Measuring DC Current and InRush Current
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Posted by Richard on April 19, 2005, 2:19 am
Please log in for more thread options 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 ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- | |||||||||||||
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Posted by Lord Garth on April 19, 2005, 7:49 am
Please log in for more thread options > Max is 3.8 amps draw @ 5 volts.
my
> > 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 > power lead to ground and I measure the voltage across the shunt?
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. | |||||||||||||
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Posted by Ban on April 19, 2005, 8:11 am
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Richard wrote: > 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? 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. > 2) Where does one buy shunts?
Digikey among others > 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? > 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) -- ciao Ban Bordighera, Italy | |||||||||||||
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Posted by Don Kelly on April 19, 2005, 8:25 am
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> I have a project that utilizes a 5 amp DC/DC Converter. The specs on the
the
> Single Board computer states the normal draw is 2.8 amp @ 5 volts, while > Max is 3.8 amps draw @ 5 volts.
my
> > 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 > power lead to ground and I measure the voltage across the shunt?
-------
> > Thanks, I'm all ears. > Richard > 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 | |||||||||||||
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Posted by Richard on April 19, 2005, 8:47 am
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Thanks to all for the help. Richard ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- | |||||||||||||
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> Single Board computer states the normal draw is 2.8 amp @ 5 volts, while