Author Topic: My first mains AC/DC supply... help?  (Read 2333 times)

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Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #25 on: March 07, 2018, 08:23:15 am »
Transformer is one of these:
https://docs-emea.rs-online.com/webdocs/0f3c/0900766b80f3c2a1.pdf

The 2x18V 100mA (each) variant.

No it's not limited and it does mention powering a DC load off it will not cater for the full 100mA DC.  The then tell you to phone their customer services to find out LOL.  I assume it can be calculated.  Besides I don't think I will load it anywhere near to 100mA, I don't intend to power much off it, maybe a headphone amp to see if it's smoother than a switch mode one.

Remember this is a test dummy to play with while I wait on the RCore from China.

EDIT:  For a basic "play" rectifier, can I use basic 1N400x diodes?  1N4002 or higher reverse voltage rating?
« Last Edit: March 07, 2018, 08:26:59 am by paulca »
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Offline Jwillis

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Re: My first mains AC/DC supply... help?
« Reply #26 on: March 07, 2018, 08:39:10 am »
Not sure if you looked but theirs a few videos on the FY3224S Sig Gen.Granted they may not be the same supply but it might give you some insight on the subject.



also one on USB isolation

 
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Online Monkeh

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Re: My first mains AC/DC supply... help?
« Reply #27 on: March 07, 2018, 08:46:13 am »
EDIT:  For a basic "play" rectifier, can I use basic 1N400x diodes?  1N4002 or higher reverse voltage rating?

There's exactly nothing wrong with a bridge rectifier made out of 1N400x diodes. 1N4001s are fine for this voltage.
 

Offline rsjsouza

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Re: My first mains AC/DC supply... help?
« Reply #28 on: March 07, 2018, 09:08:34 am »
It's supposed to be a 2x18V transformer and while I understand it will read high without a load and even putting it through a bridge rectifier and a cap will drop it considerably before a regulator can bring it down to 15V...

No, putting it through a bridge rectifier and cap will bring it up to the peak voltage minus diode drops. Your regulator best be comfy with seeing up to 40V unloaded.

Measured it was 7Vpp at 10x, so that's 70Vpp.  Through a bridge rectifier will it lose one or two diode drops?  Anyway, worst case is it loses one, so it's still 34.3V peak.  Positive regulator 7815 is good for 35V, negative 7915 is good for -40V.  I know it's close, but it should be fine.  The regulators themselves will put 3.5mA + and 8mA - load on it which will bring the voltage down I expect to something well under the regs ratings.

I assume it also means though that the secondary side is worth being careful around too.
It seems your transformer has a center tape, which means you will be using FULL BRIDGE RECTIFIERS with 4 diodes - in this case, it is two diode drops, which may be ~1.2V (if Si) or ~0.4V (if Schottky).

Regarding the input voltages to the regulators, be careful if the mains voltage goes too much above what you measured, as the regulators may operate above their specs. Sometimes simply adding some bleeder resistors on the tank capacitors before the regulators can create enough load to reduce the transformer's output voltage.
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Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #29 on: March 08, 2018, 06:17:04 am »
So, it functions.  A first prototype anyway.

Luckily I spotted that the 1000uF caps I bought were only 25V before pushing 34.4V through them.

Managed to find some 50V ones, but they were only 100uF.  Anyway they (2 in parallel) did fine to run an LED with only 50mVpp ripple.  The 25V ones will be fine on a 9V transformer to run the 5V rail on the FY6600 supply.

The bare transformer tested:


With bridge rectifier, caps and 15V regulator + test dummy LED with 1k resistor.
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Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #30 on: March 08, 2018, 07:01:53 am »
So, rough and ready schematic for the FY6600 PSU.

The question is, am I on the right track?  I'm fairly sure it can be simplified a bit.  I am showing two transformers when it will really be one that provides 4 secondaries.

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Offline rsjsouza

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Re: My first mains AC/DC supply... help?
« Reply #31 on: March 08, 2018, 07:12:49 am »
Your schematics look alright. Just check the datasheet of your regulators to see if they require specific capacitors in the input/output for stability.

