(a) Field of the Invention
The present invention relates to a circuit system, and more particularly to a variable resistance linear limit voltage circuit that may, by way of example and not limitation, be used to match the rated saturation voltage VS of different rechargeable cells during charging, and that employs light emitting diodes as both illumination and impedance devices so as to reduce heat generated by the impedance devices while still achieving the desired limit voltage control.
(b) Description of the Prior Art
U.S. Pat. No. 5,118,993 and Europe Patent No. 0487204, both granted to the applicant of the present invention, disclose a multi-voltage output circuit that uses a positive voltage drop from a diode or a zener voltage from a zener diode to control an output voltage supplied to rechargeable cells directly connected in parallel at the output of the multi-voltage output circuit during a charging process, or the positive voltage drop from multiple diodes directly connected in parallel with the cells to provide a limit-voltage-divided charging current. In either application, the circuit provides a regulated voltage V0 when the terminal voltage of the cells accumulates along with the charging current and rises up to such extent close to, and eventually becomes identical with the positive voltage drop value. However, the positive voltage drop of the diodes in such an arrangement has a gradient of approximately 0.7V difference depending on the number of diodes connected in series, and therefore it is very difficult to match the rated saturation voltage VS of the cells by changing the number of diodes connected in series when the positive voltage drop of the diodes connected in parallel is not of the same value as that of the VS. Therefore, diodes must be directly connected in parallel with the cells being charged, and such connection creates the following defects:
1. In the absence of additional connection in series of a proper limiting current, the charging current IB decreases when a composite regulated voltage V0 is generated by the positive voltage drop of the diode and the terminal voltage of the cells. As a result, the diodes are vulnerable to being burnt out due to the significantly increased current passing through the diodes, as illustrated in FIG. 1 of the accompanying drawings of the present invention; and
2. The positive voltage drop value of the diodes is not consistent with the rated saturation voltage VS required by the cells. If the value is lower than VS, the charging current IB passing through the cells gets too small and consequently, the charging process becomes too slow or the charging current is insufficient, as illustrated in FIG. 2. On the other hand, if the value gets higher than VS, the cells will be overcharged.
The primary purpose of the present invention is to provide a linearly variable resistance limit voltage circuit system including a light emitting display.
The limit voltage circuit of the invention may be used in a variety of applications, one of which is the application of matching the saturation voltage of a rechargeable cell to ensure fully saturated charging, while at the same time preventing damage to the cell due to overcharging, reducing thermal loss from the limit voltage circuit, and providing a light emitting display when required.