Missing resistor value - Page 1
Forum section: electronics / beginners

author: buzzawak
author_rank: Contributor
timestamp: October 02, 2019, 06:35:12 am
content: 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.PowerPower for the Voice Board is Provided from two separate 5V supplies. All of the RAMs and ICl, IC2, IC3, IC4, ICS and IC6 are poweredfrom 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 froman external + 12Y supply via a 100 resistor and protected against reverse polarity and excessive input voltages by 16V Zener diodeD18. A 47OuF capacitor filters any hash from the input line to the 7805. The standby supply is more complicated but uses the same componentsfor 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 isthen "dropped" by diode D21' This gives an output of + 5V which is bypassed with a 100pF capacitor. Diodes D21 and D22 arc includedso 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 thestandby regulator is Powered down due to disconnection of the offboard supply, the onboard battery maintains the data stored inmemory. It feeds the + 5V standby rail via D22 and provides about + 3.9V which is adequate to keep the memories powered up.

author: oPossum
author_rank: Super Contributor
timestamp: October 02, 2019, 06:43:30 am
content: R1 is for charging a NiCd or NiMH battery. The value would typically be calculated for at most 0.1C charging current.

author: Ian.M
author_rank: Super Contributor
timestamp: October 02, 2019, 07:52:18 am
content: 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.

author: buzzawak
author_rank: Contributor
timestamp: October 02, 2019, 08:16:14 am
content: Did we have Nicad batteries back in 1989?

author: madires
author_rank: Super Contributor
timestamp: October 02, 2019, 08:31:42 am
content: Yep, plenty of them. As replacement for standard cells, in computers and consumer electronics, RC models and so on.

author: buzzawak
author_rank: Contributor
timestamp: October 02, 2019, 08:57:23 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: October 02, 2019, 09:03:15 am
content: 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.

author: austfox
author_rank: Regular Contributor
timestamp: October 02, 2019, 09:29:23 am
content: From Silicon Chip Nov '90:

author: buzzawak
author_rank: Contributor
timestamp: October 02, 2019, 09:41:52 am
content: Austfox, this is the second time you have saved the day. I own you a beer!

author: austfox
author_rank: Regular Contributor
timestamp: October 02, 2019, 10:06:44 am
content: 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.