strange problem with transformer

[is_razi]

hi everey body,
i have recently built a board for measuring the resistance by sending a current and reading the developed voltage on the resistor. i have use a 220v transformer with three output each for a 5v , 12v and -12v supplies. but there was a strange problem. when i change the phase and null input the readings of circuit differs .
please let me know if you had such a problem and if yes or if you have a comment please tell me what is the reason and how can i solve that.
best regards.
:?::?::?::?::?:

 

[cameo_2007]

"but there was a strange problem. when i change the phase and null input the readings of circuit differs"
i really didnt understand what exactly your problem is..can u pls clarify!

Added after 2 minutes:

u mean u take reading,and then u reverse the transformer i/p line-neutral,, the reading differs??

 

[is_razi]

yes exactly.

 

[cameo_2007]

i faced a similar kind of problem long time back.I too couldnt find any relevant raason for that.I just concluded that there could be some non linearity in the transformer windings or something like that.
But i think here you could solve the problem using a constant current source ckt(a simple one using two transistors).Are u doing it? In that case there wont be any variation in the reading,as the current is constant irrespective of the i/p coltage variations.

regards,

 

[princeforw]

hi you have told about the problem so for my suggestion and knowledge you must use only +12 and -12 volts (center tapped transformer with 15 volts and dc supply voltage to +12 and -12) so that the response of the transformer may become linear.plz reply when your problem is solved

 

[wisepie]

hi everey body,
i have recently built a board for measuring the resistance by sending a current and reading the developed voltage on the resistor. i have use a 220v transformer with three output each for a 5v , 12v and -12v supplies. but there was a strange problem. when i change the phase and null input the readings of circuit differs .
please let me know if you had such a problem and if yes or if you have a comment please tell me what is the reason and how can i solve that.
best regards.
:?::?::?::?::?:

Can you describe more clearly what testing you are doing?

 

[is_razi]

hi every body,
i solved the problem by connecting the shield of sensor to gnd of circuit.
but steel i don't know what was tne problem. why the readings change with changing the phase and null.also i must say i am using a transformer with three isolated outputs(9v, 15v, 15v) and i regulate these using 79xx and 78xx regulators. and the output voltage of regulators don't change by changing the phase and null places.
:idea::|:?:

 

[ydtech]

From your last post it seems you have some capacitive leakage from the primary to the secondary and then on to the circuit. The change in reading depends on whether the part of the primary closest to secondary is connected to phase or neutral.

I've seen this before, and in high impedance high sensitivity circuits it's a real PITA to get rid of.

 

[is_razi]

yes, i think ydtech is right . the transformer is not a well built one.

 

[Meridian]

From your last post it seems you have some capacitive leakage from the primary to the secondary and then on to the circuit. The change in reading depends on whether the part of the primary closest to secondary is connected to phase or neutral.

I've seen this before, and in high impedance high sensitivity circuits it's a real PITA to get rid of.

Yeah you're right. I have seen similar problems. I guess at some point during the transformer winding process one or more windings is either damaged or the insulation coating is inconsistent which causes the 'capacitive' effect. Also, in some transformers it matters if you connect the phase to the beginning or end of that node (something to do with magnetic flux?).

 

[grittinjames]

hai all

i like to know what type of rectifier ur used for rectification . it cant be a problem with ur transformer . it will be the problem with ur rectifier side. i mean diode.

 

[rfzheng]

yes ,i think so