Measuring DC Current and InRush Current

the

my

What they want is a low value resistor (like 1 ohm) in series with the SBC. You would measure the voltage across the resistor and this allows you to know the current using V/R=I. You will need R to handle the power as well P=V^2/R.

So if you choose 1 Ohm for R, R will need to be 10 watts.

It should give a big enough signal without reducing the supply voltage below specs. If you have sense terminals on the supply, the voltage loss will be compensated for. If you can tolerate 100mV loss, you can use a value of 0.02 Ohm. This will give a 20mV/A reading.

Digikey among others

If you put the shunt in the ground return of your supply, you can measure normally. Be sure the supply is not directly connected to the chassis and the mains earth. Gnd connection should be made from the subD receptacles only.

If above is not possible due to grounding issues, you can use a 1:1 isolation transformer for either your scope or the circuit power supply. These transformers have no earth connection on the secondary side.

optional Isolation Transformer.--------. +5V .-------------. -. ,-----| Power |---------------| | )|( | Supply | | DUT | )|( | | ___ | | -' '-----| |-+--|___|--+---| | '--------' | | '-------------' | === | GND must be on this side only +---- Scope

.--------.+5V ___ .-------------. | Power |----+-|___|-+--| | | Supply | | | | DUT | | | | | | | | |----)---+---)--| | '--------' | | | '-------------' | === | | GND | | | Isolation | | .----------. Transformer | | | .-----. |----. ,----- | +--| |scope| | )|( | | | | | )|( +----------| '-----' |----' '----- scope gnd| | '----------' (created by AACircuit v1.28 beta 10/06/04

--
ciao Ban
Bordighera, Italy

the

my

------- Do NOT connect the shunt between the power lead and ground. It must be isn SERIES with the load.

The shunt is a resistor in series with the load and you measure the voltage across it. You should be aiming at a resistor of the order of 0.01 ohms which would give 0.05V at 5A. This is small compared to the 5V of the supply. You can make a shunt from a length of wire. Look up the resistance of different sizes of wire in a wire talbe and cut the appropriate length. You can also sue a wire which is a lit longer and pass a known current (say

1A) through it and measure voltage between two points, sliding one contact along until the desired voltage is reached. mark the points.

I>>

O------o----------------------o----O O terminal. --- wire o contact points |< voltage >| If I=5A make contact points where V=0.05Volts to get a 0.01 ohm shunt. (0.01V/Amp)

+-----------------------x---------------------- | power supply load ground-----------------O----o----o--O--------| | | gnd scope hot

This allows you to also measure the voltage x to ground at the same time on a second scope channel.

--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer

Thanks to all for the help.

Richard

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----

The #1 Newsgroup Service in the World! 120,000+ Newsgroups

----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Newsgroups

a fair to good DMM (digital Multimeter) will have a Peak and Hold function on it. turn it on and use the Amp meter to find the Peak Inrush current.

Yep, but that is what got me in this bind already. DMM in series only showed a peak of a little over 3 amps. When using a Fluke ScopeMeter and a calibrated shunt, I see it hitting a tad under 10 amps. The inrush is at

9.7 amps for 880us then settles down to 3 amps while the PC Boots and down to 1.5 amps running.

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----

The #1 Newsgroup Service in the World! 120,000+ Newsgroups

----= East and West-Coast Server Farms - Total Privacy via Encryption =----

.. The peak is very short so that the DMM may not even be seeing it.

a

function

News==----

Newsgroups

=----