When to use an isolation transformer - Page 1
Forum section: electronics / beginners

author: Seamus16
author_rank: Contributor
timestamp: March 13, 2017, 12:12:27 pm
content: I have watched Dave's video on isolation transformers and several others and understand the principle of them but not under certain conditions. I have looked through the forum but cannot find this particular circumstance.I am proposing building a signal generator kit, something like thishttp://www.ebay.co.uk/itm/DDS-Function-Signal-Generator-Module-Sine-Square-Sawtooth-Triangle-Wave-Kit-UK-/302042940183?hash=item4653298317:g:ZPQAAOSw9NdXsXqKIt requires various voltages so was proposing using a computer PSU that I have lying around to supply them. I will be looking at the wave forms on my analogue scope.So should I be using an isolation transformer in this case?ThanksDavid

author: Ian.M
author_rank: Super Contributor
timestamp: March 13, 2017, 12:40:20 pm
content: No. You don't need the isolation transformer for safety for this application and if you use it just to float the signal generator ground, you'll pollute the output with a lot of noise from the PC PSU via its class Y line filter caps.The +/-12V is only to power a few OPAMPS. Use DC-DC converters for the +/-12V rails running off the 5V rail. A pair of 1W ones should do it, then you can power the whole thing off a good quality USB charger, or safely float it off a powerbank if you need to,

author: jolshefsky
author_rank: Regular Contributor
timestamp: March 13, 2017, 01:20:54 pm
content: Hrmmm ... I thought I'd chime in and say yes, you should. The ground on the PSU (both the mains ground and the low-voltage ground) is tied to the earth ground of the outlet; the scope, likewise has its shield and probe grounds tied to earth ground of the outlet. (Unless there's something wrong with the wiring.) But, if you hook your scope ground to +12V in the power supply, you've shorted it to ground and you'll probably just melt your probe wires, but you might damage your scope.An isolation transformer with no ground path (oddly a transformer with an earth pin will have it tied right through for ... umm ... safety, I guess) will prevent this scenario and you can check voltages as you wish. However, as Ian.M points out, the input filtering on the PSU will be crippled if it isn't connected to ground, and—to test your circuit—you should substitute an isolated DC-DC converter or other lab power supply rather than probing around in a circuit that is mains-earth-connected.

author: bktemp
author_rank: Super Contributor
timestamp: March 13, 2017, 01:30:55 pm
content: I wouldn't use a computer power supply for such a circuit. The function generator draws a couple of 100mA at most. A computer power supply can supply >10A, so every mistake will probably destroy your circuit, melt wires or even burn down your house.Use a wall wart power supply, or even better get a cheap power supply with a decent current limit function. Most wall wart power supplies are not connected to mains ground, so you don't need any isolation transformer.

author: madires
author_rank: Super Contributor
timestamp: March 13, 2017, 02:03:24 pm
content: I agree, using a >300W ATX PSU for a tiny signal gen is nonsense. If you go for a SMPSU or DC/DC converters add a simple LC filter to each rail to reduce the switching noise and ripple. You can buy cheap DC/DC converter modules with 5V input and +12V & -12V output rated for a few Watts.

author: rstofer
author_rank: Super Contributor
timestamp: March 13, 2017, 03:10:27 pm
content: I have been looking at these Power Bricks. I don't know if they are large enough for the signal generator but they should be.http://store.digilentinc.com/powerbricks-breadboardable-dual-output-usb-power-supplies/

author: Seamus16
author_rank: Contributor
timestamp: March 13, 2017, 04:15:53 pm
content: Thank you for all the replies and am glad I asked the question. I did realise that the ground on the PSU was connected back to the earth which prompted the question. The reason for using the PSU was that it supplied the -12v.Presumably you are talking about one of thesehttp://www.ebay.co.uk/itm/USB-Boost-Single-Turn-Dual-Power-Supply-Linear-Regulator-Kit-w-Multi-Output-H4F4/131876008280?_trksid=p2045573.c100642.m3226&_trkparms=aid%3D555017%26algo%3DPL.CASSINI%26ao%3D1%26asc%3D20160706104836%26meid%3Defca0ff3380d4519abcfe4be69c0c02f%26pid%3D100642%26rk%3D1%26rkt%3D1%26It still needs a supply so could possibly use a phone/USB charger though I am always suspicious of them after watching BigClive's tear down on them. Is there a test you can put on them? Might be safer to use a 12v battery.Thanks for some very useful advice.David

author: james_s
author_rank: Super Contributor
timestamp: March 13, 2017, 05:08:37 pm
content: You can easily get -12V. Just build a simple linear power supply yourself, all you need is a transformer, handful of diodes, some capacitors and a 7812 and 7912 to get +/-12V. I wouldn't expect your signal generator to draw very much, you ought to be able to salvage a suitable power transformer out of something. Junk clock radio, microwave oven (with digital controls), etc, or just look on ebay.

