calculating current

[ash20]

hi all,
i have made a variable voltage output circuit, and it is producing .85v - 19.24v. now someone asks me what is the current it delivers.
now i am confused.
i think that current depends on the load connected and when he asks me the current rating of my circuit, there is no load attached to it, then how can i calculate current?

and same case for the mobile chargers. they give the voltage and current value at output . so how do they give it when they don't know the resistance of the mobile connected??

 

[ronv]

I think what they are asking you is what current is the regulator capable of producing at all the voltages between .85v and 19.24 volts.

The same with the mobile chargers. That is the current they can supply. If you connect to big of a load it will overheat or the voltage will go to low.

 

[simonbramble]

Generally bench power supplies are different to battery chargers. Power supplies provide a regulated voltage and the output current changes according to load (but the voltage doesn't). Battery chargers are a voltage limited current source. they will produce a fixed current (regardless of the battery voltage), but make sure the voltage does not rise above a certain value (in the case of mobile phone batteries this is normally 4.2V).

 

[alec_t]

Since you have made this circuit you will know the voltage and current ratings of what is powering it. Multiply those ratings together to get the maximum power your circuit can provide. Divide that max power by the output voltage of the circuit to get the max current the circuit can provide assuming it is 100% efficient. Then derate that figure to allow for circuit inefficiency.

 

[MrAl]

Hi,

This is a question i hear quite a bit, especially when the power supply is marked on the case something like "12 volts, 1 amp". People ask, "How can it be putting out 1 amp when i havent even connected anything to it yet?"

The answer is, the 12v is the voltage it can output and it can output that voltage up to and including 1 amp, but not over 1 amp. Thus it will work with loads that draw less than or equal to 1 amp. With no load connected it outputs zero amps.

 

[ash20]

@ronv: yeah they are asking the same but how to calculate it? you didn't tell that !!!

@simon: i want to ask how to calculate the current at output when i dont know the load there?? i can just give the required voltage between the mentioned limits. so what is the current when i dont know the load value?

@alec: i have made a variable voltage circuit. now which output voltage should i divide from the maximum power?

@mral: ok i agree that they give the rating . but how can i give it?? i want to know that.... you know my circuits output voltage. now tell me what is the current at output assuming that i used a 18-0-18 center tapped Xmer and its current output is 1 A.

 

[Reloadron]

now tell me what is the current at output assuming that i used a 18-0-18 center tapped Xmer and its current output is 1 A.

Then it is capable of 1 amp. That does not mean it will always deliver one amp it means based on the transformer used it should be capable of 1 amp and that assumes the rest of the circuit (regulator) can support one amp. So you would say Output: .85v - 19.24 VDC 1.0 ADC Since you built it you should know the output specificartions based on the parts used in the build.

Ron

 

[MrAl]

@ronv: yeah they are asking the same but how to calculate it? you didn't tell that !!!

@mral: ok i agree that they give the rating . but how can i give it?? i want to know that.... you know my circuits output voltage. now tell me what is the current at output assuming that i used a 18-0-18 center tapped Xmer and its current output is 1 A.

Hi,

How are you connecting that winding, with a bridge rectifier to make plus and minus voltages?
If so, then if EACH winding is rated for 1 amp then you should be able to get 1 amp out of it, 1 amp for each supply that is.
Often the transformer is rated in VA (volt amps). That means the voltage times the current equals the VA rating.
An 18VA transformer that has an 18v output can put out 1 amp continuous.
An 18VA transformer that has a 36v output can put out 1/2 amp continuous.
If it is an 18vA transformer and it has two 18v windings, then each winding can put out 1/2 amp.
If it is marked 18v-0v-18v and 1a-0a-1a or similar, then each winding can put out 1 amp (it's a 36VA transformer).
If you dont know the VA rating or are not sure, you can measure the stack and figure out the approximate rating that way.

 

[ash20]

@reloadron: thnx for telling

@mral: yeah, it has plus and minus voltages. +- 12, +-5, and +.85v to +19.24v
and it is printed 18-0-18 v and 1000 A on it.

ok now with all your help, i get that it has 1 A output current. thanx for this. but now how can i calculate it (to be sure it is giving that much or some less amount)? i mean, suppose i use the terminal producing +5 v. then i cant directly use the ammeter to calculate that that terminal is giving me 1 A or less. so how to find out that?

 

[Reloadron]

If you wanted to test a power supply to see if it can deliver it's rated current you would apply a known load to it and measure the voltage across the load and the current through the load. For example if I place a known 5 ohm load across a 5 volt output I should see 5 volts across the load and 1 amp through the load. No, placing an ammeter directly across the supply would be wrong!

Note: You really have to know what you are doing when testing the supply. There is always a possibility that if not done correctly or if the supply can't deliver it's rated current you could burn up the supply.

Ron

 

[ash20]

@ron: okey. thanks a lot. now i completely know everything.

thnx to everyone for sharing their knowledge.