Author Topic: My first mains AC/DC supply... help?  (Read 2709 times)

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Offline paulca

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My first mains AC/DC supply... help?
« on: February 27, 2018, 10:50:37 pm »
So I need to replace a PSU in a Chinese product because it has leakage.  In particular the FY6600 Sig Gen.  I don't like a BNC ground with 120v on it, regardless of the safety or what not.

So I want to replace it with a transformer with linear regulators.  The FY6600 has +-12V rails for the output and a 5V rail for it's internals.

I have got part the way there with help from the guys in the FY6600 thread, and am basing it on these components:

Transformer:
https://www.ebay.co.uk/itm/131588122667?_sp=p2488212.m41214.l9765&_trkparms=itemid%3A131588122667

5V regulator:
https://www.ebay.co.uk/itm/271656192244?_sp=p2488212.m41214.l9765&_trkparms=itemid%3A271656192244

+-12V regulator (after adjustment):
https://www.ebay.co.uk/itm/302371834320?_sp=p2488212.m41214.l9765&_trkparms=itemid%3A302371834320

My question is around anything I have forgotten.  For example I can think of a few things.

Primary side fuse.
Primary side caps, scrubbers, X, Y caps if I need them.

I googled but I found so many different kinds an different implementations for different countries I figured it would better to ask. 

I will happily accept a link to a guide which can be vouched for.  I'm in the UK and while the device will never leave my bench I'd it to be safe and as close to "code" as possible.
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Online Ian.M

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Re: My first mains AC/DC supply... help?
« Reply #1 on: February 27, 2018, 11:59:07 pm »
Line frequency leakage current in SMPSUs isn't inherently due to it being a SMPSU, its due to the class Y capacitor from secondary 0V to the negative side of the primary DC bus, that is required as a return path for the HF leakage current capacitively coupled (by the inter-winding capacitance) from the 'hot' end of the primary to the secondary if there isn't an inter-winding screen. 

If your proposed linear PSU's transformer doesn't have either a grounded inter-winding screen, or a split bobbin construction, it may still have enough interwinding capacitance to pass enough leakage current to be troublesome, causing its secondary to float at a significant AC voltage.

Unfortunately that transformer on EBAY doesn't look like it has a split bobbin construction and probably doesn't have an inter-winding screen.   Unless it has comprehensive manufacturer's data, you'd have to buy it and test it to find its leakage current.  If you are floating the secondary, and it doesn't have a proper datasheet, I'd also want to do a 1KV Hi-Pot test between the primary and secondary sides before I'd trust its insulation.

If the leakage current is excessive, the only option is to ground the secondary side 0V rail, which will result in a ground referenced output, though if you need it floating for DC, its possible to use a capacitor between 0V and ground(+ a series resistor to limit the surge if its ever hot-plugged).

If you haven't stated buying parts yet you may want to re-think the PSU design.   The +5V rail doesn't need to be extremely quiet so a switching regulator can be used there, with heavy filtering on its output to minimise switching frequency breakthrough,.  That means you don't need the 9V windings.  I'd also bet you don't need a lot of -12V current, so you *may* be able to capacitively couple a bridge rectifier for the negative rail, so it can all run off a single 15V RMS secondary, though a 15V-0-15V transformer would be preferable.

Have you got figures for the load current required on each rail?    If not, I would suggest measuring them using the existing PSU, with both channels analog outputs loaded with 50R 1W resistors (47R would be close enough), while outputting a maximum amplitude squarewave.    Don't keep the load on at max output for longer than half a minute (the FY6600 short circuit overload rating).

For the primary feed I'd run the mains through a 1A slow blow fuse in the Line connection, then straight to the primary.   If your mains supply is particularly dirty, you may want a suitably rated varistor across the primary.   The transformer *should* have a non-resettable thermal fuse buried in the winding.  If you add a mains power switch, don't use one with any exposed metalwork.

 On the secondary side, use good quality rectifiers rated for at least three times the required output current, and fuse the unregulated rails after the bulk decoupling caps*, with fast-blow fuses rated for the expected maximum output current.   To prevent damage to the load if a fuse blows and a rail is lost, connect three diodes between the regulated output rails in order of their voltage,each cathode to the most positive rail, so that if a secondary side fuse blows, the -12V rail cant rise significantly above 0V and the +12V rail cant drop below the 5V rail etc.

