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Old 6th February 2003, 02:53 AM   #11
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Quote:
Originally posted by sam9
A- If you find you come up short on current at voltages, you can use parrallel the regulators (2 or each). I tried this once when I needed more current that the LM317 could provide. Therewas probably sub-optimal about my approach, but it worked. They ran cooler, too.
(especially if current limited).
this is a point i really want to achieve (getting more current or even to make the regulators provides thier max limits cooler)
but itsnt that obvious how to parallel em! , can u explain more?




Quote:
Originally posted by Sud
* the lower the output voltage, the more power the regulator is dissipating at a given output current, as Christer pointed out
* the higher the output current, the lower the minimum voltage across the input caps.[/B]

those are a must to know notes about the regulators of course
but is there anyway that we can make the regulators less to suffer in the power dissipation issue?? "sure by using agood cooling methode (heatsinks and fans which i already took into consid.)" but any other ways?
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Old 6th February 2003, 04:10 AM   #12
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Quote:
Originally posted by SkinnyBoy
I wanted to make a split rail adjustable powersupply, except the only transformers I have are a 32volt 6amp one, and a 24volt 10 amp one.... and the first will exceed the max input of adjustable regulators..... I only just thought of using 1 transformer per rail, but always limiting the 32volt one to the same as the 24volt one.... But.... That would be one PRETTY big powersupply.... and if I wanted to get full power into say a +-28 volts, thats a PRETTY BIG drop for the regulator on the 32volt transformer, and thats ALOT of heatsinking needed.. lol
Choke-input?

Yeah, throw on some 2N3055s for some extra current, pull 5A through that motha.

- FWB the 24V, should get around 35VDC, derate to around 6-7A after rectification... FWB + choke input on the 32V tranny, current needn't be derated as much, but DC voltage is about .9 times the AC value, so they'll end up almost the same voltage and current.

Tim
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Old 6th February 2003, 05:19 AM   #13
Bob03 is offline Bob03  Canada
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Default Bench Top Power Supply

Check this one:

http://www.uoguelph.ca/~antoon/circ/ps3010/ps3010a.html
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Old 6th February 2003, 06:23 AM   #14
sam9 is offline sam9  United States
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Default How to wire regs parallel

When I did it it was simple: Take two LM317s, Connect both input leads and then both output leads to the same place as you would if it were only one reg. With reference to your schematic, each reg has its own R1. Also each has it's own R3 to ground. This is a little tricky in your case you need to find dual ganged pots but at least they don't need to be the expensive variety. (I was using a dual switch with specific resistor values.)

Although it may look tempting, I don't think you should use a common single pot - if there were to be any imbalance between regs, one of them might see a reverse voltage on the adjustment pin - probably a bad thing. (PS: watch the pinouts - in and out are reversed between the pos and neg versions. Getting one bassakwards can be exciting
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Old 6th February 2003, 07:02 AM   #15
Sud is offline Sud
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Default wiring regulators in parallel

I suggest you place a low resistance at the output of each regulator and connect the ends of the resistors together. A resistance of 0.1 Ohms would be adequate.

Also, upon second look, I suggest placing a 1uF to 10uF tantalum capacitor in parallel with R3 and R4 - it will help improve line requlation.
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Old 6th February 2003, 11:30 AM   #16
Bobken is offline Bobken  United Kingdom
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Default Re: wiring regulators in parallel

Quote:
Originally posted by Sud
I suggest you place a low resistance at the output of each regulator and connect the ends of the resistors together. A resistance of 0.1 Ohms would be adequate.

Also, upon second look, I suggest placing a 1uF to 10uF tantalum capacitor in parallel with R3 and R4 - it will help improve line requlation.
Hi,

With respect, the usual reason for bypassing the resistors from the adjust pins to ground (R3 & R4) with a suitable capacitor, is to increase *ripple rejection*, and I have not noticed any improvement as far as the suggested *line regulation* goes in cases such as this.

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Old 6th February 2003, 12:10 PM   #17
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Default Higher voltage regulators

the TL78C from Texas Instruments is a little more expensive than the LM317, but will stand off 125 volts -- you will have to outboard a PASS transistor as the '783 will only take 700ma.

The easiest way to regulate a higher voltage is to "roll your own". Here's a high voltage <b>negative</b> regulator you can control with a potentiometer and an LM317 used as an adjustable reference. The same design can be adapted to use of a PNP transistor for a positive regulator. It's borrowed, loosely, from Horowitz and Hill and doesn't show some of the compensation, etc. which should be put in. The voltage sense pot actually characterizes the gain of the circuit. Disadvantage of an LM317 vs a DAC is that you can't get down to "zero" volts unless you bias the adjust pin below the reference voltage -- but it's simple and cheap:
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Old 6th February 2003, 12:25 PM   #18
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Default Re: wiring regulators in parallel

Quote:
Originally posted by sam9
When I did it it was simple: Take two LM317s, Connect both input leads and then both output leads to the same place as you would if it were only one reg. With reference to your schematic, each reg has its own R1. Also each has it's own R3 to ground. This is a little tricky in your case you need to find dual ganged pots but at least they don't need to be the expensive variety. (I was using a dual switch with specific resistor values.)

Although it may look tempting, I don't think you should use a common single pot - if there were to be any imbalance between regs, one of them might see a reverse voltage on the adjustment pin - probably a bad thing. (PS: watch the pinouts - in and out are reversed between the pos and neg versions. Getting one bassakwards can be exciting
u mean that every regulator (total of 4) should has its own
pot. ,so how can i adjust the pots. to provide a spec. volt. per rail
cant i use only one so it gonna be more simple and logic??



Quote:
Originally posted by Sud
I suggest you place a low resistance at the output of each regulator and connect the ends of the resistors together. A resistance of 0.1 Ohms would be adequate.

Also, upon second look, I suggest placing a 1uF to 10uF tantalum capacitor in parallel with R3 and R4 - it will help improve line requlation.
ive seen a schematic once that places a 33ohm thermal resistor
before the regulator, dont know what its used for , but here u
said to place a v.small one (0.1ohm) at the output...!
can u explain why??


ps. ill get all of ur valuable comments & corrections and then
redraw the schematic again and re-post it of course
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Old 6th February 2003, 12:30 PM   #19
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I agree with jackinnij, if you need to go near or above the limits
of what a 317 can handle, don't use it, but do you own regulator.

You might find the following handbook from On Semi helpful,
even though it is about IC regulators, it will teach you some
of the basic theory of voltage regulators.
http://www.onsemi.com/pub/Collateral/HB206-D.PDF
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Old 6th February 2003, 02:16 PM   #20
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Default the resistor before the regulator

is current sense for the regulator and pass transistor combo -- as the current drawn increases (and the voltage drop across the transistor increases) it reduces the amount of work which the regulator has to perform and shifts this function to the transistor.

if you tie two LM317's together as described you can get some nice oscillations -- in fact, getting an LM317 to oscillate isn't difficult at all. At any rate, it's using a more expnesive part to do a less expensive job.
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