Replacement for 4-pin voltage regulator LM78GCP - Page 1
Forum section: electronics / projects

author: Frankentronics
author_rank: Regular Contributor
timestamp: June 16, 2020, 01:34:39 am
content: Greetings,Does anyone know, is there an equivalent replacement for the discontinued 4-pin voltage regulator LM78GCP?https://www.datasheets360.com/pdf/1306101402837245463Thanks...

author: bin_liu
author_rank: Contributor
timestamp: June 16, 2020, 01:50:30 am
content: LM317 reform

author: Vgkid
author_rank: Super Contributor
timestamp: June 16, 2020, 01:52:18 am
content: Quote from: bin_liu on June 16, 2020, 01:50:30 amAgreed, or lm317hv, if you need a higher volt out.Note: you will need to change the adjustment resistors.

author: Frankentronics
author_rank: Regular Contributor
timestamp: June 16, 2020, 02:32:16 am
content: Thanks guys.I was hoping to not have to change the circuit, though.So, just for my further information, I am assuming that 4-pin voltage regulators are outdated, correct?Thanks...

author: floobydust
author_rank: Super Contributor
timestamp: June 16, 2020, 03:26:20 am
content: UA78GU1C, LM78GCP, NTE953The 4-pin IC's were popular in the 80's for a short time, still around as old stock for arcade games.LM78GCP control (reference) is 5.0V compared to 1.25V for the LM317HV, so one resistor would have to be changed, and the pinout seems flipped though. It is possible to stuff one in place.

author: Frankentronics
author_rank: Regular Contributor
timestamp: June 16, 2020, 03:54:15 am
content: I'll just use the 317, then. I guess there's really no practical reason to track down old stock as it's probably less work to just figure out what the proper resistors should be.This is actually for a 12V lead battery charger, so I don't believe I need the HV version.Thanks...

author: Frankentronics
author_rank: Regular Contributor
timestamp: June 20, 2020, 01:53:44 am
content: Here, I traced the relevant part of the schematic off of the PCB.With my limited experience I do not know where to go form here, to replace the LM78GCP by a 317.Any help would be appreciated.Thanks...

author: floobydust
author_rank: Super Contributor
timestamp: June 22, 2020, 04:44:06 am
content: Is this a new project or a repair on older gear? Input voltage is high around 30VDC so this voltage regulator IC needs a good heatsink and is stressed.It's a bit messy, you cannot easily put in an LM317 here because of two features.One, the charger has current-limiting to around 300mA with Q2/R6.Two, the charger has low (off) current drain across the battery with R3, R4, R5 all much higher value than an LM317 would like.If you put in an LM317 but add two transistors it would work. That's about the best I could think of.Maybe other people have a simpler way.

author: bin_liu
author_rank: Contributor
timestamp: June 22, 2020, 05:43:29 am
content: LM317 can do this.

author: Frankentronics
author_rank: Regular Contributor
timestamp: June 24, 2020, 02:43:24 am
content: Quote from: floobydust on June 22, 2020, 04:44:06 amThis is a circuit I traced from a battery powered guitar amp that I want to clone.I thought the simplest solution would be to just copy the existing PCB layout and populate it with the same components, but since this obsolete voltage regulator is an issue perhaps I should just use some other existing 12V lead battery charger circuit for that part of the PCB.The reason I want to use a lead acid battery is because I happen to have a few of them collecting dust. But I guess it could also be some other type of battery if that would make things somehow easier.Thanks...

author: floobydust
author_rank: Super Contributor
timestamp: June 24, 2020, 03:19:01 am
content: Does the amp take power off the 12V battery or does it get powered by the raw DC from the transformer when plugged in? What size battery?I would use the generic LM317 battery charger circuits (with output backfeed diode) that use a transistor for current-limiting. They use a resistor in the (-) to GND which would not work if the amp gets power from the transformer/bridge rectifier ground, like on your schematic.

author: Gyro
author_rank: Super Contributor
timestamp: June 24, 2020, 08:51:34 am
content: Quote from: bin_liu on June 22, 2020, 05:43:29 amThe LM317 can also do this (from the LM317 datasheet):

author: Frankentronics
author_rank: Regular Contributor
timestamp: June 24, 2020, 11:39:29 pm
content: Quote from: floobydust on June 24, 2020, 03:19:01 amThe amp works strainght from the battery. But I do not know if it still gets power from the battery when the charger is plugged into AC. The "charger" is just a wall wart transformer.Quote from: floobydust on June 24, 2020, 03:19:01 amThe battery that was originally inside the amp was smaller and it died. I put in an 8Ah battery and I believe the old one (which I no longer have) was half that physical size, so I guess it was 4Ah.Was it OK to just put a 2x larger battery inside?Quote from: floobydust on June 24, 2020, 03:19:01 amWould that work if the amp was being used at the same time as the battery was being charged?In fact, it always puzzled me if batteries can actually be charged at the same time as they are being used. Can they?Quote from: Gyro on June 24, 2020, 08:51:34 amI'm not sure how to interpret this.Thanks...

author: Gyro
author_rank: Super Contributor
timestamp: June 25, 2020, 07:57:43 am
content: Quote from: Frankentronics on June 24, 2020, 11:39:29 pmI'm not sure how to interpret this.Thanks...It deliberately degrades regulation by adding series resistor Rs between the output an voltage setting divider. Effectively it gives the regulator some basic current source characteristics to reduce the charging current when the battery is discharged but still maintains accurate voltage regulation for float charging (look up an LM317 2 terminal current source for comparison - basically just a resistor between the output and adjust pins. This is a primitive combination which is fine for LA batteries).It prevents excessive load on the mains transformer etc. at the expense of longer charging time (not a problem when the battery is normally maintained fairly full anyway, it just keeps it nicely float charged).