* Its possible to fuse between the secondaries and the rectifiers but you'd need four fuses, its *MUCH* more difficult to calculate suitable fuse values, and you'll either have to use slow-blow fuses or go to a significantly higher current rating to handle the initial surge when mains is connected and the bulk decoupling capacitors initially charge.
« Last Edit: February 28, 2018, 01:02:36 am by Ian.M »
 
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Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #2 on: February 28, 2018, 12:33:32 am »
Thanks Ian.  A lot to take in.

From the FY6600 I believe the +-12V rails only need to be around 200mA max.  Basically enough to power whatever you are driving with the signal generator and in my case right now that would be high impedance audio stuff, but...  that could change.

Reports are that he 5V is well under 1A will run off one of the 9V secondaries and no need to parallel the second 9V winding... but it's there, why not?

At least one other user has successfully used this transformer in the FY6600.

I have a spare mains switch, all plastic.  I will need to buy a slow blow fuse and holder... or could I get away with the 3 Amp fitted fuse in the UK mains plug of the IEC "kettle" lead?

I also need an IEC 3 pin socket although I not sure about grounding requirements.  I would like to leave the output floating if possible so I can connect it to virtual ground devices with elevated virtual grounds without sinking current to earth.  (Do I understand that right?)

I'm out of my depth here.  AC and Mains... my signature says it all.
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Offline cowasaki

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Re: My first mains AC/DC supply... help?
« Reply #3 on: February 28, 2018, 08:42:06 pm »
Paul,

I built a power supply for a similar signal generator (3224) and had boards made to test the supplier.  I have 9 left !  If you want a board you are welcome to it and it should sort you out.  It is a fully isolated supply using a normal transformer. 

Just ignore the SMD components and it needs a 2x15v transformer such as:

http://cpc.farnell.com/camdenboss/ctfc6-15/transformer-6va-2x-15v/dp/TF01334?st=transformer%2015v

If you want one just PM me.
 

Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #4 on: February 28, 2018, 10:49:00 pm »
cowasaki, thank you for the kind offer. 

I have already ordered the 2x15V 2x9V transformer.  I have also ordered the +-15V regulator board and 5V regulator board.  That said, I might go the route of making a single board for it.  Depends on how things fit or don't fit in the FY6600.

I am going to wait for things to arrive and start testing things step by step.

I have a small enclosure that the transformer while hopefully fit in, so once that is tested to be working and wired as expected, I'll seal it away to keep the mains safe while i work on the rest.

Your board looks nice though :)
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Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Online BrianHG

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Re: My first mains AC/DC supply... help?
« Reply #5 on: March 01, 2018, 12:24:28 am »
Place .1uf caps across you diodes in parallel and in parallel across the transformer outputs.  It prevents a leaking buzz coming from switching of the diodes which may happen both during small narrow stages of the AC cycle and nowadays additional potential ringing during the time cheap CFL/LED lamps generate EMI on the mains.

Also, yes, get a split bobin transformer for a pre-amp.  If you were to go switching, your going to have difficulty getting a medical grade PSU which was designed with the super HI-Pot and no X cap, yet still has approval and is affordable.

See photos of split-bobin transformers:


Here are some cheap ones with uni:versal input and output: 1 xformer for +/-12v, a second for +5v, and the last one for +/-15v with a good 350ma to boot.
https://www.digikey.co.uk/product-detail/en/triad-magnetics/FS24-250-C2/237-1502-ND/3986393
https://www.digikey.co.uk/product-detail/en/triad-magnetics/FS12-200-C2/237-1542-ND/3986412
https://www.digikey.co.uk/product-detail/en/triad-magnetics/FS36-350-C2/237-1480-ND/3986352

Here is the complete SplitBobin 4.2kv hipot series.  Really well priced and up to 2 amps at +/-18v...
https://www.digikey.co.uk/products/en/transformers/power-transformers/164?FV=ffecd9db%2Cffe000a4&quantity=&ColumnSort=1000011&page=1&pageSize=100

Chassis mount split bobin, but only 4kv hipot:
https://www.digikey.co.uk/product-detail/en/triad-magnetics/VPS36-700/237-1288-ND/666174
« Last Edit: March 01, 2018, 12:48:27 am by BrianHG »
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Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #6 on: March 01, 2018, 12:47:50 am »
Thanks but I already have an R-Core audio transformer in the post.
"What could possibly go wrong?"
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Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #7 on: March 01, 2018, 12:53:35 am »
Also, yes, get a split bobin transformer for a pre-amp. 

