Question about Low current Voltage Regulator circuit - Page 1
Forum section: electronics / beginners

author: Topband RF
author_rank: Contributor
timestamp: September 06, 2012, 08:37:42 am
content: I am working on a project that may ( I want it to ) require a voltage regulator circuit. The circuit has been designed to operate at 13 volts DC.. i know 13.8 would be ideal.. but that's not happening. Now please keep in mind I'm a super noob @ all this pieces parts stuff so I'm going to try and Draw & explain very simply what I need (want) to do.. With parts.. and I'm going to attempt the proper schematic symbols ( I hope.. 1st one I ever drew ), as it cant hurt to brush up & learn this stuff too.From my understanding of the LM387 data sheet it is well suited for low current applications such as mine, which should run between 75mA +/- 5 mA at the very most & needs to be run at 13 vdc.I do have a few questions however.I know I can also use a 2nd LM317 in a current limiting circuit. That may be nice too to protect the active device (2N5109) from running off into oscillations and prevent over current damage. In this particular circuit if its drawing more than 80mA its oscillating and that's no good. Is it possible to use one LM317 to do both functions.. Limit voltage to 13.00 vdc (in my poor attempt at a circuit) as well as limit current draw to 80mA or less? If so.. what actually happens when the LM387 protects from excess current? Does the device pop.. or does the circuits input voltage need to be essentially cycled? Does the LM387 just simply limit the current to say 80mA as the 2N5109 'attempts' to oscillate but never quite gets there due to the current that its asking being limited?Ideally I would like to have the circuit run as close to 13.00 Vdc as possible with 13.5-15 vdc input and have some current limiting involved to protect the device. The base and collector of the 2N5109 are rated @ 400 mA @ 100% duty and I want nowhere near that from the device. I am trying desperataly to keep costs minimal and incoperate longevity of the device as a whole.Am I on the right track?Thanks in advance! Oh and if my jpg attachment dosent work.. ill try to put it some where online and re-post.

author: Rerouter
author_rank: Super Contributor
timestamp: September 06, 2012, 09:01:51 am
content: you are mixing and matching 2 parts numbers ?as for the circuit your lm317 needs minimum 1.25V higher on its input to its output, equally without voltage below your supplies ground your output can never fall beneath 1.25Vand as for the current limit, as you have a nice and low current, you implement something similar to daves usupply, where you have a current sense resistor on the input a op amp measuring the differential across it, and it driving a npn transistor that pulls down the set pin on your 317, give me an hour and i'll replay back with a more visual representation of what i am getting at,

author: Topband RF
author_rank: Contributor
timestamp: September 06, 2012, 09:17:57 am
content: Quote from: Rerouter on September 06, 2012, 09:01:51 amI may be messing up Part #'s.. but I dont think so, with the exception of any voltage control devices I use, the 2N5109 is the only 'active' component in my circuit. I may have not worded what i was meaning properly And if there needs to be a 1.25v higher input that output.. that LM387 wont work. If I try to make a few to sell, myself as well as most end users will be using 13.8 vdc supplies and reluctant to buy/make/alter their current 13.8 regulated voltage sources regardless of how cheap or easily they can do it. I had to edit this reply.. its way to late for me to be trying and make any logical sense.

author: Rerouter
author_rank: Super Contributor
timestamp: September 06, 2012, 09:56:02 am
content: ah ok now i better understand you, though the lm387 is very unsuited, as it cannot reach higher than vcc-2V, and for an npn you really need it to be 0.7V above the input, the 0.5 dropout really calls for a rail to rail outputs op amp and a pnp active pass element, (you can get transistors for under 100mA that have C-E jucntions lower than 0.3V,what i would reccomend is an op amp with inputs that include the negative rail, and using a resistance divider to make sure your feedback wire never exceeds 2V below its supply, say a OPA2170, while a tad expensive rail to rail higher than 16V isnt the easiest to find cheapon the current limiting, a decent pass transistor affords you 0.2V for the current sense maximum, so lets assume 0.1 and give you some headroom, a 1 ohm resistor leading the pass element will give you a signal of 80mV at 80mA, more than enough to escape the input offset of the op amp, with this, you have the other op amp in your package measure across it and with some gain trigger a transistor on its output which pulls down your first op amps non inverting input, like thishttp://www.oocities.org/overmindxx/pictures/Projects/CurrentLimit2.gifwhen it approaches 80mA (this is known as a differential configuration)you may want to bias your transistor so it works more like an on off switch, and there is also documentation online to help you with that, i hope this helpsi have concenrs about my own choice in op amp not having a full rail to rail input, but as your only aiming for a fixed limit, (or so it seems) you can use diodes to bring the voltage down if you wish,

author: Topband RF
author_rank: Contributor
timestamp: September 06, 2012, 10:31:17 am
content: Quote from: Rerouter on September 06, 2012, 09:56:02 amIm not sure I wanna fool with the original design of the circuit as far as Transistor bias, its a known circuit to perform well for its use. (RF receive pre-amplifier with max gain in the 1.7-2.0 Mhz range) Although if I had more test equipment scope, spectrum analyzer ect.. I may consider tinkering with it. I have a feeling I ran across some one using a similar circuit with a pair of 2N5109's in push-pull. I am concerned with adding devices to the circuit that will adversely affect the following specs - OIP3 and Noise floor.. those are of the utmost importance and in order. In the known circuit OIP3 is at or just above +41dBm any less in my particular situation is intolerable. Gain of the circuit is in the 19-21 db area with a NF of between 2-3 dB ish.I think what I am encountering is im trying to make it 100% perfect and better than the original before even building one for myself & not even having a fully functioning understanding of even basic electronics im in this weird zone between practicality and cost and ideas. As it stands I can build the circuit fairly cheap, however by my nature.. I want to try and improve. Add all the extras and the cost triples if not quadruples very easily. I do know a hand full of people who have built one.. and are happy with results. Most have just built it dead bug style and that works well for them or used a dremel to etch pads for components on 2x sided board. I recognizing I'm rambling.. I should probably sleep.. its 6:30 AM thanks for the ideas on the power rail.

author: Rerouter
author_rank: Super Contributor
timestamp: September 06, 2012, 10:55:55 am
content: now there is a key missing pointyou only mentioned a power supply, while i noticed you where using high frequency parts, i was uncertain if it was only due to your beginner understanding or some other now arisen reasoning,unless i am once again misunderstanding this, you are trying to make a HF preamplifier and not a voltage regulatorif so this is well out of my area of understnading but some of my points still hold,

author: Topband RF
author_rank: Contributor
timestamp: September 06, 2012, 08:15:05 pm
content: Quote from: Rerouter on September 06, 2012, 10:55:55 amYes 100% correct..while the circuit has been replicated many many times by many people I would like the ability to input 13.8v to a voltage regulation circuit so that the remainder of the circuit is given 12.8-13.0 vdc. I would like to implement this on the PCB and not have some other external circuit limiting voltage. I guess the quick way of saying it is I want to reduce the input voltage when fed with 13.8vdc to 12.8-13.0 vdc.As far as the current limiting that I would like to implement also..yes when i build it I can read that current drawn from the circuit.. however if the 2N5109 starts pulling more than 80mA that would indicate a problem as the circuit is designed. I would like to be able to limit current to a maximum of 80mA as well. But one step at a time