Small Power Regulated Power Supply

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Hi All,
I am in the process of building this power supply,which is published in http://orca.st.usm.edu/~jmneal/dual_psu

I am building this as my "Work Bench Power supply" to test small projects..mainly guitar effects etc.

My question is;

1.I've used 0.10uF "RIFA" PHE450 caps as in C1b & C2b.. (please refer to attachment)
But these are quoted as"for use in high frequency applications with high current stress,such as in deflection & TV protection circuits as well as in SMPS & electronic ballasts".

So, could someone please tell me if it's ok to use in C1b/C2b?

Secondly would it give additional out put protection if I include additional 1N4003 diodes between
Vout & Adj terminals of each regulators,ie.LM317/LM377?

Thanks.
 

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Hi,
the decoupling caps on the input are there to absorb/attenuate high frequency variations brought about by interference coming in from the mains or due to sudden changes in the current through the regulator.
High frequency caps are ideal for this duty. I think the reg manufacturer alludes to this if not specifically demanding it.

The diodes across R1 and R3 are there to discharge the 10uF without damaging the regulator. If the manufacturer recommends it then fit them.

Be careful with the pot on the voltage adjuster. The current through the track must not exceed the maximum current defined as sqrt{Pot Power/Pot resistance} eg. 400mW and 5k <=8.9mA and safer to use half power current i.e. 8.9/1.4 <~6mA

A nice to have addition would be a current limiter. Even better would be an adjustable current limiter. Just a 317 set up as a CCS on the input to the regulator. Switched 20mA, 50mA and 100mA would be better than unlimited.

The regulator dissipation increases markedly as the output voltage is turned down. It would be very nice to have a multi tapped secondary to help with reducing the supply voltage when working at lower output voltages. Could you cope with rewinding the secondary to give 0-6-12-18-24 & 0-6-12-18-24Vac?

Another nice to have is dual current and voltage meters on the outputs.
 
AndrewT,
Thank you indeed for the invaluable info you have given here.

so, do you recommend/approve using these caps? Although these are rated at 600vDC & are "bulky"...they never the less fit well on my board,avoiding long "links" to the common rail

As for using diodes across the adj/out put,I actually read in the article published on http://www.acoustica.org.uk/t/3pin_reg_notes1.html.

This PSU is mainly intended for testing/running small guitar effects & such,hence will be fixed at +/- 15v only.

As for your excellent suggestions on current limiting ideas,i will incorporate them (gradually) at a later juncture,as all this has been a very steep learning curve for me, in a very positive direction may i add.

thanks again
Cheers!
 
regulator

Hey, that is a nice regulator circuit, you have a protection diode and cap on the adj which will reduce noise.

Additional diodes across the adj resistor will protect the regulator in the event either the in or out is shorted. Did you look at page 9 of the LM317 Data Sheet? This describes the function of the diodes, says you need on if you have > 10uF of capacitance.

These regulators have voltage drop so you may not get the performance you expect without switching to the other transformer.
 
Thanks Guys,
I did overlook the importance of this technical detail when i bought this trafo...well i'm stuck with it for now,so will try this first & see how it performs.
It is rated @ 12VA 12x2 sec.
Thanks for pointing this out,'cause "not many people know that",,...quoting Michael Caine here.

I'll be testing/running mostly small anologue effects mic/piezo preamps,stomp boxes.So I'd reckon these will not consume more than 200-300 mA ..but i do see the point you're making.Couldn't agree more!

I thought i'd include an EMI filters in the PSU, the reason being I'm in the process of designing a Piezo disc/mic blender preamp for acoustic guitar,& all the piezo data sheets strongly recommend EMI shielding both at the component level as well as EMI & RF shielding externally.ie; casing,leads etc.

I found this EMI filter with the following specs;

Make - MURATA
Part No; - MDSS6NZ82A103
Type -11
Max.voltage -100v dc
Max i - 6A
Capacitance -0.01uF (this is the largest value available here)

Do you guys think this would help in reducing EMI or would it just be an overkill?

I presume the EMI filter sits next to the filter cap/rail?

I've also salvaged a few ferrite beads from an old board.(small cylinderical beads to go on the legs of these filters) Can I just use these arbitrarily or do I need the exact values?

I would be grateful for any good suggestions or help.

Thanks.
 

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a 12VA 12+12Vac transformer is a 500mAac output.
The maximum continuous DC current that this transformer can supply when feeding a capacitor input filter is only 250mA.

To reduce the operating temperature I normally suggest that the continuous DC current be reduced to approximately 125mA.
A 12Vac transformer will allow 12Vdc regulated.

