4.Load, electrical life
22.Contact protective circuit
4
Use of contact protective devices or protection circuits can suppress the counter emf to a low level. However, note that incorrect use will result in an adverse effect. Typical contact protection circuits are given in the table below.
Also, note that release time will slow down due to sneak in the circuit and may cause the electrical life to shorten and
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| Diode circuit | |
Circuit |
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| The diode connected in parallel causes the energy stored in the coil | |
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| to flow to the coil in the form of current and dissipates it as joule | |
Features/Others | heat at the resistance component of the inductive load. This circuit | ||
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| delays the release time. (2 to 5 times the release time listed in the | |
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| catalog) | |
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| Use a diode with a reverse breakdown voltage at least 10 times the | |
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| circuit voltage and a forward current at least as large as the load | |
Devices Selection | current or larger. | ||
In electronic circuits where the circuit voltages are not so high, a | |||
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| diode can be used with a reverse breakdown voltage of about 2 to 3 | |
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| times the power supply voltage. | |
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| Diode and zener diode circuit | |
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Circuit |
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Features/Others |
| It is effective in the diode circuit when the release time is too long. | |
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Devices Selection |
| Use a zener diode with a zener voltage about the same as the power | |
| supply voltage | ||
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In the actual circuit, it is necessary to mount the protective device (diode etc.) in the immediate vicinity of the load. If it is mounted too far away, the effectiveness of the protective device may diminish. As a guide, the distance should be within 50cm.
Avoid using the protection circuits shown in the figures below.
Although it is usually more difficult to switch with DC inductive loads compared to resistive loads, use of the proper protection circuit will raise the characteristics to that for resistive loads.
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Although it is extremely effective in arc | Although it is extremely effective in arc |
suppression as the contacts open, the | suppression as the contacts open, the |
contacts are susceptible to welding since | contacts are susceptible to welding since |
energy is stored in C when the contacts open | charging current flows to C when the |
and discharge current flows from C when the | contacts close. |
contacts close. |
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