For example, you mention LM7805, LM7812 and LM7912:
http://www.ti.com/lit/ds/symlink/lm7800.pdf
http://www.ti.com/lit/ds/symlink/lm79.pdf

If you want to minimize the ripple with what you have, simply put two 1000uF/25V capacitors in series and you will have a 500uF/50V capacitor.

http://farside.ph.utexas.edu/teaching/302l/lectures/node46.html
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Oh, the "whys" of the datasheets... The information is there not to be an axiomatic truth, but instead each speck of data must be slowly inhaled while carefully performing a deep search inside oneself to find the true metaphysical sense...
 

Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #32 on: March 08, 2018, 07:22:41 am »
If you want to minimize the ripple with what you have, simply put two 1000uF/25V capacitors in series and you will have a 500uF/50V capacitor.

http://farside.ph.utexas.edu/teaching/302l/lectures/node46.html

I googled that and found while it was possible I needed to put meg resistors in parallel to ensure the caps don't charge to different voltages, but maybe they are being pedantic. 

I decided to try it with just the 100uF 50V caps in parallel, giving 200uF.  I might try the 1000uF 25s in series with 1 meg resistors across each, just takes up more breadboard room.

I'll order 63V 1000uFs for the final version. (Edit: I'll calculate what capacitance I need for a guesstimated load and aim a little higher) 

Toying with making a strip board version, but laying out a PCB anyway if my first 2 boards actually arrive without too many customs charges that is. 

The FY6600 hasn't arrived yet though, it should be here soon.
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Online Ian.M

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Re: My first mains AC/DC supply... help?
« Reply #33 on: March 08, 2018, 07:27:26 am »
The +/-12V supply only needs one bridge rectifier (although it should have a minimum 70V rating).  Connect the 15V RMS windings in series, use the resulting center tap as 0V, and connect the bridge AC terminals to the ends of the two windings.  The bridge + and - terminals  will provide the raw DC + and - rails.

Connect a 10nF capacitor across each diode in each bridge rectifier as close to the bridge rectifier as possible to suppress impulse noise due to rapid current cut-off during reverse recovery.

Connect 100nF ceramics as close to each regulator as possible in parallel with their input and output decoupling capacitors.
 

Offline rsjsouza

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Re: My first mains AC/DC supply... help?
« Reply #34 on: March 08, 2018, 07:28:10 am »
I only see a huge benefit in using equalizing 1M $\Omega\$ resistors in more extreme cases - i.e., not with the voltage and current levels you are using, but others may disagree.

At any rate, the series association was just a suggestion for you to experiment and play around with the circuit.
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Oh, the "whys" of the datasheets... The information is there not to be an axiomatic truth, but instead each speck of data must be slowly inhaled while carefully performing a deep search inside oneself to find the true metaphysical sense...
 

Offline rsjsouza

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Re: My first mains AC/DC supply... help?
« Reply #35 on: March 08, 2018, 07:30:53 am »
The +/-12V supply only needs one bridge rectifier (although it should have a minimum 70V rating). 
Gosh, how did I miss that? You are right (I need more coffee).

(edit) Something like:
« Last Edit: March 08, 2018, 07:32:45 am by rsjsouza »
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Oh, the "whys" of the datasheets... The information is there not to be an axiomatic truth, but instead each speck of data must be slowly inhaled while carefully performing a deep search inside oneself to find the true metaphysical sense...
 

Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #36 on: March 08, 2018, 07:41:00 am »
I did think about that, but wasn't sure how to get the common 0V rail or how the rectifier would be wired. 

When you say "the centre tap", if they are two separate secondaries, I assume I just take the point I put them in series, the common point?
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Offline rsjsouza

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Re: My first mains AC/DC supply... help?
« Reply #37 on: March 08, 2018, 08:03:18 am »
I did think about that, but wasn't sure how to get the common 0V rail or how the rectifier would be wired. 

When you say "the centre tap", if they are two separate secondaries, I assume I just take the point I put them in series, the common point?
You need to find out the "polarity" of the windings, or you will end up with zero volts.

To find the polarity of the windings you can use your oscilloscope: tie two wires together and measure the output of the two disconnected wires - if it is close to zero volts, you need to disconnect and tie the other wire of one of the windings.

