# Measuring DC Current and InRush Current

Discussion in 'Electronic Basics' started by Richard, Apr 19, 2005.

1. ### RichardGuest

I have a project that utilizes a 5 amp DC/DC Converter. The specs on the
Single Board computer states the normal draw is 2.8 amp @ 5 volts, while the
Max is 3.8 amps draw @ 5 volts.

The DC/DC Converter fails to power up all the time and the manufacturer
request I install a shunt and a digital scope to verify the Current
requirements.

I have never done this before and I have a few questions.

1) What Size shunt should I use?
2) Where does one buy shunts?
3) Since I'm measuring the voltage with a scope, does the shunt go from my
power lead to ground and I measure the voltage across the shunt?

Thanks, I'm all ears.
Richard

2. ### Lord GarthGuest

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.

3. ### BanGuest

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
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 www.tech-chat.de)

4. ### Don KellyGuest

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

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---------------------- |
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.

5. ### RichardGuest

Thanks to all for the help.

Richard

6. ### JamieGuest

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.

7. ### RichardGuest

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.

8. ### Don KellyGuest

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

--
Don Kelly