Tested with 10R resistor in place of lamp, volts were measured across the primary and the current at the same time in series between the supply and load?is this incorrect??
Hi,
the 10r in the primary circuit is to allow Vdrop to be measured using the Vac scale of your DMM.
From Vdrop you can estimate the primary current.
eg. 870mVac ~ 87mAac, 53mVac ~ 5.3mAac
Start with the highest Voltage scale and select the more sensitive scales after you know the AC voltage is low enough not to damage your DMM.
Once again, that 10r is at LIVE voltage.
the 10r in the primary circuit is to allow Vdrop to be measured using the Vac scale of your DMM.
From Vdrop you can estimate the primary current.
eg. 870mVac ~ 87mAac, 53mVac ~ 5.3mAac
Start with the highest Voltage scale and select the more sensitive scales after you know the AC voltage is low enough not to damage your DMM.
Once again, that 10r is at LIVE voltage.
Ok, still not sure if this is right?
For a start I had the wrong resistor! New 10r fitted ( 2 watt ) variac voltage set from 0Vac to 250Vac, DMM probes connected to transformer primary (no load). results attached😕
Seems to be all over the place. not the steady rise and fall as I would have thought.
For a start I had the wrong resistor! New 10r fitted ( 2 watt ) variac voltage set from 0Vac to 250Vac, DMM probes connected to transformer primary (no load). results attached😕
Seems to be all over the place. not the steady rise and fall as I would have thought.
You have posted the voltage at the output of the Variac and the voltage at the primary of the transformer. Then subtracted two big (similar) numbers. You are bound to get experimental error using difference of two similar numbers.
I need to see the current fed to the primary through the 10r resistor. Do this by measuring the voltage across the 10r resistor. Divide that measured voltage by 10 to get amperes.
Plot Amperes vs Variac supply voltage.
I need to see the current fed to the primary through the 10r resistor. Do this by measuring the voltage across the 10r resistor. Divide that measured voltage by 10 to get amperes.
Plot Amperes vs Variac supply voltage.
![Transformer_Test_2_-_Graph_-_[iTools.SubhashBose.com].png](/community/data/attachments/190/190900-48edd565b001c6ca9e872aad7ea9c323.jpg?hash=SO3VZbABxs)
Hi,
that graph is telling you that this transformer is not for 220/240Vac.
@220Vac the transformer is drawing ~1.2Aac when idling (open circuit).
That is a dissipation of 260W. It will overheat very rapidly.
@ 120Vac the transformer current is ~0.4Aac. This is a dissipation of 48W. Still very high.
Did you remember to calculate the current through the resistor using I=V/R ?
If your current measurements are correct then it would appear that the transformer is faulty. I suspect a shorted turn or three.
Did you notice the improvement in reducing experimental inaccuracy of current by measuring voltage direct across a known resistor?
It is a powerful technique that you will come to use often.
The 10r with those currents through it will have been severely overheated. Bin it. For currents that high you should have changed the resistor to 1r0, or even less, a pair of 1r0 in parallel.
that graph is telling you that this transformer is not for 220/240Vac.
@220Vac the transformer is drawing ~1.2Aac when idling (open circuit).
That is a dissipation of 260W. It will overheat very rapidly.
@ 120Vac the transformer current is ~0.4Aac. This is a dissipation of 48W. Still very high.
Did you remember to calculate the current through the resistor using I=V/R ?
If your current measurements are correct then it would appear that the transformer is faulty. I suspect a shorted turn or three.
Did you notice the improvement in reducing experimental inaccuracy of current by measuring voltage direct across a known resistor?
It is a powerful technique that you will come to use often.
The 10r with those currents through it will have been severely overheated. Bin it. For currents that high you should have changed the resistor to 1r0, or even less, a pair of 1r0 in parallel.
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Thanks for all the help and patience
Nearly 1.4 amp's at 240vac seems high with no load?
Cheers
you have a 120volt transformer there, the 66mA idle current seems to be just about right...
Thanks Andrew
The transformer is designed for a 230v supply, I had it wound by Airlink Transfomers. link attached (VAL0692 : 120VA 230V 200-0-200 0-6v 0-10v)
Yes I used I=V/R to calculate the current. The 10r resistor was smoking so yes will bin it😀
Airlnk have asked me to return the Transformer, postage will be a killer😱 but if they find the problem to be a manufacturing fault then they will replace the unit.
The transformer is designed for a 230v supply, I had it wound by Airlink Transfomers. link attached (VAL0692 : 120VA 230V 200-0-200 0-6v 0-10v)
Yes I used I=V/R to calculate the current. The 10r resistor was smoking so yes will bin it😀
Took me a while but yes I see now. I have a couple of other units to test so will try them against know working units and compare the results. I am going to change my Variac set-up again so that different fixed resistors can be added (safely) in place of the lamp for this type of testing.
Airlnk have asked me to return the Transformer, postage will be a killer😱 but if they find the problem to be a manufacturing fault then they will replace the unit.
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