hi,

i am testing a modem circuit..when i connect the circuit to voltage=5V and current=0.3A,the current suddenly drops to zero(as indicated at the power supply's lcd display)..what could be the reason of this?is the circuit short circuited or open circuited??

 

If the current suddenly drops to zero and the voltage remains at 5v, it means that the circuit is open.
Ohm's Law as applied in this case:
I = E/R (or, Current = Voltage / Resistance)
So if E=5 and I=0, you know that resistance must be infinite, or an open circuit.

This condition may be temperature related. Try using an electronic circuit chilling spray; they are available in aerosol cans.

Examine the circuit with a high-powered magnifier. Sometimes you can observe cracked traces or cracked solder joints.

 

sgtWookie is correct, but there is one other possible situation that might be occuring if you are using a regulated, protected bench-type power supply. I know from experience that my own bench supply will "crowbar" if the load current exceeds a set value. When that happens, the output voltage and current both drop to zero, and a "Fault" light on the front panel comes on. The supply will reset itself when the fault is cleared by removing the load. I don't know what type of supply you are using, but this could be something to consider.

Question: When you see the current go to zero, does the output voltage remain at 5 volts, or does it drop to zero too?

 

Xray,
Good possibility on the crowbar. However, if the output from the supply is still 5v, then it isn't in crowbar state.

At this point, we really need more input from the OP (original poster) before we can make more suggestions.

 

Xray,
Good possibility on the crowbar. However, if the output from the supply is still 5v, then it isn't in crowbar state.

At this point, we really need more input from the OP (original poster) before we can make more suggestions.

Exactly. That's why I asked him the question about the voltage level. The answer will gives us a better hint as to what's going on.

Thanks

 

thanx for the feedback..the voltage remains at 5V when the current drops..i've checked the connections using multimeter to see if any tracks are craked or disconnected but there isn't any..im not sure the type of power supply i use..
anyways,can ESD(eletrostatic discharge) cause the circuit to be faulty and open circuit??how do i check if ESD had happened?

 

ESD is highly likely, particularly if it was connected to a telephone line when it originally started having problems.

Lightning strikes power and telephone lines all the time. You can get surge protectors; they help a great deal if the strike isn't too close. However, there isn't much you can do to protect against a direct hit, save enclosing the circuit in a seamless copper sphere and burying it about 50 feet underground. Hard to get much use out of it that way, but it'll be safe from ESD.

Carbon tracking is a sure sign, but you can't always see it. The damage may be microscopic, or internal to IC's on the board.

 

this must be the dumbest thing ever..the reason why the current droped to zero is the wire connecting the power supply and the circuit was torned..cheyhh..wasted all my time and frustration for a lousy thing..glad the circuit all working now..

anyways..thanx guys

 

I've done dumber things in my time

Things like power supply wires are very easy to take for granted. After all, they're so simple, how can they be bad?

Well, they can. Very easy to overlook.