author: Seamus16
author_rank: Contributor
timestamp: March 13, 2017, 05:24:17 pm
content: Thanks - yes I do have some lying around but am not confident that I will get it right. Thought I would learn more by building up a kit.

author: james_s
author_rank: Super Contributor
timestamp: March 13, 2017, 05:36:01 pm
content: Well there are kits too, I don't have any specific recommendations but I know they exist.

author: Ian.M
author_rank: Super Contributor
timestamp: March 13, 2017, 06:52:32 pm
content: I was thinking of a pair of these:http://www.digikey.com/product-detail/en/cui-inc/PCSA1-S5-S12-S/102-3947-ND/6181042or similar(+ LC Pi network output filtering), to get +/-12V rails from 5V, then a genuine brand-name USB phone charger or similar to get the main 5V supply. Look for it second hand rather than buying a cheap Chinese fake new.

author: james_s
author_rank: Super Contributor
timestamp: March 13, 2017, 07:01:03 pm
content: Personally I prefer a linear power supply for this sort of thing, they tend to be cleaner. You don't want switching noise riding on the waveform from a signal generator. It's possible to make a SMPS clean enough but it's not trivial.

author: bktemp
author_rank: Super Contributor
timestamp: March 13, 2017, 07:35:02 pm
content: The signal quality from this DDS function generator is probably quite bad (there is no reconstruction filter after the R2R DAC). The even omitted all capacitors on the opamp supply rails to make it as cheap as possible.Therefore I wouldn't spend much money on a clean power supply. The +/-12V are only used for the output buffer, so 25mA should be enough. The 12V rails also don't need to be well regulated, anything between +/-11 and +/-15V should be ok. The 5V rail should be more stable, because it is the reference voltage for the DAC. Using a +/-12V power supply and adding a 5V regulator is a good idea. The current consumption on the 5V rail is probably <50mA.

author: james_s
author_rank: Super Contributor
timestamp: March 13, 2017, 07:36:32 pm
content: That is true of course, it's just a personal preference, I'm sure a decent switchmode supply will work just fine.

author: Ian.M
author_rank: Super Contributor
timestamp: March 13, 2017, 07:55:30 pm
content: Its a budget DDS synthesiser using an ATmega MCU and a HD44780 LCD. You can almost guarantee its going to have digital noise all over its output and I doubt that some well filtered DC-DC converters for the OPAMP power rails are going to make a significant difference. (as bktemp already pointed out). I'd certainly add 0.1uF decoupling caps to ground right at the OPAMP - originally omitting them is crazy!However if you are a purist, start from a 12.6V 10VA transformer, which if you are scavenging from old wall-warts needs to be able to deliver at least 14V into a 500mA resistive load e.g., a 5W car bulb, and half-wave rectify it to get +/-15V rails with enough headroom for a pair of LM78L12 and LM79L12 regulators for the OPAMP +/-12V rails, and a LM7805 (to handle the extra dissipation) for the 5v rail. Its all too easy to under-spec the transformer voltage, and then you'll be in *BIG* trouble if your mains supply voltage droops a bit (but still within its permitted limits).There may be some justification for going for 9V DC in, and boosting that to +/-15V to feed a pair of 12V linear regulators via some LC PI filters for the OPAMP, and using it 'straight' to a LM78M05 ot LM7805 for the ATmega and LCD.

author: Seamus16
author_rank: Contributor
timestamp: March 13, 2017, 08:39:36 pm
content: Thank you for all the suggestions.I realise that these kits are going to have compromises on them but I feel I will learn more by going this route.On a point of clarification re power plugs, since they are all insulated and no earth connection then their output is safe to use without using an isolation transformer.

author: james_s
author_rank: Super Contributor
timestamp: March 13, 2017, 08:49:33 pm
content: Yes, the only time you need an isolation transformer is when you want to poke around in something that has a "hot chassis" design where the circuit ground may be floating at a substantial voltage. Primary side of switchmode power supply, many CRT TV sets, stuff like that.

author: Ian.M
author_rank: Super Contributor
timestamp: March 13, 2017, 08:58:01 pm
content: Quote from: Seamus16 on March 13, 2017, 08:39:36 pmBy power plug, I assume you mean plug-in power adapter - what the leftpondians colloquially refer to as a 'wall-wart'. Yes, assuming you haven't got a 'China Export' CE marked fake and you don't float its output more than +/-50V away from ground.Things *NOT* to do - anything that resembles using a wall-wart PSU to directly power an Arduino control or monitoring circuit riding on top of the +300V HT rail of your new valve amp build!

