FeelTech FY3200 PSU upgrade - Page 1
Forum section: electronics / testgear

author: leonerd
author_rank: Regular Contributor
timestamp: June 19, 2016, 06:43:51 pm
content: I've got one of these popular DDS signal gens, the FT3200 - mine's the full 24MHz unit. As a lot of people have discovered, the switching power supply inside it suffers a lot of mains leakage. I measured about 95Vac between its "ground" outputs and earth mains, and it seems capable of supplying nearly 1mA of current!I've therefore decided to undertake a little project to upgrade the power supply, on a similar project toI measured the current requirements of the lines - a little more directly than the above video (watch from around the 2m40 point onwards), but I get very similar results, namely:+5V - current draw increases with output frequency, independent of output voltage or load. Maximum occurs at 24MHz, of 260mA.+12V and -12V - current draw increases with output current, independent of frequency. Maximum occurs at 20Vpk into 50ohm, of 80mA on each.I'm thinking rather than 3 linear supplies, what I'll do is make 2 linear supplies for the +/-12V lines (as they power the analog output frontend), but go with a simpler efficient switching design for the 5V rail. In particular, most of the logic chips on the board are 3.3V anyway. The 5V line into the actual board mostly just powers a bank of 3.3V regulators, and possibly the LED backlight of the display. I think that will be fine with some switching noise.I expect I'll post a few photos and diagrams and the like on this thread. Wish me luck...

author: Kevin.D
author_rank: Frequent Contributor
timestamp: June 19, 2016, 09:37:20 pm
content: Quote from: leonerd on June 19, 2016, 06:43:51 pmApart from being enough to fry sensitive components every time you connectedsome node of your own circuit to earth this also would not pass any electrical safety laws inEurope (or most of the world I think).Since this is a class II device (accessible unearthed conductors) max leakage should be .25 mACopy/Paste from Standard IEC950 entitled “Safety of Information TechnologyEquipment Including Electrical Business Equipment"Earth Leakage Current: Maximum earth leakagecurrent must not exceed the limits shown in the following tableunder the most unfavorable (highest) input voltage. For class IIequipment when output is not connected to earth ground, thetest shall be made on accessible conductive parts, and to metalfoil with an area not exceeding 10 cm x 20 cm on accessible nonconductiveparts.Class Type of Equipment Maximum Leakage CurrentII All 0.25mAI Hand-held 0.75mAI Movable 3.5 mA(other than hand-held)

author: leonerd
author_rank: Regular Contributor
timestamp: June 19, 2016, 10:16:17 pm
content: Some back-of-envelope calculations... Anyone see anything wrong with the logic here?If I go with a typical LM317/LM337 linear regulator combination for the 12V lines they'll want 3V of dropout on each rail for the regulators. Add a bit for rectifiers, account for RMS vs. peak at input-side smoothing cap, and I think I can use a 15/0/15V secondary winding on the transformer. I need 80mA at 15V which means 1.2VA, and again for the negative rail, is 2.4VA required by the linear regulators.If I tap off from the input to the +ve regulator and ground to supply the LM2597 board to generate 5V, I'll need more power here. To generate 5V at 250mA in a buck regulator that I'd guess is around 80% efficient needs 1VA.So in total I need at least 3.4VA of power from the transformer.Is 4VA going to be sufficient, or might I want more spare capacity? Are there any downsides to overspecifying the transformer and getting a 6VA (or more) model?Is a 15/0/15V going to be sufficient for these regulators? 15V is 21.2Vpeak which is 20.2Vpeak after the rectifier (1 diode drop because it's a split rail), so even letting the smoothing capacitor droop to 80% of that still gives us 16.1V into the regulator, giving it a helpful 4.1V of headroom to generate the 12V output. So I think that sounds fine.And finally, is the transformer/rectifier going to be OK with me asymmetrically loading it? The LM317 and LM2597 between them will be pulling about twice the current from the +ve end of the rectifier as the LM337 will be sinking into the -ve.

author: StillTrying
author_rank: Super Contributor
timestamp: June 20, 2016, 12:10:01 am
content: If these are sustained currents I think you're missing a VA or two and 4VA won't be enough.I need 80mA at 15V which means 1.2VAThe 80ma is drawn from a ~20V source so 1.6VA.To generate 5V at 250mA in a buck regulator that I'd guess is around 80% efficient needs 1VA.It must be at least 1.5VA. @ 20V.And finally, is the transformer/rectifier going to be OK with me asymmetrically loading it?Yes, you're not really asymmetrically loading the transformer at all.

author: leonerd
author_rank: Regular Contributor
timestamp: June 20, 2016, 12:15:22 am
content: Quote from: StillTrying on June 20, 2016, 12:10:01 amOh, because of peak vs. RMS?Quote from: StillTrying on June 20, 2016, 12:10:01 amRighty. So 3.2VA for the linears and another 1.5VA for the buck; that's 4.7VA. Turns out that 6VA is about the smallest transformer anyone sensibly sells, so that should suffice, yes?Quote from: StillTrying on June 20, 2016, 12:10:01 amWell, not the transformer, but definitely the rectifier. I'll be putting twice as much current through the two diodes generating the +ve output, as I am the -ve.

