# Transformer current

tanaygupta2000
Homework Statement:
An electrical device operates on 9V and has a resistance of 21 ohm. It is connected to a power supply of 120 V output through a transformer. The current in the primary of the transformer is? (a) 0.032 A, (b) Cannot be determined from the information supplied, (c) 0.23 A, (d) 2.32 A
Relevant Equations:
Primary power = Secondary power
VpIp = VsIs
Okay, so according to the transformer equation, VpIp = VsIs
In this question, I know I have to calculate Ip.
I think given, Vp = 9V, Vs = 120 V
Now I don't know what to use in the value of Is.

Mentor
Homework Statement:: An electrical device operates on 9V and has a resistance of 21 ohm. It is connected to a power supply of 120 V output through a transformer. The current in the primary of the transformer is? (a) 0.032 A, (b) Cannot be determined from the information supplied, (c) 0.23 A, (d) 2.32 A
Relevant Equations:: Primary power = Secondary power
VpIp = VsIs

Okay, so according to the transformer equation, VpIp = VsIs
In this question, I know I have to calculate Ip.
I think given, Vp = 9V, Vs = 120 V
Now I don't know what to use in the value of Is.

The more traditional direction for such a power transformer is to call the 120Vrms input the Primary side, and the 9V output the secondary side.

You have the secondary/output power determined by the 9V output and 21 Ohm load resistance. Equating input and output powers (for a first approximation which does not include losses in the transformer) will let you calculate the input current at 120Vrms to generate that power.

That looks very wrong to me. Can you post an image of the problem to be sure that you have transcribed it correctly? Thanks.

• tanaygupta2000 and cnh1995
tanaygupta2000
The more traditional direction for such a power transformer is to call the 120Vrms input the Primary side, and the 9V output the secondary side.

You have the secondary/output power determined by the 9V output and 21 Ohm load resistance. Equating input and output powers (for a first approximation which does not include losses in the transformer) will let you calculate the input current at 120Vrms to generate that power.

That looks very wrong to me. Can you post an image of the problem to be sure that you have transcribed it correctly? Thanks.
Sure sir

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tanaygupta2000
Please note sir that these answers are not always correct, as I found many wrong.

Homework Helper
Gold Member
You have the secondary/output power determined by the 9V output and 21 Ohm load resistance. Equating input and output powers (for a first approximation which does not include losses in the transformer) will let you calculate the input current at 120Vrms to generate that power.
@tanaygupta2000, I think this is what the problem is all about. Power balance.
If you follow this procedure, your answer will match one of the options (not d of course ;))

• berkeman and tanaygupta2000
tanaygupta2000
Option - (a)

• berkeman
Homework Helper
Gold Member
Option - (a)
Yes.

• tanaygupta2000
Vince Kamboj
Homework Statement:: An electrical device operates on 9V and has a resistance of 21 ohm. It is connected to a power supply of 120 V output through a transformer. The current in the primary of the transformer is? (a) 0.032 A, (b) Cannot be determined from the information supplied, (c) 0.23 A, (d) 2.32 A
Relevant Equations:: Primary power = Secondary power
VpIp = VsIs

Okay, so according to the transformer equation, VpIp = VsIs
In this question, I know I have to calculate Ip.
I think given, Vp = 9V, Vs = 120 V
Now I don't know what to use in the value of Is.
Secondary power = Vs^2/Rs = 9×9/21 = 3.86W
Primary power = Vp×Ip = 120I
VpIp = VsIs
I = 3.86/120 = 0.032A

• Delta2
Mentor
Secondary power = Vs^2/Rs = 9×9/21 = 3.86W
Primary power = Vp×Ip = 120I
VpIp = VsIs
I = 3.86/120 = 0.032A
Welcome to PF, Vince. Since the OP has solved the problem a number of months ago, it is okay to post your solution to the problem. In general, though, in the schoolwork forums it is not allowed to post solutions to schoolwork/homework problems until after the OP has solved the problem correctly. The student must do the bulk of the work here at PF.

Again, Welcome to PF. • Delta2