You could also check with an analog multimeter or a digital one with a good bargraph.
http://www.eevblog.com/forum/beginners/analog-meter-vs-a-dmm/msg210926/#msg210926
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Oh, the "whys" of the datasheets... The information is there not to be an axiomatic truth, but instead each speck of data must be slowly inhaled while carefully performing a deep search inside oneself to find the true metaphysical sense...
 

Online Ian.M

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Re: My first mains AC/DC supply... help?
« Reply #38 on: March 08, 2018, 08:10:38 am »
Yes. the junction of the two secondaries is the center tap.

As RJSouza just posted, be careful about phasing - if in doubt connect one end each of a pair of secondaries and check the voltage across the remaining ends before wiring them up any further - its especially critical for your paralleled secondaries for the 5V supply.   For the +/-12V supply, it wont instantly destroy the transformer, but getting it wrong doubles the ripple and stresses the transformer more for the same current.  Any multimeter on AC V should give a clear indication, I wouldn't bother scoping it.   Don't expect *exactly* zero volts between the ends you are going to parallel - if its down around 1% of the nominal secondary voltage you shouldn't have any issues.
 

Offline cowasaki

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Re: My first mains AC/DC supply... help?
« Reply #39 on: March 08, 2018, 10:10:56 am »
The 7912 I used required a tiny load in order to regulate the voltage. I created that load by using the -12v rail for the power LED. At least that way you can test it without connecting it.
 

Offline Jwillis

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Re: My first mains AC/DC supply... help?
« Reply #40 on: March 08, 2018, 02:57:52 pm »
Hate to say this but those transformers are a bit  to high.You have to calculate the voltage after the filter caps.You do this by taking say your 15 volt transformer and multiplying 1.4 that will give you the voltage that will be going to the regulator .You'll end up with around 20 volts off your 12 volt transformer and 12.6 of the 9 volt transformer.Remember the more voltage you put to the regulator the hotter it will get .So typically you want to shoot for about 4 -5 volts above the required voltage.So a 12 volt transformer on the 12 volt side will get you around 16-17 volt after rectifier and filter cap.And I would use a 6 volt on the 5 volt side which would be around 8 volts after rectification and filtering.You also don't want to go to high on the filter cap either .Because on startup to high uF can put to much load on the transformer shortening it's life and it will blow fuses.Capacitor at zero charge acts like a short circuit before they begin to charge.
 

Online Ian.M

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Re: My first mains AC/DC supply... help?
« Reply #41 on: March 08, 2018, 04:27:30 pm »
Its *FAR* more complex than that.  You *MUST* allow for the ripple on the unregulated rail as the regulator input needs tp stay above its dropout voltage at the maximum load current, to avoid loss of regulation and excessive output ripple. 

The reservoir capacitor charges up to the peak of the full-wave rectified AC (less diode losses) and then has to supply the load current until the next full-wave peak, 10ms later.   The discharge slope can be calculated from Q=CV.  You can approximate the ripple by Vripple=I/(2fC), (where f is the mains frequency) though for a more accurate solution you have to account for the discharge phase ending earlier than the peak and waveform distortion due to the transformer source impedance and the diodes dynamic resistance, which makes a pure analytic solution rather awkward.  Its easiest to simply simulate it to check out the effect of load current and capacitance value.

A LTspice sim (attached) shows that for 9V RMS in (loaded), 1000uF of reservoir capacitance, and 1A silicon diodes, you wont even be able to draw 600mA without the ripple trough dropping below 7V.   You also have to worry about the ripple current rating of the reservoir capacitor - the sim shows 1A RMS ripple current, which is a *LOT* for a 1000uF capacitor, so it will cook itself to death.   More capacitance usually gets you a higher ripple current rating, (especially if you use several smaller capacitors in parallel to increase their surface area and thus their heat dissipation capability), but as JWillis has pointed out, that can get you in trouble with the startup surge current so you'll probably have to uprate the diodes and use a slow-blow fuse.

You also have to consider the effects of mains voltage variation - can your design maintain the ripple trough voltage above the regulator dropout voltage with the mains voltage at the permitted lower limit, and will the peak input voltage, with no load and the mains voltage at its permitted upper limit, exceed the regulator's maximum input voltage?
(see http://www.twothirtyvolts.org.uk/pdfs/site-info/Explanation_230Volts.pdf )



 

Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #42 on: March 08, 2018, 07:07:43 pm »
Thanks.  So I have some things to consider.