The overall result is that your 12VA 12+12Vac transformer is capable of giving you a regulated +-12Vdc upto 130mA continuous or 250mA intermittent.

Go and read Decibel Dungeon and/or Elliot Sound Products on what's important in designing/building a power supply.
 
Yes,I stand corrected!It is indeed rated @500mA.(12VA /2x12v)

I can of course easily run this PSU at a reduced voltage, around +/-12-13v as suggested by another forum member(mjf).I don't really need to run any of my current projects at +/-15v at all!
besides, the max. current requirement is not more than 50-60mA at the most at the moment.then, I can of course change to a bigger "trafo" at a later juncture.

I would also need a bigger "casing", as the present one would be too small for a larger trafo!
 
teleman said:
lgreen, Thanks!
With regard to the 10uF caps in series with the adj.resistor,one of the articles I've mentioned earlier even suggests a 100uF cap instead for better noise specs!

I'm not too sure ..so i'll read it again before confirming.

Bigger caps aren't necessarily better a reducing noise. The best caps for filtering noise are the caps with the lowest ESR. Some of the larger caps can sacrifice ESR for capacitance. To reduce noise usually a set of small Tantalum Caps is added in parallel to the electrolytic Caps.

*ESR: Equilavent Series Resistance
 
Here are two passages from the same article,which I'd like to quote directly from;

ON USING A MUCH HIGHER VALUE C2 (100uF)

1."The 317's reference voltage is based on holding an accurate 1.25v across R1.This is multiplied by (R2/R1) to get an output voltage of 1.25 (1+(R2/R1)) In effect, R2 adds POSITIVE FEEDBACK from the sum of R1 current and the internal voltage reference current! What C2 does is bypass (reduce) the positive feedback:effectively reducing 'noise gain' to the internal amplifier.

At DC, the voltage gain remains R2/R1, but to AC signals the voltage gain tends towards 0/R1 since C2 'looks like' a very small impedance at higher frequencies! THIS LOWERED NOISE GAIN is where the much better ripple rejection shown in the LM317 (and similar parts) datasheet comes from.So a BIG HINT: MAKE Z (the impedance) of C2 much smaller than R2 at twice your AC mains frequency to suppress ripple better!

2.WHY LOW-ESR CAPACITORS CAN BE A BAD THING!

An error amplifier tries to force the output up or down by comparison with an internal referance. As the error amplifier's gain falls off with increasing frequeny,so too does the regulator's accuracy and output impedance!

This rise in output impedance with frequency then just looks like an inductor-and that is how it behaves:as a synthesised inductor.

At the output of a regulator,then using a cap with too low an ESR risks forming a "peak" in the overall response, because it can resonate with this inductive output impedance!

Ladies & Gentlemen, the floor is YOUR'S!
 
or go the whole hog and fit an ordinary 220uF to the adjust pin of the 317.
Even better is to fit a cap//Zener to define a voltage at the adjust pin. The 317 sends an almost perfect constant current through the Zener and thus holds a very stable voltage at the adjust pin. Now that internal amplifier stands a good chance of giving a nice stable output voltage.
 
Andrew

What jbau's results (and others I've seen - I can provide refs if necessary) seem to show is that the conventional wisdom of putting a big cap on the adj pin is wrong. There best phase response is obtained with something around 60-120nF, allthough this will significantly reduce ripple rejection, something jbau hasn't considered so far.

Another thing - since the o/p Z of an LM317 depends heavily on current draw, isn't all this fine tuning a bit pointless anyway?

Any thoughts on this?
 
Gopher said:
jbau's results
I have been following that thread since the beginning. I have said little, deliberately, since his philosophy is at odds with what I have come to learn. But, I listen and watch intently to give him (and others) a chance to re-educate me.
I think I am in for a long wait.

Anyone who puts in that much experimentation and time and is prepared to post the bad results along with the good results along with the unexpected/unexplainable results is a brave soul who deserves to be listened to.
 
This seems a minefield. So many variables.

Let's see - the LM317 output Z varies with both frequency and output current (I've no info on whether the impedance curves even look the same shape for different Iout), also in/out Vdiff if jbau is correct, o/p capacitor size and ESR and Vadj capacitor size and ESR. Then there's the effect of the impedances of local Vsupply traces/decoupling caps to consider if one reg supplies several different chips. The latter of course will be different from one circuit to the next.

How on earth can one optimisation scheme fit all cases?

Oh, and we haven't even considered snubbers on the o/p of the reg yet!!

Oi vey.
 
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