Trying to figure out a battery-backup circuit - Page 1
Forum section: electronics / projects

author: philpem
author_rank: Frequent Contributor
timestamp: January 25, 2023, 04:03:56 am
content: This one's giving me a headache!This is the circuit for the battery-back-up on the BSB Eurocypher access control module. I'm trying to figure out what the heck Q1 is meant to be doing, as the "switch between battery and 5V-in" task seems to be handled by the two diodes.I'm assuming the diodes are both 1N4148 style small silicon diodes as they're in the red glass package typical of those. They could be some other kind of small diode if that makes more sense. I don't want to desolder them as that'd likely cost me the contents of the RAM (i.e. the encryption keys and control data)AS_P25 and AS_P1 go to an ASIC which contains an embedded 65C02 processor and some I/O ports. I'm guessing P1 is either to back-up some RAM in the ASIC, or to monitor the battery voltage. I assume P25 is an output as it's driving the base of Q1.Any insight would be appreciated!Cheers

author: moffy
author_rank: Super Contributor
timestamp: January 25, 2023, 04:55:28 am
content: I assume you mean "Q1" as there is no "Q2" in your circuit. The 2N2907 is a low saturation moderate current PNP transistor, so I would assume that when switched it would reduce the voltage drop to BB_VCC from 5VA from about 0.6-0.7V through CR1 to possibly 0.1-0.2V through Q1 when switched.

author: David Hess
author_rank: Super Contributor
timestamp: January 25, 2023, 02:37:59 pm
content: I think moffy has it right; Q1 is being used to avoid the voltage drop of CR1 after operation from the 5 volt input has started. I assume they had some kind of problem operating at full power from 4.4 volts, or there was some requirement for the full 5 volts.The glass diodes could also be small signal schottky diodes for lower voltage drop. Measuring their forward voltage drop should reveal if this is the case.

author: philpem
author_rank: Frequent Contributor
timestamp: January 25, 2023, 03:00:19 pm
content: Quote from: moffy on January 25, 2023, 04:55:28 amWoops, typo. Sorry about that.I figured the Vce(sat) would be more like 0.4V to 1.6V, as they're the only numbers in the PN2907 datasheet I found, and there wasn't a Vce(sat) vs. Ib performance graph.I've tracked down a 2N2907 datasheet with more information --https://global.oup.com/us/companion.websites/fdscontent/uscompanion/us/pdf/microcircuits/students/bjt/2N2907-motorola.pdfThe other important factor is the RAM in use, it's an MK48H64UN:https://www.silicon-ark.co.uk/datasheets/mk48h64-datasheet-stmicroelectronics.pdfStandby current is quoted as max 7mA, operating current max 100mA (more than I expected!)And it looks like you're right -- figure 4, collector saturation region (Vce vs. Ib and Ic), the voltage drop is 0.1V to 0.2V with 2.5mA base current (the 2k resistor with 5V supply) and 100mA collector current.With a 100mA load (Ic), Vce would be around 0.175V.With the 7mA max standby load, Vce would be less than 0.1V.With the diode alone, a 100mA load would have a Vf of around 1.1V -- which gives Vcc=5V - 1.1V = 3.9V, far outside the 5V +/- 10% operating figure quoted in the MK48H64 datasheet.So it's there to bypass the diode and reduce the voltage drop in an operating condition.Thanks for the help!As for whether they're Schottky diodes, I'm not sure. I'm guessing this whole setup is to avoid using a Schottky diode, I assume due to cost?The board was made by General Instrument in 1988 or 1989, the cost of a Schottky diode may have been more than the transistor and bias circuitry at the time.

author: Peabody
author_rank: Super Contributor
timestamp: January 25, 2023, 03:46:53 pm
content: They could even be germanium diodes, which have a lower Vf than silicon. It seems it would almost have to be something like that for the battery backup to do any good.

author: David Hess
author_rank: Super Contributor
timestamp: January 25, 2023, 04:39:14 pm
content: Quote from: Peabody on January 25, 2023, 03:46:53 pmModern safety specifications when using lithium batteries like that require two diodes in series, or a diode and current limiting resistor, so it is an old design, or the designer ignored that.Lower voltage operation is usually allowed when only backing up memory.

author: philpem
author_rank: Frequent Contributor
timestamp: January 25, 2023, 06:17:03 pm
content: Quote from: Peabody on January 25, 2023, 03:46:53 pmI'd be surprised if they were -- OA91s usually come in huge glass packages. These look like 1N4148s, in the small red glass package.Quote from: David Hess on January 25, 2023, 04:39:14 pmThere is a current limiting resistor - a 10k from the battery-ground to the main ground. OSDGND connects to the main ground via a zero-ohm link.The design dates back to 1988 or 1989.

author: langwadt
author_rank: Super Contributor
timestamp: January 25, 2023, 06:25:15 pm
content: Quote from: David Hess on January 25, 2023, 04:39:14 pmR1 is going to limit the current quite a bit

author: harerod
author_rank: Frequent Contributor
timestamp: January 25, 2023, 06:27:52 pm
content: [quote author=philpem link=topic=361597.msg4660624#msg4660624...The design dates back to 1988 or 1989.This would explain Q1 being a BJT. Another option could have been a relais.At any rate, nowadays Q1 would be a p-channel MOSFET, ideally replacing the BJT saturation drop by a low Drain-Source-resistance.

author: David Hess
author_rank: Super Contributor
timestamp: January 25, 2023, 09:33:34 pm
content: Quote from: langwadt on January 25, 2023, 06:25:15 pmMy eyes are not getting any younger.