Author Topic: Missing resistor value  (Read 519 times)

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Offline buzzawak

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Missing resistor value
« on: October 02, 2019, 06:35:12 am »
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.



« Last Edit: October 02, 2019, 06:37:29 am by buzzawak »
 

Online oPossum

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Re: Missing resistor value
« Reply #1 on: October 02, 2019, 06:43:30 am »
R1 is for charging a NiCd or NiMH battery. The value would typically be calculated for at most 0.1C charging current.
 

Online Ian.M

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Re: Missing resistor value
« Reply #2 on: October 02, 2019, 07:52:18 am »
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.
 

Offline buzzawak

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Re: Missing resistor value
« Reply #3 on: October 02, 2019, 08:16:14 am »
Did we have Nicad batteries back in 1989?
 

Offline madires

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Re: Missing resistor value
« Reply #4 on: October 02, 2019, 08:31:42 am »
Yep, plenty of them. As replacement for standard cells, in computers and consumer electronics, RC models and so on.
 

Offline buzzawak

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Re: Missing resistor value
« Reply #5 on: October 02, 2019, 08:57:23 am »
Ok, seeing that there is no mention in the project text on needing to use a rechargeable battery I guess the R1 resistor is meant to be left out. 
 

Online Ian.M

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Re: Missing resistor value
« Reply #6 on: October 02, 2019, 09:03:15 am »
Sealed NiCds have been arround since post-WWII.  In the '80s they filled the same market niche that LiPOs do today.  They were far better if neglected than the 2V miniature sealed Lead acid cells that preceded them, which would die if left fully discharged for more than a few days.
 

Offline austfox

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Re: Missing resistor value
« Reply #7 on: October 02, 2019, 09:29:23 am »
From Silicon Chip Nov '90:

 

Offline buzzawak

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Re: Missing resistor value
« Reply #8 on: October 02, 2019, 09:41:52 am »
Austfox, this is the second time you have saved the day. I own you a beer! :-+
 

Offline austfox

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Re: Missing resistor value
« Reply #9 on: October 02, 2019, 10:06:44 am »
No worries, happy to help.

Silicon Chip keep an excellent Notes and Errata database on their website, as you have mentioned, but unfortunately this project was not mentioned.
« Last Edit: October 02, 2019, 10:08:25 am by austfox »
 


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