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Old 16th July 2009, 09:22 AM   #1
teleman is offline teleman  Norway
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Default Small Power Regulated Power Supply

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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Old 16th July 2009, 09:29 AM   #2
AndrewT is offline AndrewT  Scotland
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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.
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Old 16th July 2009, 10:46 AM   #3
teleman is offline teleman  Norway
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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!
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Old 16th July 2009, 11:21 AM   #4
AndrewT is offline AndrewT  Scotland
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for a regulated +-15Vdc output, you need a 0-15 + 0-15Vac transformer.
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Old 16th July 2009, 01:50 PM   #5
teleman is offline teleman  Norway
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The one i've got is 0-12/0-12 Vac @1amp...which would be roughly 16.92v dc .Do you think this would suffice?
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Old 16th July 2009, 01:58 PM   #6
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You really need a little more voltage from your transformer. 15-0-15 volts would work well.
You might get by with your existing transformer if you don't draw more than about 500 ma AND if the transformer voltage doesn't sag under the 500 ma load.
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Old 16th July 2009, 02:45 PM   #7
lgreen is offline lgreen  United States
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Default 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.
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Old 16th July 2009, 03:04 PM   #8
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Quote:
Originally posted by teleman
The one i've got is 0-12/0-12 Vac @1amp...which would be roughly 16.92v dc .Do you think this would suffice?
A rule of thumb is to have the same AC voltage as the desired stabilised DC voltage. You have measure the DC voltage without a load?
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Old 16th July 2009, 03:14 PM   #9
teleman is offline teleman  Norway
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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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Old 16th July 2009, 04:51 PM   #10
AndrewT is offline AndrewT  Scotland
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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.
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