Ah, you might be assuming this PSU is for my pre-amp project which makes your post make more sense :)

This one is actually for a signal gen to replace a dangerous chinese switching supply with mains leakage.  The Audio/Preamp box I will just see if I can filter the PSU better :)

Thanks gain.
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Online BrianHG

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Re: My first mains AC/DC supply... help?
« Reply #8 on: March 01, 2018, 01:08:33 am »
Thanks but I already have an R-Core audio transformer in the post.
I bet you will have trouble finding that primary/secondary isolation spec...
Just because you see a bobin on the left and right doesn't mean one side is primary and the second side is the secondary.
The transformer ring would be un-balanced power wise.  What's nice about the R-Cores is that they are relatively flat for the amount of power they give out.  Hence a low profile AV device.

Also, there exist ones with shielding:
« Last Edit: March 01, 2018, 01:15:19 am by BrianHG »
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Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #9 on: March 01, 2018, 03:12:55 am »
Thanks but I already have an R-Core audio transformer in the post.
I bet you will have trouble finding that primary/secondary isolation spec...
Just because you see a bobin on the left and right doesn't mean one side is primary and the second side is the secondary.
The transformer ring would be un-balanced power wise.  What's nice about the R-Cores is that they are relatively flat for the amount of power they give out.  Hence a low profile AV device.

Do I need to know the isolation spec?  Assuming I have reports of people using this and it survives 240V, then... do I really care?

As long as it doesn't zap me.  I mean is it likely to be worse than a switch mode with a cap between the hot side and device side and 120V on the BNC GND?

"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline Gyro

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Re: My first mains AC/DC supply... help?
« Reply #10 on: March 01, 2018, 03:29:17 am »
It does appear to have an inter-winding screen. As long as it's properly implemented, then Earthing that will increase safety and reduce primary-secondary leakage and noise coupling.
Chris

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Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #11 on: March 01, 2018, 03:42:18 am »
It does appear to have an inter-winding screen. As long as it's properly implemented, then Earthing that will increase safety and reduce primary-secondary leakage and noise coupling.

When you say "Earthing". Do you mean the SCN connection?  I was hoping that was "Screen".  Or do you mean just earthing the transformer chassis?

I'd rather like to avoid earth referencing the secondary side and thus my signal generator.
"What could possibly go wrong?"
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Online BrianHG

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Re: My first mains AC/DC supply... help?
« Reply #12 on: March 01, 2018, 03:55:42 am »
Thanks but I already have an R-Core audio transformer in the post.
I bet you will have trouble finding that primary/secondary isolation spec...
Just because you see a bobin on the left and right doesn't mean one side is primary and the second side is the secondary.
The transformer ring would be un-balanced power wise.  What's nice about the R-Cores is that they are relatively flat for the amount of power they give out.  Hence a low profile AV device.

Do I need to know the isolation spec?  Assuming I have reports of people using this and it survives 240V, then... do I really care?

As long as it doesn't zap me.  I mean is it likely to be worse than a switch mode with a cap between the hot side and device side and 120V on the BNC GND?

No, obviously the transformer has better isolation than an X cap.  I'm sure the transformer is at least 1500v like the one I've attached in this photo.  It is safe to touch without feeling a thing.  However, I know touching the RCA input on my amp, which uses a toroid transformer, makes a nasty 60Hz hum and I don't feel that current on my finger.  On a battery powered amp, without any other connections to anything but the speakers, no hum, if not a very tiny one, not full blase.

The purpose of the HV isolation is so your pre-amp's GND on the RCA ins and outs will sit right there without any current, even electrostatic, bridged from the mains.  You want to get as close as possible to a battery powered pre-amp.  In fact, you don't need to regulate your +/-15v, even a slight 50/60hz there wont affect your amp's performance since the op-amps reject the power supply pin noise.  It's crucial that your GND takes on the reference to what ever device you plug you pre-amp into.  Split bobin transformers are known to separate you from the mains as much as possible except for the addition of additional shielding spacing between the windings, or specially insulated windings and layout found in plug in desktop 7-1/2 digit multimeters.
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Offline Gyro

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Re: My first mains AC/DC supply... help?
« Reply #13 on: March 01, 2018, 04:02:13 am »
It does appear to have an inter-winding screen. As long as it's properly implemented, then Earthing that will increase safety and reduce primary-secondary leakage and noise coupling.

