Voltage and Current source.
- Thread starter lquadros
- Start date
- Status
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Voltage and Current source.
Neither term is defined in the IEEE Standard Dictionary of Electrical and Electronic Terms (IEEE Std 100 ? 1996), so in my mind, the simplest way to describe them is which term, at a given set of conditions, would the source ?attempt? to maintain regardless of the load.
A voltage source would attempt to maintain its ?open circuit? voltage and a current source would attempt to maintain its ?short circuit? current.
Neither term is defined in the IEEE Standard Dictionary of Electrical and Electronic Terms (IEEE Std 100 ? 1996), so in my mind, the simplest way to describe them is which term, at a given set of conditions, would the source ?attempt? to maintain regardless of the load.
A voltage source would attempt to maintain its ?open circuit? voltage and a current source would attempt to maintain its ?short circuit? current.
bphgravity
Senior Member
- Location
- Florida
Re: Voltage and Current source.
The voltage source is the location in which an electromotive force is established. This pressure creates the action of current which is effectively the flow of electrons in a conductor.
So, a point where current can be taken (current source) is a location that has conductors with a potential between (voltage source).
The voltage source is the location in which an electromotive force is established. This pressure creates the action of current which is effectively the flow of electrons in a conductor.
So, a point where current can be taken (current source) is a location that has conductors with a potential between (voltage source).
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Voltage and Current source.
But to really address the customer?s question, I would need to know why they are asking. Are they, for example, contemplating buying a device from you, and you need to explain the purpose or function of the device?
I agree with this description. We used to have to assume one or the other, when performing calculations in our college Electrical Engineering classes. A ?Voltage Source? would maintain a constant voltage across its output terminals, no matter what you connected to it. A ?Current Source? would maintain a constant current through itself, no matter what you connected to it. That is the simple difference.
But to really address the customer?s question, I would need to know why they are asking. Are they, for example, contemplating buying a device from you, and you need to explain the purpose or function of the device?
Re: Voltage and Current source.
I was trouble shooting a problem with voltage drop and was using a power source 12VDC with voltage and current adjustment. The customer wanted me to crank up the current to compensate for the voltage drop.
Here is a brief history - It so happened that the motor which has a 4.5A running current took about 90A of starting spike that I measured with chart recorder. This was triggering a low voltage detection circuit.I had to upsize the wiring.I cannot figure out if this is normal for a motor of that size.There is another make of motor that does not do this when powered up.
I was trouble shooting a problem with voltage drop and was using a power source 12VDC with voltage and current adjustment. The customer wanted me to crank up the current to compensate for the voltage drop.
Here is a brief history - It so happened that the motor which has a 4.5A running current took about 90A of starting spike that I measured with chart recorder. This was triggering a low voltage detection circuit.I had to upsize the wiring.I cannot figure out if this is normal for a motor of that size.There is another make of motor that does not do this when powered up.
physis
Senior Member
Re: Voltage and Current source.
To start with he needs to understand that the current is what caused the voltage drop.
Editted Typo.
[ October 20, 2004, 12:36 PM: Message edited by: physis ]
I think you'd have to get into the mathmatical relationship of voltage and current a little bit.
To start with he needs to understand that the current is what caused the voltage drop.
Editted Typo.
[ October 20, 2004, 12:36 PM: Message edited by: physis ]
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Voltage and Current source.
Its definitely unusual for a motor to draw 20 times its running current unless the motor usually operates significantly underloaded. What is the motor HP? Starting current is generally unaffected by load, the load just changes the time it takes to accelerate to operating speed.
Re: Voltage and Current source.
I would modify a little the Charlie good definition:
A ?Voltage Source? would maintain a constant voltage across its output terminals, no matter what a non-zero impedance you connected to it. A ?Current Source? would maintain a constant current through itself, no matter what a finite impedance you connected to it.
I would modify a little the Charlie good definition:
A ?Voltage Source? would maintain a constant voltage across its output terminals, no matter what a non-zero impedance you connected to it. A ?Current Source? would maintain a constant current through itself, no matter what a finite impedance you connected to it.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: Voltage and Current source.
In repairing electronic equipment I use a power supply that has an adjustable voltage and current regulation.
If I need to test a piece of equipment and I need 13.6 volts but I don't want the equipment to draw more than 2 amps to keep from blowing the 3 amp fuse that is soldered to the circuit board I will set the current to 3 amps and it will not go over this. it does this by allowing the voltage to drop off as it nears the limit. so lets say this motor needs 24 volts DC to function but requires 90 amps to start up, then it will need a power supply capable of delivering this amount of current other wise the voltage will drop off at the power supply's limit. If it has an adjustment to safely allow more current and the power supply can handle it it should be adjusted to this level. But if the voltage drop is caused by resistance in the wires feeding the motor then raising this current limit will do nothing to solve this voltage drop as it cannot compensate for the voltage loss after it's own terminals unless it has remote voltage sensing. which the power supply will keep turnning the voltage up to maintain the voltage at the remote sensing point. but if the drop exceeds the voltage limit of the power supply then again it will stop at the max it can put out. I would check at the power supply to see if the voltage is dropping at it. if it is then adjust the current to the max the power supply can deliver without going over what the motor requires or it's own limit. If it has remote sensing and the voltage goes up at the power supply then it is a resistance in the wires causing the voltage drop.
