Z3801A Power Supply (38-60V version)

Disclaimer: the features described below are taken from my own Z3801A. They are intended for academic purposes only and may not be used for businesses of any kind. The descriptions below are believed to be correct. However, I cannot be held responsible for any damage or personal injury that results from using these descriptions. In other words, you are welcome to ask questions, suggest alterations and use this information at your own risc - but basically you are on your own if problems of any kind arise ;-)

Overview

The power supply consists of two DC-DC converters, a window comparator to check the input voltage, a separate 5V supply, and filtering. The input circuit has no direct connection to any of the DC-DC converters outputs.

The power supply consists of the following parts:

• a DC-DC converter Datel UWR-5/4000-D48 (U16 or U106) for the outer oven heater
• a DC-DC converter Lucent (now Tyco) CW025ACL-M (U5), delivering +5V, +15V and -15V to the oven controller and main CPU board
• a 5V supply (Q1, U1) for the window comparator
• a dual operational amplifier  LM393D (U3) used as window comparator (with U4)
• two separate 2.5V references LM317 (U2, U7)
• a main switch Q2 with driver stages Q3, Q4, controlled by the window comparator
• filtering (C22-24, L1, L5), fusing (F1, F2) and overvoltage/reverse polarity protection (VR1, CR10, CR7, CR8; CR1, CR2)

Also the three output voltages to the main CPU board (-15V, +5V, and +15V) are filtered with toroidal chokes, L2, L3, L4.

(for better readability of the schematic diagram you might want to right-click on the picture and re-open it in a new window)

Normal operation

Q1 is used for stepping down the input voltage to ~9V which is safe and a sufficient input voltage for the linear regulator U1, creating a stable 5V supply for the window comparator U3, U4A.

U3b compares the voltage on TP10 to a first reference voltage of 2.5V at TP2. U2 is used to generate this reference voltage. Due to the positive feedback through R6, R7 and the choosen values for the input  voltage divider R8, R9, R18, R21 the output of U3b goes to +0V for input voltages exceeding ~62.29V on TP13. To get the output back to +5V, the input voltage must decrease below ~59.72V on TP13.

Similarly U3a compares the voltage on TP3 to a second reference voltage of again 2.5V at TP9. The input voltage is divided by the network consisting of R10, R11, R12, R19 and R26. R26 is variable and can be used to adjust the lower trip voltage of the comparator. The switch-off range lies between 32.57 ... 37.15V. To switch on again the voltage at TP13 has to exceed 42.09 ...46.67V. In any case the hysteresis is about 9.52V.

As long as both outputs from U3 are high, the output of U4a is low (TP4) and Q2 switches on. In turn both DC-DC converters get a correct input voltage and begin to work. The converters have a nominal minimum input voltage of 36V, so R26 should be adjusted such that TP4 goes high for input voltages < 36V ( at TP13). The minimum switch on voltage is then ~46V at the input connector.

Notes:

• all voltages are measured from TP12, common ground (= minus lead of input connector)
• input voltage is one diode drop higher than the voltage on TP13, for the MURS340 this is about 0.9V @ 0.7A, 25°C.
• values or brand of L1 are unknown
• Q3, Q4 could be BSP123, at least these would work
• Q2 could be a MTP12P10, or any  p-channel MOSFET with V_GSmin ~ -3V and R_DSon < 0.5 Ohm
• CR6, CR7, CR8, CR9 are only there to protect U3, they have no functional purpose
• it is not understood why there are two separate 2.5V references instead of just one
• functionally F1 and F2 are in series (i.e. all current flowing through F1 must also flow through F2). Several readers noted that it is usual practice for e.g. mobile equipment to fuse the negative wiring, too. In case of a short to ground and some other equiment wanting to pass excessive current to the common ground return, the fuse will prevent damage. In addition, telco equipment uses positive voltage as ground reference, therefore F2 is the main fuse (3A) and F1 is there to protect the (positive) ground wiring.
• 'earth' (C22 and C23) is connected to the housing of the Z3801A by one of the mounting screws.

A description of the outer oven controller supplied by U16 is here!

last updated: 07 March  2004         mailto: stefan(dot)hegnauer(at)gmx(dot)ch