On the load, I believe the expected load is lower than 1A.  I haven't seen any true characteristics of the FY6600, but in bench tests it was only something like 250mA on the 12V rails and not much more than that on the 5V rail.

As long as I don't connect a low impedance device to the BNCs....

The video posted, I believe took those measured loads and calculated the cap values based on that, but they had a much lower 5V transformer rated at 6V, so they had to use a 400mV LDO regulator.  They ended up with (I believe from memory) 2200uF caps. 

As my xformer will be putting out higher voltage I shouldn't need the caps to be that large.  Though I might swap the 1A fast blow for a 0.5A slow blow.

So plenty of things to work out.  I have basic TO-220 heatsinks and thermal grease, but I'll need to work out the actual heat dissipation of the regulators due to the high input voltage.  I have the options of buying (or hacking out of something) a much heavier heatsink and/or fitting a 3cm 5V fan to the back of the case.  Another option is making a heatisnk, I have a 3" by 2" sheet of what looks like thick aluminium which was the heat sink from a power bank battery bay, I could bend it in some corigated fashion and mount all 3 regulators to it.

Note, this device will not be an always on type thing.  It will come out once in a while, run for an hour and go back on the shelf.  So it will be full supervised and I'm hoping the worst it can do is fry the FY6600, blow it's fuses, let out some smoke and pop my RCD.  Oh... of course it could potentially dump 120V/240V onto the BNC and fry me and anything connected, but that seems unlikely.  Of course I'd like to do what I can to avoid all of the above.
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Online Ian.M

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Re: My first mains AC/DC supply... help?
« Reply #43 on: March 08, 2018, 08:55:35 pm »
If in the LTspice sim, you put a voltage source equal to the output voltage in series with the current source  that is the load (below it is best), the load's dissipation will be almost exactly equal to that of a LM7xx regulator passing the same current (neglecting the regulator's quiescent current).  You can then  alt-click the current source to plot its dissipation, and ctrl-click the resulting trace legend to calculate the average power.   

Of course, that only helps if you know the FY6600's current consumption on each rail - but you can measure that with the existing PSU, and your new transformer's output voltage when loaded with a RMS current of 1.6 times the DC current, or with the actual DC current after the bridge + reservoir capacitor.

Taking the example of your nominally 18V RMS @100mA transformer, its nominally 230V in so at 240V the output voltage will already be 18.8V, nearly 5% high.   You measured its no-load voltage as 25V.    At 80mA RMS, for 50mA DC out, I estimate the voltage (by interpolation) will be 20V RMS.   Plugging that into the sim in place of the 9V the source Vac was set to, changing the cap to 100uF, decreasing Iload to 50mA and adding a 12V source so the regulator dissipation can be plotted, I get 645mW for the regulator dissipation.  That does *NOT* include the power dissipation due the the quiescent current (from In to Gnd), which is another 126mW, so the total  LM7812 dissipation at 50mA load current with 20V RMS in will be 0.77W.  Revised sim for attached.

Add up the expected regulator dissipations for all the rails, and the result should be close enough to choose a heatsink size.     I strongly suspect you'll need an external heatsink, or fan cooled as I expect you'll have a worst-case dissipation of not much under 10W

Don't put any fan on the 5V rail as it makes the regulator dissipation problem worse.   The full wave rectified 9V rail should be in the right ballpark to run a 12V fan a bit slower than normal.
 

Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #44 on: March 09, 2018, 07:03:35 pm »
Thanks Ian. 

Remember the test dummy xformer is 18V regulated to 15V, the FY6600 xformer is only 15V regulated to 12V.  Thought that might result in similar heat disapation.

So if an 18V xformer gives me 25Vrms / 35Vpk the 15V might be around 20.5Vrms (based on a extrapolated 29Vpk).  That's unloaded.

Your spice sim won't run for me, simulate and most other things are greyed out, but my LTSpice currently has issues in Linux, might work natively in windows if I reboot.

The xformer arrived, still no FY6600.  So I could end up with a rather nice audio PSU and no sig gen :)  Nor has the +-12V PSU kit board I ordered.

