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