Take you actually to calculate transformer of an electric current!

 

We will design transformer of an electric current. Transformer of use electric current can reduce the loss when measuring electric current of transformer former edge, for instance high-power switch power source, because electric current is too big,so coil of mutual inductance of need use electric current monitors electric current in order to reduce loss.

 

Where is the distinction of voltaic transformer and general voltage transformer? This problem even if staff of design of elder magnetism component replies very hard also. Main distinction depends on: Transformer tries to alternate voltage to deputy edge from former edge, and voltaic transformer tries to alternate electric current to deputy edge from former edge. The voltage size of voltaic transformer is decided by load.

 

We pass a concrete design example, can understand the working principle of voltaic transformer better.

 

The former edge electric current that measures transformer with voltaic transformer suppose, voltage of 1V of correspondence of electric current of former edge 10A. Of course, we can be measured with the resistor of a 1V/10A=100mΩ , but resistor is the loss that cause 1V×10A=10W, so big loss cannot be accepted to almost for all designs. So, want to choose voltaic transformer, if pursue,1 is shown.

 

Graph 1 detect with electric current transformer reduces loss

 

Of course, to reduce winding resistance, we take the v/LIT all over the ground number of former edge for 1 v/LIT all over the ground, compare low level to to make electric current falls at the same time, deputy edge v/LIT all over the ground number should be more. If deputy edge circles number is N, can get by Ohm’s law (10/N)R=1V, the power that consumes in resistor is P=(1V)^2/R. The power that we consume suppose is 50mW(that is to say, we can use the resistor) of 100mW norms, this must not be less than 20Ω with respect to requirement R, if use the resistor of 20Ω , can get N=200 of deputy edge v/LIT all over the ground number by Ohm’s law.

 

We see magnetism core now, hypothesis diode is common general diode, on-state voltage is 1V about, electric current is 10A/200=50mA. Transformer outputs voltage to be 1V, add the on-state voltage 1V of diode, total voltage about 2V. When 250kHz frequency works, the magnetic induction intensity on magnetism core won’t exceed

 

 

Among them the time that 4us is a cycle, actual affirmation is less than a cycle.

 

Because former edge has shed voltaic time,exceed switch cycle impossibly (otherwise, magnetism core cannot restoration) . Because this Ae is OK very small, and B also very won’t big. The dimension of core of the magnetism in this example cannot ask through loss or magnetic flux saturation asks to decide, larger possibility is by the segregation between former deputy edge voltage is decided. If keep apart voltage to do not have a requirement, what the size of magnetism core occupies bulk to decide by 200 v/LIT all over the ground winding commonly. You can have shed the peak value electric current of 500mA with 40 lead, but this kind of wire is real too fine, general transformer manufacturer won’t be circled for you make.

 

Unless practical clew must be used, do not use norms to be less than the lead of 36 lines usually.

 

We are analysed now why cannot replace voltaic transformer with voltage transformer? Had known deputy edge voltage has 2V only, because this former edge voltage is 2V/200=100mV. If input volts d.c. to be 48V, so voltage of former edge 10mV is opposite voltaic transformer is negligible —— for 48V voltage in that way the electricity that you can get 50mA in deputy edge, and affect without what almost to former edge. Suppose another kind of circumstance (not actual) , the input volts d.c. of former edge has 5mV only, so the voltage that the former edge of transformer has 10mV impossibly, at the same time as a result of former edge impedance (if reflex deputy edge impedance) bigger also, decided deputy edge produces the electricity of 50mA far from possibly. Although whole 5mV voltage is added in former edge entirely, deputy edge also can produce the tension of 200×5mV=1V only: Enough tension cannot produce on changeover resistor. Accordingly, voltage transformer can use as only transformer, cannot use detect electric current.

 

From the point of another angle: When although input the voltage of power source,be 48V, but the size that has shed voltaic transformer electric current is not by this 48V of former edge voltage is decided, however other factor decides.

 

Voltaic transformer is the voltage transformer that has impedance restriction.

 

Finally, how will we treat the error condition of voltaic transformer? The answer depends on the fundamental definition of voltaic transformer going up: Inductive is electric current.

 

The resistor of the diode in mutual inductance of practical clew electric current and deputy edge winding won’t affect voltaic measurement, because (wanting impedance only is not infinity) the electric current in series circuit everywhere equal, have nothing to do with the component of series connection.

 

In real work, using diode of Xiao Te radical to regard rectification as diode did not concern: The low on-state voltage of diode affects transformer only, won’t affect voltaic transformer.

 

If the inductance of transformer deputy edge is too small, measure an error to will increase. Namely excitation inductance is too small, the biggest error that assumes we ask to measure electric current is 1% , former edge electric current is 10A, electric current of so deputy edge is 50mA, this means requirement magnetizing current (deputy edge) should be less than 50mA×1%=500μA. Magnetizing current had not shed changeover resistor, we also cannot detect this electric current, such errors increased. We are OK the least value of inductance of cipher out deputy edge

 

 

Present v/LIT all over the ground number is 200, we need the magnetism link of AL=16mH/200=400nH, it is OK to use annulus of common magnetism of Xiaotie oxygen body, link of this kind of ferrite magnetism is found very easily.