High-Frequency CT - Sirio Inductive Components

Current sense transformers

Current sense transformers are typically inserted into the primary power circuit of a high-frequency transformer, but their use can be extended to AC current measurements in applications that do not require particularly accurate detection.

By connecting a resistor to the transformer’s secondary winding, a floating voltage signal is obtained, proportional to the primary current. The signal can then be electronically conditioned or processed via software and used to protect electronic switches or perform current-based controls.

The turns ratios of these transformers are typically 1:50, 1:100, 1:200, 1:500, or 1:1000.

As the number of turns increases, the transformer’s optimal operating frequency shifts toward lower frequencies. Since the load resistance determines the transformer’s output voltage, once the minimum operating frequency is known, the maximum output voltage can be determined, allowing the sensing circuit to operate without signal distortion.

The maximum operating frequency is primarily determined by the performance of the core material and the capacitive effect created by adjacent turns of the winding, which limits the transformer’s bandwidth.

From a construction standpoint, these current transformers are built either on a toroidal core, or an E core with molded-in primary turn. This solution is particularly suitable for applications where, in order to contain costs, robust and easy-to-mount components are required.

The typical operating temperature ranges from –25 to +85°C.

Some standard codes in the toroidal version with through hole

Rated primary current I_p = 20Arms

Code	I_p [Arms]	n	Ls [mH]	fmax [KHz]	Vt [μVs]	Drawing	D [mm]	U_is [Vrms]	U_p [Vrms]
TA 150511	20	(1) : 50	1,9	1000	180	1508FA	5	1000	4500
TA 150512	20	(1) : 100	7,5	400	400	1508FA	5	1000	4500
TA 150513	20	(1) : 200	30,0	100	850	1508FA	5	1000	4500
TA 150521	20	(1) : 50	4,5	500	160	1508FA	5	1000	4500
TA 150522	20	(1) : 100	19,0	200	360	1508FA	5	1000	4500
TA 150523	20	(1) : 200	76,0	50	750	1508FA	5	1000	4500
TA 150524	20	(1) : 500	475,0	10	1800	1508FA	5	1000	4500

Rated primary current I_p = 70Arms

Code	I_p [Arms]	n	Ls [mH]	fmax [KHz]	Vt [μVs]	Drawing	D [mm]	U_is [Vrms]	U_p [Vrms]
TA 153201	70	(1) : 50	11,5	500	650	153A	9	1000	4500
TA 153202	70	(1) : 100	45	200	1300	153A	9	1000	4500
TA 153203	70	(1) : 200	180	50	2700	153A	9	1000	4500
TA 153204	70	(1) : 500	1130	20	7500	153A	9	1000	4500
TA 153205	70	(1) : 1000	4500	10	15000	153A	9	1000	4500
TA 153206	70	(1) : 50	11,5	500	650	153B	9	1000	4500
TA 153207	70	(1) : 100	45	200	1300	153B	9	1000	4500
TA 153208	70	(1) : 200	180	50	2700	153B	9	1000	4500
TA 153209	70	(1) : 500	1130	20	7500	153B	9	1000	4500
TA 153210	70	(1) : 1000	4500	10	15000	153B	9	1000	4500

Rated primary current I_p = 100Arms

Code	I_p [Arms]	n	Ls [mH]	fmax [KHz]	Vt [μVs]	Drawing	D [mm]	U_is [Vrms]	U_p [Vrms]
TA 152009	100	(1) : 100	31	60	2250	152A	13	1000	4000
TA 152059	100	(1) : 200	124	25	4500	152B	13	1000	4000

Some standard codes in the linear version with the loop inserted

Rated primary current I_p = 20 Arms

Code	I_p [Arms]	n	Ls [mH]	fn [KHz]	Vt [μVs]	Drawing	D [mm]	U_is [Vrms]	U_p [Vrms]
TA 150621	20	1 : 100	10	5 ÷ 100	550	15062X	1,2	440	4000
TA 150622	20	1 : 200	40	1 ÷ 50	1100	15062X	1,2	440	4000
TA 150625	20	1 : 1000	1000	0,05 ÷ 10	6000	15062X	1,2	440	4000

Rated primary current I_p = 50 Arms

Code	I_p [Arms]	n	Ls [mH]	fn [KHz]	Vt [μVs]	Drawing	D [mm]	U_is [Vrms]	U_p [Vrms]
TA 150641	50	1 : 100	10	5 ÷ 100	850	15064X	2,3	480	4680
TA 150642	50	1 : 200	40	1 ÷ 50	1900	15064X	2,3	480	4680
TA 150645	50	1 : 1000	1000	0,05 ÷ 10	9000	15064X	2,3	480	4680

Rated primary current I_p = 70 Arms

Code	I_p [Arms]	n	Ls [mH]	fn [KHz]	Vt [μVs]	Drawing	D [mm]	U_is [Vrms]	U_p [Vrms]
TA 150671	70	1 : 100	10	5 ÷ 100	1400	15067X	3,5	600	5000
TA 150672	70	1 : 200	40	1 ÷ 50	2800	15067X	3,5	600	5000
TA 150675	70	1 : 1000	1000	0,05 ÷ 10	14000	15067X	3,5	600	5000

The characterization of the high-frequency current transformer refers to the following quantities:

n ratio between primary current and secondary current (it typically corresponds to the number of turns of the secondary winding)
R_S secondary winding resistance
I_Prms primary current rating
L_S secondary winding inductance
f_n nominal operating frequency (or optimal frequency range)
V_t minimum secondary transfer area referred to a temperature of 25°C
U_is maximum operating voltage
U_p insulation voltage
D diameter of the through-hole or primary turn (in the version with molded-in turn)

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