Rectifier diode voltage push - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 04, 2021, 04:51:07 pm
content: I don't know if the heading is the correct term. But I want to add a single rectifier diode to the 0 Volts rail of a linear regulator to increase the voltage by about 0.7 Volts above the regulators rating. So a 12 Volt L7812 would be 12.70 Volts when powered up. The thing I'm unsure about is the power rating of the diode, the power supply circuit would be capable of 20 Amps continuous, and 25 Amps for short periods. Obviously I can't put that much current through a 10 Amp rectifier diode. So was thinking I might use a single diode junction from a 50 Amp bridge rectifier package, but leave the three other junctions unused. The circuit is a typical linear power supply with multiple PNP BJT power transistors. I just want the output around 0.5 Volts or so just above the rated 12.00 Volts of the linear voltage regulator. Thoughts appreciated, and thank you for reading.

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: January 04, 2021, 05:09:14 pm
content: For the diode in the reference pin of the regulator any from the 1N400X series or even a 1N4148 should do as there isn't that much of current flow there.

author: David Hess
author_rank: Super Contributor
timestamp: January 04, 2021, 05:13:42 pm
content: The common pin current from a 78xx series regulator, or the resistive divider used with an adjustable regulator, is less than 10 milliamps so any signal or rectifier diode will work.

author: B J
author_rank: Contributor
timestamp: January 04, 2021, 05:24:07 pm
content: I've done that several times, works great. Only other thought, bypass the diode with a cap and with a resister to Vcc, helps keep the new reference voltage solid.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 04, 2021, 07:51:04 pm
content: Thank you for your replies. That's great, I've done this on power supply circuits under 10 Amps. It was just a larger output current I wasn't completely sure of. Years ago I found a CB radio PSU with this diode on the regulator negative rail, intact it had two diodes in series. I measured the voltage from, and in between these diodes and found the voltage increase from the regulator. It was an old harrier CB power supply.Thanks again for your help.

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2021, 09:07:48 pm
content: I can confirm that it works with a small diode, such as the 1N4148, but I prefer to use the LM317, if it's a new design, because it has better regulation and temperature stability. It's one more resistor, but they cost nothing.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 05, 2021, 02:47:07 am
content: I have used the LM317 quite a few times, its my understanding that they don't perform well with PNP pass transistors. Otherwise i would use one instead of an L7812, but use fixed value resistors to obtain the output voltage i need. Anywhere between 12.50 Volts and up to 13.80 Volts, i don't need it to be variable voltage, so maybe it would be stable at a selected voltage with fixed value resistors.

author: David Hess
author_rank: Super Contributor
timestamp: January 05, 2021, 04:48:54 am
content: Quote from: davelectronic on January 05, 2021, 02:47:07 amThe LM317 has the option of bypassing the adjustment pin to common which makes it more likely to oscillate because it is operating at a lower noise gain, but the gain and phase margin would be checked in a production design anyway.

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2021, 09:13:43 am
content: I haven't had any problems using the LM317, with a PNP pass transistor, but it's not something I do often. If I want more than an Amp or so, I normally go for a switching regulator.