How to lower the supply by one volt. - Page 1
Forum section: electronics / beginners

author: DanKalc
author_rank: Newbie
timestamp: February 14, 2014, 01:42:04 pm
content: Hi,I have a device with can operate only up to 16.5V and i want to hook it to 17V DC supply. Is there a simply way to lower my voltage?My chip is digital so it can spike in supply voltage, so using a diode it a bad idea (its drop out voltage will change with current consumption, and i need precise input voltage). Maybe using constant current source will help with the diode.Anyone know other way to lower my voltage?

author: Christopher
author_rank: Frequent Contributor
timestamp: February 14, 2014, 02:02:32 pm
content: 3 terminal Linear regulator, as long as you watch your power dissipation?Series diode is out of the question, what about a Zener shunt? (Again watch P diss)

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 14, 2014, 02:14:25 pm
content: Doesn't a linear 3-pin typically have about 1V voltage drop?Maybe there are better ones, but I doubt you can get 16.5V out of 17V with a LM317 or 78xxHow precise does it need to be. Diode does change a little with current, but not terribly much, especially not if you are a bit further from the 0 current axis. And 0.5V is in some of the diode ranges (the lower drop ones, otherwise they too go more to about 0.8.If you really must have 16.5V precise, and it must come from 17V, maybe there exists FET based 3 terminal regulators?PS: title is wrong: How to lower by 1/2 volt is what you seem to be asking.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 14, 2014, 02:24:00 pm
content: Op says the device can operate up to 16.5V but doesn't tell what the lower limit is. If it is happy with 12V just use a 7812.

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 14, 2014, 02:36:57 pm
content: Oh right. Maybe that is what OP meant.But if that is how to interpret the request, maybe please re-explain why a (or two) diodes is not possible?Or what is meant with the remark on "precise input voltage"

author: vk6zgo
author_rank: Super Contributor
timestamp: February 14, 2014, 03:09:13 pm
content: Quote from: DanKalc on February 14, 2014, 01:42:04 pmWhy not just wind the 17v supply down to 16v?Most supplies are adjustable enough to handle such a "tweak".

author: DanKalc
author_rank: Newbie
timestamp: February 14, 2014, 04:14:19 pm
content: My device consumps few hundreds of uA, and if i put a diod on the supply its drop will be about 0.2V. When current rise to mA diode drops will increase to 0.6V, so my supply voltage will change (from 16.8 to 16.2V).I cannot change my supply voltage because im using old laptop supplier with fixed 17V.Is there any better (simpler) option than using a voltage stabilizer like LM317?

author: tszaboo
author_rank: Super Contributor
timestamp: February 14, 2014, 04:27:56 pm
content: Quote from: DanKalc on February 14, 2014, 01:42:04 pmYou connect slightly discharged a battery in series.

author: ivan747
author_rank: Super Contributor
timestamp: February 14, 2014, 04:29:55 pm
content: Try an LM1117 low dropout voltage regulator

author: Sigmoid
author_rank: Frequent Contributor
timestamp: February 14, 2014, 04:30:34 pm
content: Add a regular silicon diode in series. If you can get it to drop .6V, it should have you exactly where you want to be. They can be surprisingly stable if there's enough current.Now as you say your circuit doesn't consume enough current, and the diode doesn't drop enough voltage, you just need to make sure to always draw enough current through the diode: After the diode, add a 15k resistor to ground. That will act as a pulldown, and always keep the current through the diode above 1mA.In case your chip can draw more than that at a peak, just drop the value of the resistor.And make sure to add a decoupling capacitor (or two) at the power pin of the chip.There, done.

author: Rudane
author_rank: Regular Contributor
timestamp: February 14, 2014, 04:36:33 pm
content: Here's a crazy idea. Get a BJT based op-amp and wire it up as a buffer with your 17 volt supply as both the input and the rail. You should loose about 0.5 to 0.7 volts from the rail issue and the output will be 16.5 Volts'ish'

author: mariush
author_rank: Super Contributor
timestamp: February 14, 2014, 04:59:23 pm
content: A 1117 (lm1117, nxp1117 etc) adjustable linear regulator will have a voltage drop of about 1.1-1.2v at 0.8-1A, a bit less at lower currents. So the output voltage will be fairly close.Otherwise, a single 1n4017 will give you about 0.8-1v drop at 100mA+

author: Simon123
author_rank: Regular Contributor
timestamp: February 14, 2014, 06:47:59 pm
content: If you need low current you can use opamp and germanium transistor as voltage follower-set input voltage to what you need.