author: StillTrying
author_rank: Super Contributor
timestamp: June 20, 2016, 12:49:41 am
content: I'd be tempted to try a 12-0-12 @ 10VA but many would disagree with that!

author: leonerd
author_rank: Regular Contributor
timestamp: June 20, 2016, 08:46:59 am
content: Quote from: StillTrying on June 20, 2016, 12:49:41 am12Vrms is 16.97Vpeak; minus 1V of rectifier drop that's 15.97Vpeak into the regulators. The regulators want 3V of dropout to generate 12V, so that's nearly 1V spare headroom. This is just within possible, but means the smoothing caps can droop by no more than 6%.On the plus side it does mean less work for the linear and buck regulators to do, so less waste heat generated. Good inside a passively cooled box.

author: leonerd
author_rank: Regular Contributor
timestamp: June 20, 2016, 08:56:03 am
content: Quote from: leonerd on June 20, 2016, 08:46:59 amTo achieve an acceptable ripple with the load on the +ve cap of 160mA, we're looking at ballpark of over 3000uF to smooth that. Whereas a 15/0/15 transformer works just fine with a little under 1000uF.The regulator board I'm looking at (eBay link above) comes with 2200uF caps.

author: CJay
author_rank: Super Contributor
timestamp: June 20, 2016, 09:06:30 am
content: Quote from: leonerd on June 20, 2016, 08:56:03 amI seem to remember the PSU from an old ADSL router I used to have dozens of would do for that job, +5 -12 and -12 all in one standard size open frame module.I've got a small pile of them sat in my shed if you want to try one (and if my memory is correct on the -12 rail being present)

author: leonerd
author_rank: Regular Contributor
timestamp: June 20, 2016, 11:57:24 am
content: Quote from: CJay on June 20, 2016, 09:06:30 amWell that is what I'm aiming at. The question is whether the +12/-12V lines are linear or switching though. My intended design here has those supplied by linear regulators, because they're the analog output chain. Also it leaves me with a simple two-pole transformer primary of the kind that would let the signal generator float relative to mains earth, or at the very least not let through that nearly-1mA of mains noise onto the output ground.

author: CJay
author_rank: Super Contributor
timestamp: June 20, 2016, 01:53:21 pm
content: Quote from: leonerd on June 20, 2016, 11:57:24 amFor the +12, definitely from the main transformer and I *think* the -12 will be as well but I wouldn't swear to that.They're a pretty quiet supply though, ADSL doesn't play nice in the presence of high frequency noise.

author: leonerd
author_rank: Regular Contributor
timestamp: June 22, 2016, 04:42:38 pm
content: Status update: I ordered an adjustable LM317/LM337 regulator board (http://www.ebay.co.uk/itm/262483314472) and a 2x15V 6VA transformer (http://www.ebay.co.uk/itm/121671766834).I'll post again when the bits have arrived and I've had a chance to build something up.

author: leonerd
author_rank: Regular Contributor
timestamp: June 24, 2016, 03:23:01 pm
content: Quote from: leonerd on June 19, 2016, 06:43:51 pmWaaaait a second. 20Vpk-to-pk is a little over 7Vrms. Into a 50ohm load I'd expect to see 140mA. And yet I see only 80. That doesn't add up somehow.Or am I somehow a factor-of-two off here because of two rails?

author: taemun
author_rank: Regular Contributor
timestamp: June 27, 2016, 03:27:33 pm
content: Quote from: leonerd on June 24, 2016, 03:23:01 pmWaaaait a second. 20Vpk-to-pk is a little over 7Vrms. Into a 50ohm load I'd expect to see 140mA. And yet I see only 80. That doesn't add up somehow.Or am I somehow a factor-of-two off here because of two rails?There'll be a 50 ohm resistor in series with that, inside the unit. So it's a 100 ohm load on a 7Vrms source - 70mA plus some spare change to get to 80mA.

author: hansibull
author_rank: Regular Contributor
timestamp: July 05, 2016, 09:33:59 pm
content: I got my Feeltech FYS3200 in the mail today, and decided to replace the PSU after measuring 17v AC between mains ground and the BNC connector. This is the result after a few hours of work:

author: leonerd
author_rank: Regular Contributor
timestamp: July 10, 2016, 12:04:00 am
content: Quote from: hansibull on July 05, 2016, 09:33:59 pmLooking good. That's linear all the way there I see? Quite a large heatsink required there.

author: masterx81
author_rank: Frequent Contributor
timestamp: July 11, 2016, 07:41:02 am
content: I want to use a single dual 15+15v 5+5va output transformer, 7812 + 7912, and a buck for the 5v output, i'm in the process of making it...What's the advantage of using the LM317/LM337 instead of the 7812/7912?

author: hansibull
author_rank: Regular Contributor
timestamp: July 11, 2016, 09:07:42 am
content: QuoteYep, it's linear all the way. Haven't had any heat related issues yet, but I haven't stress tested it for several hours either

author: leonerd
author_rank: Regular Contributor
timestamp: July 11, 2016, 10:14:35 am
content: Quote from: masterx81 on July 11, 2016, 07:41:02 amNo reason at all. I just happened to find a nice-looking cheap regulator board on eBay that is based on the LM317/LM337, so I went with that. I'd have been just has happy to go with a 7x12 pair also.

