Why didn't IBM just use another 1N4148 here? - Page 1
Forum section: electronics / beginners

author: humidbeing
author_rank: Regular Contributor
timestamp: September 01, 2022, 12:53:04 am
content: I'm looking at the schematic for the battery/RTC power supply for an IBM 5170 computer, and I'm wondering why IBM used a dual transistor setup to prevent backfeed to the power supply, but diodes to prevent backfeed to the battery?Why not just 1N4148s for both and simplify the BOM?

author: MikeK
author_rank: Super Contributor
timestamp: September 01, 2022, 01:02:00 am
content: Because the transistor circuit is for current limiting?

author: humidbeing
author_rank: Regular Contributor
timestamp: September 01, 2022, 01:08:27 am
content: Quote from: MikeK on September 01, 2022, 01:02:00 amWhy would you need to current limit the VCC of a 5Volt IC?

author: pgo
author_rank: Regular Contributor
timestamp: September 01, 2022, 01:14:39 am
content: Maybe Vce(sat) < Vf ?It looks like they wanted to drop the battery voltage by a couple of Vfs.

author: MasterT
author_rank: Frequent Contributor
timestamp: September 01, 2022, 02:17:29 am
content: Undervoltage cut-off? Switch power when +5V rail is above 4V.

author: MikeK
author_rank: Super Contributor
timestamp: September 01, 2022, 11:48:03 am
content: Quote from: MasterT on September 01, 2022, 02:17:29 am^ This. The voltage divider at the base of Q1 is already not much above Vf. Q1 shuts off when the +5V rail goes below 4V.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 01, 2022, 01:28:14 pm
content: Quote from: MikeK on September 01, 2022, 11:48:03 amOk that makes sense to me. But when I simulate the circuit in LTspice with no battery voltage and an input voltage of 3V, my probe still reads 3volts on the right of the PNP. So it doesn't seem to be cutting off? Am I doing something wrong in LTspice?

author: gamalot
author_rank: Super Contributor
timestamp: September 01, 2022, 01:59:55 pm
content: Quote from: humidbeing on September 01, 2022, 01:28:14 pmI was wondering if you reversed the emitter and collector of Q1.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: September 01, 2022, 02:16:27 pm
content: What is the current limit of the Q1/Q2 network?Is the battery connector always connected - is there always a voltage at the collector of Q2? Or can it float?Forward current through a 1N4148 is around 300mA, so is this comparable to the transistor network? Forward voltage is around 0.65V, so that's 1.3V for the pair; which brings a generic 6.5V battery down to around 5V.It's a mystery

author: humidbeing
author_rank: Regular Contributor
timestamp: September 01, 2022, 02:19:21 pm
content: Quote from: gamalot on September 01, 2022, 01:59:55 pmGood catch!I fixed it but no change voltage prove wise in LTspice. I made sure to re-run the sim.

author: gamalot
author_rank: Super Contributor
timestamp: September 01, 2022, 02:26:49 pm
content: You don't have any load in your simulation, add a load and you'll see the voltage change.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 01, 2022, 02:57:51 pm
content: Quote from: gamalot on September 01, 2022, 02:26:49 pmAh ha! I put a resistor to ground and now LTspice shows the behavior y'all suggested.So this is a brownout circuit, basically? Why would you need a brownout circuit when the battery is connected all the time through the 1N4148s?

author: gamalot
author_rank: Super Contributor
timestamp: September 01, 2022, 03:19:59 pm
content: Quote from: humidbeing on September 01, 2022, 02:57:51 pmI don't think it's a brownout circuit, I more believe it's just for the lower voltage drop to stop power from the battery.---When you see two diodes connected in series on the battery side, people should ask why, wouldn't just one diode also prevent current from charging into the battery?The obvious answer is that because the battery voltage is too high, it's the reason that more diodes are required, and also the reason that voltage drop of the 5V supply needs to be minimized.

author: Kjelt
author_rank: Super Contributor
timestamp: September 01, 2022, 03:59:41 pm
content: Quote from: gamalot on September 01, 2022, 03:19:59 pmIndeed, it was a 6V battery according to thishttp://minuszerodegrees.net/5170/battery/5170_battery.htm

author: humidbeing
author_rank: Regular Contributor
timestamp: September 01, 2022, 04:14:05 pm
content: So basically, the needed to drop the 6V battery to ~5. But on the power supply side they already had 5V so they went with a solution to control flow without a diode drop. Makes sense to me. Thanks, all!