author: Seamus16
author_rank: Contributor
timestamp: April 07, 2017, 08:26:37 pm
content: As an update to this, I have now got hold of an isolation transformer but cannot workout whether it is earth referenced. I am enclosing some pics below.Looking at the wiring the primary is on the right hand side and you will notice that the earth disappears into the resin and is attached somewhere. All earth points are connected together.On the secondary side (left) with the power off there is no continuity between between either outlets and earth.With the transformer powered up (I will call them live and neutral although I know that is not strictly so).Between Live & earth I get 108vBetween Neutral & earth I get 91vTo me it looks like the earth centrally tapped though probably wrong.So please let me know if this is ground referenced or not and whether I just need to remove the secondary earth wire.

author: james_s
author_rank: Super Contributor
timestamp: April 07, 2017, 08:34:33 pm
content: You can tell everything you need to know with a multimeter. Use the Ohms setting and check all input wires to all output wires, there should be no continuity between anything on the input and output side.

author: Zero999
author_rank: Super Contributor
timestamp: April 07, 2017, 09:05:10 pm
content: Quote from: Seamus16 on April 07, 2017, 08:26:37 pmGoing from your measurements, it's unlikely it's centred tapped to earth.91 + 108 = 199V which is much less than the rated voltage and indicates it's just capacitive coupling.The earth wire goes to the transformer's core and there may be a screen between the primary and secondary to cut capacitive coupling between the two windings.

author: Seamus16
author_rank: Contributor
timestamp: April 07, 2017, 09:06:06 pm
content: Thanks - that was quick.There is no continuity with if off. Are saying to check when it is powered uphttps://www.eevblog.com/forum/Smileys/default/undecided.gifBedtime here so I am off to it now.

author: james_s
author_rank: Super Contributor
timestamp: April 07, 2017, 09:13:55 pm
content: No, check with it disconnected, there is no reason to power it up. You can also try connecting an incandescent lamp between any output terminal and earth ground, it should not light up. You may measure voltage from capacitive coupling but there should not be any significant current behind it.

author: Ian.M
author_rank: Super Contributor
timestamp: April 07, 2017, 09:58:43 pm
content: It sure looks like that secondary side ground goes to the core and the primary side ground.I'd disconnect it and fit two 4mm sockets, one for 'Mains Earth', connected to the wire you just removed, and the other for 'Isolated Socket Earth Pin' wired to the socket Earth pin. Then you've got the option to ground it if you are only using it to float L and N because you are running some gadget you don't trust the PSU wiring in or to reduce mains borne noise, or leave it fully floating for test/repair bench safety with the DUT power circuit exposed.It *may* be worth adding 4mm sockets for secondary side L and N - occasionally you may need to bond the derived supply N to E, and its convenient to be able to directly plug in a multimeter if you are using it after a Variac, but make sure the L and N sockets are the deep shrouded type and safe for 240VACIf it hasn't got one, add a 2A circuit breaker. Although its rated for 750VA (3A) for half an hour, if you push it that hard for longer you may end up blowing a non-resettable thermal fuse buried in the winding under all that potting compound.

author: Zero999
author_rank: Super Contributor
timestamp: April 08, 2017, 01:43:42 pm
content: I Googled the part number.I'm certain the secondary is floating. I've used the same range of transformers before. They're very good. It even has a resettable thermal overload protection.http://carroll-meynell.com/isolation-transformer-cm7501-230http://uk.rs-online.com/web/p/safety-site-transformers/2604238/

author: Gyro
author_rank: Super Contributor
timestamp: April 08, 2017, 03:50:40 pm
content: Yes, an isolation transformer with a standard UK mains socket on the output will have a floating secondary. The only ones I know of with center tap earth (ground) are the 110V (55-0-55) building site ones with the industrial sockets.Normally the mains earth is carried through to the output socket earth, but this is sometimes disconnected for specific applications (the isntructions for my RS one mention it as a user configuration option).

author: Seamus16
author_rank: Contributor
timestamp: April 08, 2017, 07:22:17 pm
content: Quote from: james_s on April 07, 2017, 09:13:55 pmI have tried it with a normal light bulb (checked it lights beforehand) and there is no light when connected between secondary outlet and earth (see pics). This is a case where cannot always trust what a voltmeter is showing. Todd Harrison did a video on this. When the light is connected there is no voltage showing. I suppose this is a case of phantom voltage.

author: james_s
author_rank: Super Contributor
timestamp: April 08, 2017, 08:32:24 pm
content: The issue is that modern multimeters have a very high input impedance so they are very sensitive to capacitive coupling and picking up RF out of the air. You could solve this sort of situation by having a lower input impedance, say 100k across the meter in parallel with the probes.