| Current sense resistor is usually located in | | | | power section will blow and |
| the switch mode power | | | | |
| | | | most of the time the components changed are |
| supply section. Newer version of monitor-the | | | | fuse, power fet, pwm ic, |
| b+ (boost) circuit | | | | |
| | | | zener diode and current sense resistor.Do |
| also have this current sense resistor. If | | | | you know that a small change in this current |
| you repair electronic | | | | sense resistor value |
| | | | |
| equipment especially the switch mode power | | | | have a great impact on the secondary output? |
| supplies, the current | | | | A little change in value |
| | | | |
| sensing resistor is connected to the source | | | | say from 0.22 ohm to 0.44 ohm can caused |
| pin of power FET (field | | | | your ouput voltage to drop |
| | | | |
| effect transistor) and the other end to HOT | | | | into half! I have seen a lot of repairer |
| GROUND. The value of | | | | can't detect this type of |
| | | | |
| current sense resistor is in the range of | | | | problem (low output voltage) due to that |
| 0.1 ohm to about 1 ohm. | | | | they used analog or digital |
| | | | |
| The values are depends on the design of each | | | | meter that don't have the low ohms range. |
| power supply.In monitor power supply where | | | | You can't differentiate |
| pulse width modulation (pwm) ic UC3842 | | | | |
| | | | if the low ohms resistor has its value |
| are used. Pin 3 of this power ic is called | | | | changed by using normal meters. |
| I-sense which means | | | | |
| | | | I personally use the Dick Smith ESR Meter |
| current sense and this pin is connected to | | | | which have the capabilities |
| source pin of POWER FET. | | | | |
| | | | to check resistor value down to 0.01 ohm! If |
| The purpose of this current sense circuit is | | | | you don't have this |
| for overcurrent protection. | | | | |
| | | | special meter you may just direct replace |
| This circuit usually samples the voltage | | | | the current sense resistor |
| drop across a resistor in series | | | | |
| | | | to see if the generated secondary output |
| with the switching fet transistor. If the | | | | will back to normal voltage.Another tip is |
| current rises abnormally due to | | | | that if the current sense resistor in the |
| | | | monitor |
| some short circuit in the secondary side | | | | |
| such as a shorted secondary diode | | | | B+ circuit have changed value it will caused |
| | | | the output in the |
| or horizontal output transistor (HOT), the | | | | |
| voltage will exceed a reference | | | | boost circuit to become low and you will |
| | | | have a small width display |
| level and shutdown the pulse generator which | | | | |
| is the PWM ic. This circuit | | | | and of course the high voltage will become |
| | | | low as well. Instead of |
| provides protection to components in the | | | | |
| event a problem occurs. | | | | 24Kv it becomes 15-18Kv.Jestine Yong is a |
| | | | electronic repairer and a writer. |
| In some cases where overload happen-the | | | | |