author: leonerd
author_rank: Regular Contributor
timestamp: July 25, 2016, 01:50:30 pm
content: My PSU board eventually arrived from eBay, so I've been able to complete this.I'm using a piece of black perspex as a base board to mount the LM317/LM337 module on, along with the transformer, and the 5V buck regulator to provide the digital section power. New mains wiring and the addition of a fuse and a nice chunky MOV to finish it off.

author: technogeeky
author_rank: Frequent Contributor
timestamp: July 25, 2016, 01:58:08 pm
content: Very nice!Do you have any idea what effect this has on the signals generated, if any? Or is this project just to get rid of the mains leakage?-tg

author: leonerd
author_rank: Regular Contributor
timestamp: July 25, 2016, 02:49:10 pm
content: Quote from: technogeeky on July 25, 2016, 01:58:08 pmOh I didn't really have any problem with PSU switching noise on the outputs. I was purely trying to get rid of the mains leakage. I expect any nonlinearity out of the R-2R ladder (see various other posts on that subject; especially around the zero-crossing region) would outweigh any PSU noise at this point anyway.

author: StillTrying
author_rank: Super Contributor
timestamp: July 28, 2016, 02:49:27 am
content: Looks neat to me, I wouldn't have bothered with the MOV.What was the smoothed voltage from the 15V TX, I still think a 12V would just be enough.

author: Excoriator
author_rank: Contributor
timestamp: November 01, 2016, 09:46:17 pm
content: I bought a 1:1 isolation transformer and kept the switch mode supply. Mounting it was a doddle. I used brackets to spread the weight of the transformer over four bosses, but even including these brackets, the job took under an hour with minimal intellectual effort included.It allows me to 'float' the unit provided I don't connect the USB socket to the computer. This is not a serious constraint as far as I'm concerned, but for those who find it is, you can buy USB isolation cables that solve that little problem too.Chucking out the switcher for a linear PSU redesign is a bit OTT for me.If you can't find a suitable 1:1 transformer at a suitably low cost, then two small 1:n transformers back to back would do much the same thing. The power consumption is tiny, so two dirt cheap transformers should cost you peanuts!

author: S185149
author_rank: Contributor
timestamp: September 24, 2017, 04:49:31 pm
content: Very late to the game here. Can anyone tell me what type of connectors are used in the FY3200 for the power connections?Thanks,S18

author: ledtester
author_rank: Super Contributor
timestamp: April 19, 2018, 04:18:09 pm
content: I have a question about the original power calculations used to size the transformer...The output power needed for the 5V supply is 5V * 250 ma = 1.25 W. Assuming an 80% efficient buck converter means that about 1.6W are needed from the transformer. However, this power is only coming from one of the windings in the 15-0-15 transformer, so when sizing it we need to double this amount even though that power from the other winding won't ever be used.The requirements for the +/- 12V supplies has been computed to be another 1.6 VA each. So the total VA required from the transformer is 4*1.6 = 6.4 VA.Is this correct?

author: Bob Sava
author_rank: Regular Contributor
timestamp: April 19, 2018, 06:36:19 pm
content: The point of using a transformer is to be able to use linear regulators and avoid EMI and ripple from SMPS. Adding a buck converter for 5V output seems to defeat that goal.

author: DSE_Kid
author_rank: Newbie
timestamp: May 04, 2020, 10:23:13 am
content: Hey guys, I got my FY3224 almost a year ago and after stumbling on this post I decided I should test it to see if it had this leakage problem. I found that I was getting approx 3.4vac mains earth to output ground channel A, which is thankfully well within limits, so I opened the unit up and discovered that the power supply board was a different design. My real concern however came about when I took a voltage reading between the USB signal ground and computer and found a significant 85 VDC. I ended up ordering a cheap USB Cat5 extender which I'm hoping will have some sort of optocoupler built in to provide that isolation, otherwise I'll have to pony up the cash for a proper USB isolator.

author: RovDan
author_rank: Newbie
timestamp: September 25, 2020, 09:28:19 pm
content: Hi everybody,I Just bought a FeelTech FY3224S. I read many messages about the leaking risks because there's no wire to ground.I used a DVM between BNC sockets and ground. I read 1V AC. So my question : do you think the manufacter modified the supply to avoid any link between the power (230V here), and the sockets ?In fact, if really necessary, there's 2 ways to fix it : doing a DIY power unit +/-12V +5V or use a separating transformer. I think the transformer would be better.Can a guy explain me what is the use of the DC modeI see nothing on my scope in this mode !Many thanks for your advices.RovDan

author: tom66
author_rank: Super Contributor
timestamp: September 26, 2020, 09:28:14 am
content: Quote from: ledtester on April 19, 2018, 04:18:09 pmCross loading a transformer doesn't double the load requirement. You might get a difference in voltage between the windings because of the crossload, but you only need to consider the per-winding rating and the total rating of thr transformer.