Fluke 5725A High Voltage Supply Section, 4-17, Ref Des, 1.Internal Fuse Data, Assembly, Rating

Theory of Operation 4

Detailed Circuit Description

High Voltage Supply Section 4-17.

The high voltage supply allows normal operation from a pair of nominal 400V dc supplies, or troubleshooting operation in a reduced voltage mode with a pair of nominal 50V dc supplies. Switching between normal and reduced-voltage operation is done by removing the plug from J301 and inserting it into J303. See Section 7 for the procedure to do this. (Plug P301 is wired with both 400V and 50V transformer taps so that the 400V taps are connected in the circuit when P301 is plugged into J301, and the 50V taps are connected in the circuit when P301 is plugged into J303.)

Two sets of high voltage rectifiers, CR301 through CR304 and CR310 through CR313, form bridge rectifiers for the positive and negative supplies, respectively. The output from these bridges is filtered by C301/C321 and C320/C322. The capacitor pairs are configured in series to obtain the necessary working voltage. Bleeder resistors R303, R314, R315, and R 308 discharge C301/C321 when power is removed and form a voltage divider that equalizes the voltage across halves of C301. Resistors R335, R337, R336, and R339 do the same function for C302/C322.

Warning

Do not rely on the power supply LEDs as an indication of the presence of high voltage. Always use a voltmeter to check for high voltage between TP307 and TP301, and between TP307 and TP304.

Components CR317 and CR318 are high intensity red LEDs that provide visual indication of the presence of high voltage. These LEDs have high luminosity at low current levels. They are located in series with the bleeders for the positive and negative supplies, using about 1 mA of current for bias. Under average lighting conditions, these LEDs are visibly lit with as little as 40V on C301/C321 and C320/C322. Zeners VR306 and VR307 are normally off, as their zener voltages are greater than the forward voltage of the LEDs. However, should an LED fail, these zeners carry enough current to allow the bleeder resistors to function.

Fuses F301 and F302 protect the transformer against overload in case the 400V supplies are shorted. Without these internal fuses, a short-circuit would cause excessive power dissipation in the power transformer, even though ac line current would be insufficient to blow the main fuse. Table 4-1 lists data for fuses F301 and F302, and the other internal fuses in the 5725A.

Caution

For safety, replace internal fuses only with fuses of the rating and type specified in Table 4-1.

Surge arrestor E302 fires if the secondary voltage of the +400V supply exceeds 600V. This should only occur if the 5725A is plugged into an ac line voltage of 200V or more when it is set to 100-120V. E302 would then draw enough current to blow the main fuse, thereby protecting the 5725A.

REF DES.

F301

F302

F201

F202

Table 4-1. Internal Fuse Data

ASSEMBLY	RATING
Power Supply (A4)	2A, 600V
Power Supply (A4)	2A, 600V
Current Amplifier (A2)	20A, 32V
Current Amplifier (A2)	20A, 32V

TYPE

Littelfuse BLS or equivalent Littelfuse BLS or equivalent Bussman AGC or equivalent Bussman AGC or equivalent

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