[ October 27, 2004, 06:55 PM: Message edited by: hurk27 ]
In repairing electronic equipment I use a power supply that has an adjustable voltage and current regulation.
If I need to test a piece of equipment and I need 13.6 volts but I don't want the equipment to draw more than 2 amps to keep from blowing the 3 amp fuse that is soldered to the circuit board I will set the current to 3 amps and it will not go over this. it does this by allowing the voltage to drop off as it nears the limit. so lets say this motor needs 24 volts DC to function but requires 90 amps to start up, then it will need a power supply capable of delivering this amount of current other wise the voltage will drop off at the power supply's limit. If it has an adjustment to safely allow more current and the power supply can handle it it should be adjusted to this level. But if the voltage drop is caused by resistance in the wires feeding the motor then raising this current limit will do nothing to solve this voltage drop as it cannot compensate for the voltage loss after it's own terminals unless it has remote voltage sensing. which the power supply will keep turnning the voltage up to maintain the voltage at the remote sensing point. but if the drop exceeds the voltage limit of the power supply then again it will stop at the max it can put out. I would check at the power supply to see if the voltage is dropping at it. if it is then adjust the current to the max the power supply can deliver without going over what the motor requires or it's own limit. If it has remote sensing and the voltage goes up at the power supply then it is a resistance in the wires causing the voltage drop.
[ October 27, 2004, 06:55 PM: Message edited by: hurk27 ]
Re: Voltage and Current source.
Current sources are used a lot in the Telecom and CATV industry. Telephone offices have special digital copper span lines called T1 or DS1 signal lines. The digital signal can only be carried about 5000 feet before the capacitance of the twisted pair attenuates below usable levels. So line repeaters must be installed to regenerate the signal.
A current source is used to power the repeaters. The current is driven by 130 VDC source @ 65 ma. However there is a distance limitaion of 2000 ohms. It is very easy to determine line problems by simply checking the voltage leaving the office. If you see 130 VDC, you know the cable is cut. Otherwise the voltage is something less than 130 VDC.
CATV uses the same principle to power line amplifiers on thier network.
Current sources are used a lot in the Telecom and CATV industry. Telephone offices have special digital copper span lines called T1 or DS1 signal lines. The digital signal can only be carried about 5000 feet before the capacitance of the twisted pair attenuates below usable levels. So line repeaters must be installed to regenerate the signal.
A current source is used to power the repeaters. The current is driven by 130 VDC source @ 65 ma. However there is a distance limitaion of 2000 ohms. It is very easy to determine line problems by simply checking the voltage leaving the office. If you see 130 VDC, you know the cable is cut. Otherwise the voltage is something less than 130 VDC.
CATV uses the same principle to power line amplifiers on thier network.
Re: Voltage and Current source.
"I was trouble shooting a problem with voltage drop and was using a power source 12VDC with voltage and current adjustment. The customer wanted me to crank up the current to compensate for the voltage drop."
Your DC supply is limited by several factors. Linear supplies, for instance, will produce voltage drops if their transformer current capacity is exceeded or if a load surge momentarily drains the filter capacitors. Transient loads will also be effected by the supply voltage regulator response characteristics. The current limit on your supply will in effect choke your motor, so your customer was correct in a sense. If you set the current limit, voltage will vary as motor load changes.
"Here is a brief history - It so happened that the motor which has a 4.5A running current took about 90A of starting spike that I measured with chart recorder. This was triggering a low voltage detection circuit.I had to upsize the wiring.I cannot figure out if this is normal for a motor of that size.There is another make of motor that does not do this when powered up."
Motors do show a large current spike upon starting. Size & duration of the spike are determined by many factors. Your 4.5A figure may be measured under a certain load or given as a nameplate value. But here's the bottom line: your voltage drop is occurring because your DC supply is unable to supply adequate current. Increasing wire size will do little. Increase the current available from your supply and/or tune the low-voltage detector circuit by increasing its trip level or response time/delay. You may also be overloading the motor & simply need more H.P. (if your spike lasts more than a fraction of a second). footnote: chart recorders can add their own behavior to what's displayed in cases like this.
"I was trouble shooting a problem with voltage drop and was using a power source 12VDC with voltage and current adjustment. The customer wanted me to crank up the current to compensate for the voltage drop."
Your DC supply is limited by several factors. Linear supplies, for instance, will produce voltage drops if their transformer current capacity is exceeded or if a load surge momentarily drains the filter capacitors. Transient loads will also be effected by the supply voltage regulator response characteristics. The current limit on your supply will in effect choke your motor, so your customer was correct in a sense. If you set the current limit, voltage will vary as motor load changes.
"Here is a brief history - It so happened that the motor which has a 4.5A running current took about 90A of starting spike that I measured with chart recorder. This was triggering a low voltage detection circuit.I had to upsize the wiring.I cannot figure out if this is normal for a motor of that size.There is another make of motor that does not do this when powered up."
Motors do show a large current spike upon starting. Size & duration of the spike are determined by many factors. Your 4.5A figure may be measured under a certain load or given as a nameplate value. But here's the bottom line: your voltage drop is occurring because your DC supply is unable to supply adequate current. Increasing wire size will do little. Increase the current available from your supply and/or tune the low-voltage detector circuit by increasing its trip level or response time/delay. You may also be overloading the motor & simply need more H.P. (if your spike lasts more than a fraction of a second). footnote: chart recorders can add their own behavior to what's displayed in cases like this.
- Status