author: commongrounder
author_rank: Frequent Contributor
timestamp: February 14, 2014, 08:53:12 pm
content: Don't give up on the laptop supply yet. If you are into disassembling it, most have a trimmer inside that allows the voltage to be adjusted at the factory. You can use that to reduce the output to the 16.5 you want. I have done this many times with all kinds of laptop supplies to allow them to be repurposed. They tend to outlast the laptops they came with!Edit: Remember these supplies are switchers that have a hot primary. Don't poke around on the AC side of the supply!

author: codeboy2k
author_rank: Super Contributor
timestamp: February 14, 2014, 11:05:11 pm
content: I even doubt that the 17V laptop supply will be well regulated, but OP is suggesting he needs precisely 16.5 V and his device cannot run on 16.2V , and cannot run with a simple diode dropper... So it seems he needs tight regulation, even if the 17V laptop supply is a little high or a little low.However, I do not think there is an available integrated LDO on the market with Vdropout< 0.5V that can accept up to 17V input (say 20V input to be safe) and has adjustable output that can be set at 16.5V. All the LDOs with < 0.5V dropout are designed for fixed, low voltages like 1.8, 1.2, 0.8V.So, you will need to make your own regulator. I think a TL431 shunt regulator will work just right for you. The picture below is from the TL431 datasheet. Set the series resistor at the input to limit the current to < 100mA you'll be fine. The TL431A is capable to sink up to 100mA. In your case, your device needs just a few hundred microamps, I'd use a limiting resistor to set 5mA current max, say 100 ohms. The TL431A needs about 1mA max for regulation and will shunt the rest away until the output is at the 16.5V that you need. Set R1 = 402K and R2=71.5K from the E96 1% series. Even still, you may want to make the bottom resistor into a 10 turn trimmer to set the voltage exactly to 16.5V that you need.

author: mariush
author_rank: Super Contributor
timestamp: February 14, 2014, 11:11:11 pm
content: I didn't realize those were the requirements, 16.2 or 16.5 from 17v. There are regulators with low dropout.See LM2941 or MIC2941 ... dropout is 40mV at light loads and 350mV at 1A for MIC2941, 0.11v @0.1A and 0.5v @ 1A for LM2941 (typical values).

author: codeboy2k
author_rank: Super Contributor
timestamp: February 14, 2014, 11:21:41 pm
content: Quote from: commongrounder on February 14, 2014, 08:53:12 pmI've done this toowith a 12V/5A LCD monitor adapter I bumped it up to 14.2 and 13.8 volts by changing just the sense resistors and used a FET on the output for current limiting to make a 4A SLA battery charger. Works great !

author: codeboy2k
author_rank: Super Contributor
timestamp: February 14, 2014, 11:27:52 pm
content: Quote from: mariush on February 14, 2014, 11:11:11 pmgrrr.... damn, digikey!I didn't know this part, and before I posted I used digikey's search to find an LDO that can accept the high input voltage and dropout < 100mA and nothing.. I even changed it up a little, tweaked the parameters, ... nothing, except fixed low voltage LDOsYet, digikey sells this part.

author: Unknown
author_rank: Unknown
timestamp: February 14, 2014, 11:55:31 pm
content: Quote from: codeboy2k on February 14, 2014, 11:05:11 pmLT1963A-adj? Up to 1.5A, 1.2V-20V, ~350mV typical / <500mV maximum dropout.But there's probably a much simpler solution to the problem - we just haven't been given all the information yet.

author: DanKalc
author_rank: Newbie
timestamp: February 15, 2014, 12:01:49 am
content: Sorry I didnt said that the device can operate on 16V and lower, my main concern was the current consumption, which wont make the minimal current requirement of LM317 (about 5mA) and probably any linear regulator. Finally i will try with diode and a constant current source output.Thanks for help

author: Unknown
author_rank: Unknown
timestamp: February 15, 2014, 12:34:39 am
content: Quote from: DanKalc on February 15, 2014, 12:01:49 amBut that's the easy bit to solve - that's part of the calculation for the LM317's R1 & R2 values. (Also, go read the specs again, then read the many threads here & elsewhere about them. You want to aim for 10mA, not 5mA...). FWIW, the LT1963A I mentioned only requires 1mA.Or are you also trying to limit overall / quiescent current?

author: codeboy2k
author_rank: Super Contributor
timestamp: February 15, 2014, 02:32:20 am
content: I'm biased (!) but if I were you I'd still go for the TL431 solution I posted.. It's a minimal quiescent current, can operate down to 1mA, and it's only pennies to buy, plus you can often salvage a TL431 from an old switching PSU. Most of the LDOs are going to be a few dollars.Quote from: Tac Eht Xilef on February 14, 2014, 11:55:31 pmAgain, Digikey didn't give me that one either, but they sell it. I think it's because when I go to their page for that part, they actually don't have the dropout voltage parameter listed, so it's probably not searchable for that part.