Adding dual polarity rails to  power supply - Page 1
Forum section: electronics / projects

author: geekbot
author_rank: Contributor
timestamp: May 02, 2014, 08:17:47 am
content: I was inspired after viewing Dave's power supply design and bought two LT3080 and a current sense amplifier and some DACs. I was thinking of adding a +/- 15 v fixed rails to the power supply so that I can power opamps with it and a 5v positive low current rail for powering logic circuits and MCUs.I was thinking of using LT1086 for 5V supply, I want a better performance than the regular 7805. For negative and positive rails, LT has a -15v fixed regulator LT3015, but no +15v fixed regulator. Even if I use a +12v LT3016 and -12V LT3015 can i connect them like the 7815 and 7915?I will be powering the whole supply from the rectified output of a 3A transformer. I will be paralleling the LT3080 for 2A output. I need output voltages in the range 0-25v. Is there any changes I should make to Dave's power supply schematic ?He was operating it from 0-6 V and did not go higher in any of his videos, as far as i know. LT3080 can operate upto 36vs.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 02, 2014, 11:24:08 am
content: I should maybe get a rubber stamp made with the following:The LT3080 has a flaw that makes it easy to blow it up in an adjustable, "lab" or whatever, power supply. The voltage difference between the set and the output pin must not exceed the rather tiny amount of ±0.3V. That is one of the reasons Dave never finished the power supply project. The occasionally mentioned LT3083 has a similar fault, however it is a bit more robust and can stand ±10V. Still not good enough for an adjustable power supply with > 10V output.As for the missing +15V fixed regulator in LT's offerings, use a jellybean 7815 from any vendor (except maybe ST). Or, if it really needs to be from LT, use an adjustable regulator like the LT1086 with fixed resistors to set it to 15V

author: geekbot
author_rank: Contributor
timestamp: May 02, 2014, 12:01:00 pm
content: Heartbroken, Dave didnt mention that in the first 5 video tutorials. I already bought 2 of them. Can you recommend any other adjustable regulators other than the LM338/317?

author: geekbot
author_rank: Contributor
timestamp: May 02, 2014, 12:33:03 pm
content: Yeah, read about it in the forums. Going back to LM338. Can I still use the Dave's schematic for that? Only thing will be 1.25 ref min output voltage. I dont need a 0.

author: just_fib_it
author_rank: Contributor
timestamp: May 02, 2014, 01:29:36 pm
content: Quote from: Bored@Work on May 02, 2014, 11:24:08 amInteresting, could you elaborate on that? I recently got a few tubes of various 78xx and 79xx regulators and most of them are from ST...

author: mariush
author_rank: Super Contributor
timestamp: May 02, 2014, 03:54:14 pm
content: A lot of models made by ST have very thin metal backing, so they don't dissipate heat by themselves as better as other 78xx made by other companies. You have to be more careful with how how they become if you use them without additional heatsink.

author: geekbot
author_rank: Contributor
timestamp: May 02, 2014, 04:13:31 pm
content: Any one suggest me a LDO which can be driven by a MCU. LT3080 is pretty much useless from what people posted in this forum. And dave should have made a video about it in the Power supply series.

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 02, 2014, 11:09:54 pm
content: is the lt3080 safe to use in a fixed power supply at least? I was going to use these for my project as fixed +15 v rails.when you say the voltage cannot exceed +-0.3, what does this mean? a short circuit will blow it up? I already have a bunch of LT3080s that I was going to use for +5 and +15V using 500k and 1.5m resistors. that I also have. in low tempco..*strictly for powering op amps and a DAC.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 03, 2014, 07:53:15 am
content: Quote from: SArepairman on May 02, 2014, 11:09:54 pmIf you can keep it within its operating conditions. If that is possible depends on your application. Power on, power off, accidentally powering the output from some external source, and rapid load changes seem to be the critical events.Quote from: SArepairman on May 02, 2014, 11:09:54 pmLinear says this. And the defect LT3080s people happen to have indicate Linear means it.QuoteQuote from: SArepairman on May 02, 2014, 11:09:54 pmAs Linear coyly and vaguely saysQuoteQuote from: SArepairman on May 02, 2014, 11:09:54 pmIt can, it is is a gamble, because Linear doesn't say what the IC can really bear. It must be capable of surviving that condition unharmed for a short bit of time, otherwise it would blow up if you just look at it in a funny way. But from the experience people reported here it can't survive for long.Maybe clamping the output to the set pin with two diodes can prolong the life of the regulator, but might cause other problems. And you are hard pressed to find diodes switching precisely at 0.3V. And even if you find them, how accurate is Linear's 0.3V spec? And how robust need the diodes to be?Maybe some day Linear comes clean and tells us what the regulator really can and can't do (and fires the marketing joker who prevents the engineers from telling it like it is ...).

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 03, 2014, 07:58:54 am
content: oooo... so having inductors on the supply rail for a LT3080 might be a really bad thing ....accidentally powering the output from some external sourcethat seems like a perfect way to blow it up. with an inductive spike. Especially when there is a filter capacitance on the set pin, i think?im glad i caught this... 0.3 v is not much room for error.holy crap that kind of "marketing" could cost someone alot of money. what the hell is LT run out of a boiler room?its seem that the only viable alternative is the LT1965.. but god knows what the noise out of it is at 15V.......or the 3083, +-10v should be able to handle the inductive spikes that I expect to see. but damn, its like twice as expensive.and I can't really find the noise specs of the LT3080 @ 15V. Considering its 40uV rms @ 1V (who the hell would use this at 1V, another sign of boiler room sales men).. does it go up with voltage (like the LT1965 does, at least they specify it up to 3.3V........)I think the 1965 is a good replacement so long you can deal with the lower voltage rating.

author: David Hess
author_rank: Super Contributor
timestamp: May 03, 2014, 01:10:53 pm
content: If two floating sources are used for the +15 and -15 volt supplies, then you can use two of the same positive or negative regulators and do not need one of each. If you want higher performance, then use an integrated regulator for its fault protection with an external operational amplifier and reference.These days I would consider a floating variable output tracking regulator to do +/- 2.5 to +/- 15 volts because a lot of analog ICs operate with lower maximum supply voltages.

author: geekbot
author_rank: Contributor
timestamp: May 24, 2014, 04:01:51 am
content: Quote from: SArepairman on May 03, 2014, 07:58:54 amYeah! I decided to use fixed resistors to get 5v from LT3080 and use it as a USB charger.