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| Missing resistor value |
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| buzzawak:
So I am doing a project from a December 1989 issue of Silicon Chip Magazine. "Digital Voice Recorder Board" and there is one resistor that I can't find the value for. No reference in the article or on the website Errata list. The article does talks about the power supply. Power Power for the Voice Board is Provided from two separate 5V supplies. All of the RAMs and ICl, IC2, IC3, IC4, ICS and IC6 are powered from the standby + 5V supply while the analogy circuitry, IC7, IC8 and IC9 are powered from the main + 5V supply. Both supplies are based on 7805 3-terminal regulators. The main + 5V supply is the simplest. Its 7805 regulator [REG1) is fed from an external + 12Y supply via a 100 resistor and protected against reverse polarity and excessive input voltages by 16V Zener diode D18. A 47OuF capacitor filters any hash from the input line to the 7805. The standby supply is more complicated but uses the same components for input Protection and filtering. However, the standby regulator (REG2I has diode D20 in series with the GND leg to jack up the output voltage by 0'6V which is then "dropped" by diode D21' This gives an output of + 5V which is bypassed with a 100pF capacitor. Diodes D21 and D22 arc included so that a 4.5V backup battery can be included. The diodes provide isolation of the normal 5V regulator output from the 4.5V battery. If the standby regulator is Powered down due to disconnection of the offboard supply, the onboard battery maintains the data stored in memory. It feeds the + 5V standby rail via D22 and provides about + 3.9V which is adequate to keep the memories powered up. |
| oPossum:
R1 is for charging a NiCd or NiMH battery. The value would typically be calculated for at most 0.1C charging current. |
| Ian.M:
Modern NiMH batteries tend to die if trickle charged for long periods, and even NiCds wont like unterminated C/10 charging. You *might* consider trickle charging at C/50, but IMHO its not worth it compared to primary cells - if its powered enough to keep a rechargable backup battery charged, the drain on a backup 3x alkaline AA will be pretty small, and the expected backup life far more predictable. |
| buzzawak:
Did we have Nicad batteries back in 1989? |
| madires:
Yep, plenty of them. As replacement for standard cells, in computers and consumer electronics, RC models and so on. |
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