When you say "Earthing". Do you mean the SCN connection?  I was hoping that was "Screen".  Or do you mean just earthing the transformer chassis?

I'd rather like to avoid earth referencing the secondary side and thus my signal generator.

Yes, the SCN connection. It will (should) be between the primary and secondary winding (insulated from both), so earthing it provides screening/shielding without compromising your floating secondary.
« Last Edit: March 01, 2018, 04:06:12 am by Gyro »
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Online BrianHG

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« Last Edit: March 01, 2018, 04:24:13 am by BrianHG »
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Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #15 on: March 01, 2018, 06:01:44 am »
Looks nice too.

As the xformer will take forever and a day to get here, I ordered a cheap xformer from RS which should arrive Friday.  It's +-18V sealed PCB mount, 4W.  Also got some -15 +15 regs, and Nichicon 1000uF caps.  The rest I think I have in stock.  It will mean I can have a play transformer and experiment which not blowing shit up.

Was reading a marketing PDF for some chinese transformers and something lost in translation which made me laugh was the line, "Better explosive voltage than other transformers".  Not sure that's what they meant LOL.
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Offline rhb

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Re: My first mains AC/DC supply... help?
« Reply #16 on: March 01, 2018, 10:36:28 am »
I'd suggest reading the LM317 datasheet.  There may even be an old application note from when they first came out.  I built a 5 V supply for TTL work based on that general line of regulators and used an LM317 to fix a burned out 12 V CB radio PSU.  I think I modified the latter to include an extra tantalum capacitor to reduce ripple that was suggested.  But it was a very long time ago.
 

Offline Bassman59

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Re: My first mains AC/DC supply... help?
« Reply #17 on: March 01, 2018, 01:46:43 pm »
Looks nice too.

As the xformer will take forever and a day to get here, I ordered a cheap xformer from RS which should arrive Friday.  It's +-18V sealed PCB mount, 4W.  Also got some -15 +15 regs, and Nichicon 1000uF caps.  The rest I think I have in stock.  It will mean I can have a play transformer and experiment which not blowing shit up.

Get some heat sinks for your regulators! 18 Vrms is ~25 V peak, which will be approximately the input to your regulator after the smoothing caps. (Simulate to see!) You’ll drop 10 V in the regulator which is a watt with a modest 100 mA load.
 

Offline cowasaki

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Re: My first mains AC/DC supply... help?
« Reply #18 on: March 02, 2018, 12:34:22 am »
Here is the video -
 

Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #19 on: March 02, 2018, 02:27:14 am »
I did watch that.  I'm not really after creating a fully variable bench supply.  The goal here is to play with transformers, make a fixed dual rail supply and when I get that working I can build the supply for the FY6600 to replace the dodgy crap it comes with.
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Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #20 on: March 07, 2018, 07:46:15 am »
So I got my "play" transformer through from RS.  No regulators yet, but I put the transformer onto some strip board with a 250V switch and a 0.1A fuse primary side.  Shrink wrapped and electrical taped up the primary side to make it at least resembling "safe".

Power it on, no smoke, no bangs.

Measured the voltage on the secondary in AC Trms mode.  It reads 25V on each output.  Fine.

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...

So the question is... can I connect my little mini bench scope to it.  It has a 50V pk printed on the BNC.  So if that's 50V peak it's fine as 25Vrms should be 35V peak.  If it's 50V peak to peak it's not fine.

I have my OWON scope which has a much higher input rating, but it needs the laptop which is elsewhere tonight.
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #21 on: March 07, 2018, 07:52:59 am »
Actually, I think I'm okay.  It's 50V peak with 1x probe.  So it should be fine with 70Vpp with a 10x probe.  1MOhm input impedance.
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Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline Monkeh

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Re: My first mains AC/DC supply... help?
« Reply #22 on: March 07, 2018, 07:57:45 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.
 

Offline paulca

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Re: My first mains AC/DC supply... help?
« Reply #23 on: March 07, 2018, 08:12:58 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.
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Offline Monkeh

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Re: My first mains AC/DC supply... help?
« Reply #24 on: March 07, 2018, 08:15:49 am »
Two drops.

The secondary side is always worth being careful around. The voltages aren't dangerous, but you're presumably not dealing with a current limited (yes, those exist - they're really low power to begin with) transformer - which means shorting it will bring smoke, fire, and sadness if the fuses aren't sized right. Don't assume a primary side fuse can protect the secondary effectively!
 


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