a 9V battery case:
http://www.hackaday.com/2005/12/06/supercap-9v-battery/

The type to get for high energy density is "carbon aerogel foam". They
operate by a chemical effect known as "electrochemical double layer".

Manufacturers:
Maxwell Technologies Boostcap: http://www.maxwell.com/index.html
Cooper Bussmann Powerstor:
http://www.cooperet.com/products/products.cfm?page=supercapacitors
Epcos Ultracapacitors:
http://www.epcos.com/web/generator/Web/Sections/ProductSearch/Hitlist/Capacitors/Ultracapacitors/Page,locale=en.html?param_c_r=¶m_v_r=¶m_terminals=¶m_headline2_en=&from=searchform&doAction=searchform&distributorId=0&cssurl=&shop=no
Panasonic Gold:
http://www.panasonic.com/industrial/components/capacitive/cap_gold.htm

Digikey pages:
http://pdfcatalog.digikey.com/T063/1182-1185.pdf

Newark pages:
http://newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp?N=1001767

On 9/14/06, archie road <jeep_32@...> wrote:
> I am currently working on a retrofit of a lagun cnc mill. i was
> wondering where i might find large capicaters (sp) and info on test
> equipment. such as a way to mesure very low resistances as in the
> impedence of motors and what type of osillascope people are using.