I haven't powered it up, maybe over the weekend, it looks fairly well done.  The primary and secondard windings are around what looks like high temp plastic or ceramic bobbins which wrap each side of the core.  The chassis protrudes up the middle, but not fully.  So the windings do have separation.  It's hard to see but it's not much, maybe a millimeter.  That might suggest it would not have a high isolation value and could potentially arc across with a HV surge.  But I'm grasping in the dark there.

Winding outputs are not polarized in any way.  Same colour wire both ends, so I will need to use the meter to find the polarity for commoning.

Ordered some 63V 1000uF and 470uF caps.  If I need to replace the capcitors in the +-12V and +5 regulator boards.  They will also allow me to make the test xformer circuit simpler and test commoning it's two 18V secondaries.

Out of interest, my mains measures at 239V (from memory), been a while since I tested it.
« Last Edit: March 09, 2018, 07:06:56 pm by paulca »
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Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #45 on: March 18, 2018, 12:42:26 am »
So I got a pair of regulator boards.  An LM317/LM337 +-12V (adjustable) AC-DC regulator and am LM7805 5V regulator AC/DC regulator.

I want to test the +-12V regulator, however, using the "test" transformer as I have not wired the other one up yet.  It has two 18V windings which unloaded put out 35V.

So I checked the polarity of the windings and they were as the pinout on the transformer would intuitively lead you to.  Swapped the bridge connection and got 0 volts on the scope to confirm and went back to the correct polarity.

The LM317 board is rated for +-35V and takes the AC input as +, - and 0.  I was assuming I would connect the outer windings to + and - and the centre point to 0.

The scope however is making me wonder.  When measuring the voltage across both windings I was getting....

VMax: 70V
Vpp: 115V

This sounds like twice what it should be.  Is this because I was measuring the outer windings?  Would that not mean while one winding is+35V the other winding is -35V, giving 70V output? it The 115V is because the bottom of the sinewave was squashed.  Not entirely sure why.

Obviously when connected to the regulator board the + side will see 0-35V and the - side will see 35-0V

It's just confusing trying to work with + and - terms with AC.  I'm confuddled.

EDIT: The reason I'm being overly cautious is the the caps on the board are only rated to 35V and I don't want one popping in my face.  I also intend to put some form of load on it to hopefully bring the voltage down a bit.
« Last Edit: March 18, 2018, 12:46:13 am by paulca »
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Online Ian.M

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Re: My first mains AC/DC supply... help?
« Reply #46 on: March 18, 2018, 04:11:58 am »
Oh dear.  35V unloaded is right on the bleeding edge for those caps.  If you want a long and happy life from them, replace with 50V or even 63V rated ones. The LM317 and LM337 are only rated for 40V Abs. Max. voltage differential in to out so there's *very* little margin for supply voltage variation at startup.   OTH. once its powered up, there's 15V less across each regulator as long as their outputs never get shorted so they should be OK.   Do re-check the measurements and this time check the mains supply voltage as you want to be certain it wasn't significantly under the UK's customary 240V at the time of measurement.

Worst case, it may benefit from a capacitance multiplier circuit on each of the raw + and - rails, which will vastly reduce the ripple 'seen' by the regulators, and if you clamp the multiplier transistors' bases with 27V Zeners, will also act as a preregulator when it is lightly loaded.
 

Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #47 on: March 18, 2018, 07:43:52 am »
It shouldn't be a big deal, the RCore has 15V windings rather than 18V.  Maybe I should wire it up instead.  I was just going to test with this 18V xformer as it's wired and I have it handy.

I have got Nichion 63V 1000uF caps, I could swap them, better quality and they would be well overrated too.  Probably means taking the risk of drilling the holes back out as I can never get them clear after removals.

The high reading on the scope, would this have been because I was in AC coupling mode while measuring across two AC windings?

Quote
Worst case, it may benefit from a capacitance multiplier circuit on each of the raw + and - rails, which will vastly reduce the ripple 'seen' by the regulators, and if you clamp the multiplier transistors' bases with 27V Zeners, will also act as a preregulator when it is lightly loaded.

I assume when the power supply is in it's new home powering the device it will not spend much time unloaded.  Of course it depends on the output mosfets on the sig gens outputs as to how unloaded the +-12V rails are.

The SigGen is still on the slow boat from China.
« Last Edit: March 18, 2018, 07:47:32 am by paulca »
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