Current through a coil in a DC circuit

[UneXisted]

Hey guys,

I would like to measure the current through a coil in a DC circuit, but I'm not sure how to accomplish this. I cannot put a sense resistor in between and the coil is my only option to indicate for the current. I was thinking about measuring the voltage over the coil, but I'm not sure how the voltage relates to the current for this coil.

Do you have any ideas?

 

[colin55]

The coil behaves exactly like a resistor when DC flows.

 

[Reloadron]

The coil behaves exactly like a resistor when DC flows.

Meaning you have to know the DC resistance of the coil and with that known and the voltage drop across the coil using Ohms Law you calculate the current through the coil.

Ron

 

[UneXisted]

In this case I do not know the DC resistance of the coil, as it is assembled on a motherboard. Can I use the multimeter to measure the resistance of the coil while the coil is still assembled on the motherboard?

 

[Reloadron]

No because that will not take into account other circuit components that may place a parallel resistance across the coil. This may go better if you could explain exactly what this coil is and what the circuit is. The more detail you can provide the more people here can help you.

Ron

 

[MrAl]

Hi,

Sometimes you can measure but only when the inductor is connected to a transistor that does not get forward biased by the voltage put out by the multimeter, and there are only capacitors connected to the inductor or resistors that are much higher in value than the inductor equivalent series resistance. You have to know more about the circuit to know if this works or not though. Many inductors in power supplies have very low resistance so this can work if you have a meter that can measure resistance that low without putting out too much voltage.

 

[UneXisted]

The circuit is a VRM of a motherboard and the coil is part of a buck-buck converter. I want to know the current that flows through the coil so I have an indication of the energy usage of certain components.

My initial thought was to unsolder the coil, but its throughhole and with a 7-layer PCB im not sure if that's a good idea. If the coil can be seen as a very low resistor then I could use Ohms law, but I dont know the coil's resistance. I can also use a hall effect sensor.

I'm not sure what to do... still contemplating

 

[morse w]

if you can lift one side off the board and set your multimeter to amps that should help you..... i would suggest using a solder sucker or wick on that kind of pcb.

 

[MrAl]

Hi,

I would never try to unsolder a 7 layer board. I would first cut the lead in a place where i could connected it back together again using perhaps a very small coil of thinner wire wrapped around both leads (like a metal bandage) and soldered carefully. I'd only do this if it was really a necessity to do this in the first place. I'd hate to do that too though.
Probably isnt room for a current probe either.

 

[colin55]

Simply add a 1R or a few 1R in parallel to the input of the buck inverter and measure the voltage across it.

 

[UneXisted]

Simply add a 1R or a few 1R in parallel to the input of the buck inverter and measure the voltage across it.

Hey Colin,

can you explain me why this would work? I dont understand

**broken link removed**


Another way would be to derive the current by the number of turns of the coil. Deriving the current by the resistance is not possible you guys say? due to the AC current. I have a spare motherboar with the same coil that i can unsolder. I can use it to measure the characteristics if needed.

The only option I have is to use a Hall Effect sensor I think, but thats my last solution because i have to place it inside a computer. THere is a great chance that interference will occur.



**broken link removed**
This is the VRM and there are three coils near each other. The PWM controller is the upper left chip. I think the 2 chips belong to each other. They are called FET FDD6680S and MOSFET 7030BL. I am quite sure that coil 3 belongs to another circuit, but coil 1 and 2 definitely different kind of coils. Can someone clarify the purpose of these coils two coils?

 

[misterT]

Why do you wan't to measure the current?