Planar multi-junction thermal converters were fabricated for precise measurements of the ac voltage and current by an ac-dc transfer method. A heater and a thermocouple array were fabricated onto a sandwiched membrane, $Si_{3}N_{4}$ (200 nm) / $SiO_{2}$ (400 nm) / $Si_{3}N_{4}$ (200 nm), a thickness of $0.8;{mu}m$ and a size of $2{ imes}4;mm^{2}$, which is supported by a surrounding frame. The NiCr heater is located at the center of the membrane vertically. Hot junctions of $48{sim}156$ pairs of thermocouples (Cu-CuNi44) are located near or onto the heater, and cold junctions are located onto the silicon frame. Output of the thermal converters for 10 mA dc input was $76;mV{sim}382;mV$ dependent on a model, and short term stability of the outputs was ${pm}5{sim}15;ppm$/ 10 min with 5 mA dc input. Responsivity in air was in the range of $3.9{sim}14.5V/W$. Responsivity of the model BF48 in air which has 48 thermocouples was 2 times or greater than that of 3 dimensional multi-junction thermal converter in vacuum which has 56 thermocouples. AC-DC transfer differences with an input of 10 mA or less were less than ${pm}1;ppm$ in the frequency range from 5 Hz to 2 kHz, and about $2{sim}3;ppm$ at 5 kHz and 10 kHz.