Power supply for voltage references

[branadic]

Hi all,

triggered by the W/F7000 I thought it would be interesting to talk about power supplies for voltage references. Battery power is indeed a proper solution and Andreas showed a design that provides the voltage out of a set of 12x NiMH for the LTZ reference via an LT1763, while applied with BMON (battery monitor) a charge circuitry is present.
But what about mains power, with extrem low coupling capacity?

John Pickering obviously developed a DC-DC converter solution that seems to work pretty well. So I was looking for info about it on the web and came across the following papers, describing the solution. Carefully reading all three of them about the xformer together with the pictures of W7000 available on xdevs provides an overall picture how it's done:

Design of a 10 mA DC current reference standard
Design and evaluation of a 10-mA DC current reference standard
A new compact isolated power supply for electrical Metrology at low signal levels



So I wonder if anyone else created a similar solution, providing high isolation between primary and secondary, so that it is possible to power the reference from mains. What xformer was used?

Edit: Guidance on eliminating interference from sensitive electrical circuits

Edit: Design and Evaluation of a 10-mA DC Current Reference Standard states a frequency of 8 kHz for the DC-DC converter, which agrees with the frequency I can measure on my F7000 based on LT1533

-branadic-

[Kleinstein]

For extremely low capacitance / high voltage, there are acoustic transformers (piezo - isolator - piezo). However they are relatively exotic and usually for low power (like 1 W).

The usual small higher frequency transformers work better with a low voltage. So this would likely be in 2 stages: 1st a conventional mains transformer to some 5 V DC and than a special, low capacitance ferrite or similar transformer. Dave sowed a nice example from the Clever-scope.

If efficiency is not that important, on can have the primary and secondary windings on opposite sides of a relatively large ring core.

[exe]

Am I correct that you want a tranny with low capacitance between windings to avoid getting noise from mains?

R-core transformers?  I resist buying one, although I may pull the trigger. Alternatively, a toroid with a shield between primary and secondary might also be an option. May be even a conventional EI transformer with a gap between windings would suit you?

Anyway, even if we eliminate capacitance between windings, magentic coupling is still quite strong up to some tens of kHz or even beyond. I know this because people use small EI-trannies as injection transformers.

[doktor pyta]

Yup.
Resonant current fed push-pull operating at about 20kHz.
As far as I remember parameters were:
Supply:12 V, output 16V @0.3A. Efficiency 50-60%.
Capacitance C(in-out)=4pF.
Pros: very easy to build with commercial components, I've never observed problems with start-up.

[BravoV]

For extremely low capacitance / high voltage, there are acoustic transformers (piezo - isolator - piezo). However they are relatively exotic and usually for low power (like 1 W).

The usual small higher frequency transformers work better with a low voltage. So this would likely be in 2 stages: 1st a conventional mains transformer to some 5 V DC and than a special, low capacitance ferrite or similar transformer. Dave sowed a nice example from the Clever-scope.

If efficiency is not that important, on can have the primary and secondary windings on opposite sides of a relatively large ring core.

This one ?

[tszaboo]

I have two ideas for you:
- multi stage transformer (230 to 48V, 48 to 12)
- Jim Williams magic:
https://www.analog.com/media/en/technical-documentation/application-notes/an29f.pdf

There is a circuit, where he drives a transformer with a 1-2 KHz sine wave that comes from a current boosted opamp. Crap efficiency, but small and not noisy, so who cares.

[notfaded1]

Does anyone know why they refer in the IEEE paper to "The value of 10 mA, based on engineering considerations, seemed suitable for all of the following applications, allowing a single design of dc current reference, referred to as the PBC, to cater for each."  Anyone have any idea why they called them PBC?  Just curious.

Another couple questions... maybe dumb but it says "The five prototype units were each connected to a 100Ω resistor and measured daily with an MIL 8000 against Fluke 732B standards traceable to METAS/Bern. The long term drift can be seen in Fig. 2. When the measurement resistors were replaced by 1 kΩ , the inter-comparison could be made directly with the Fluke 732B 10-V standards through the DataProof scanner and uncertainty reduced. It is estimated that a group of four units can easily provide the 10 mA with an uncertainty of five parts in 10^7 constantly maintained.

What's a MIL 8000 and a DataProof scanner?  I'm guessing the DataProof scanner comes from here: http://www.dataproof.com/

Bill

[Vgkid]

Measurements international binary voltage divider
https://mintl.com/products/model-8000b-10-volt/

[Conrad Hoffman]

The construction details of the GR shielded bridge transformers are interesting- https://www.ietlabs.com/pdf/GR_Experimenters/1935/GenRad_Experimenter_Oct_1935.pdf
You can also shield a toroid with copper tape.
Me, I'd probably use a couple solar cells, lit up with some big incandescent lamp! Can't get much better isolation than that.

[dietert1]

After experiments with high frequency converters (art of electronics) i am now using something very simple and old fashioned. It's based on a small two chamber mains transformer with low coupling (20 pF) plus two extra 2K2 resistors on the input side (RF suppression). Like all two chamber transformers the secondary needs a proper snubber. Then there is a LM723 regulator with a 30 mA current limit. Outputs are adjusted to 15.000 V with fixed resistors. Temperature drift remains below +/- 3 mV in our lab. Image shows a 750 Ohm resistor i used for test (20 mA at 15 V). This circuit will inject less than 1 uArms current at 50 Hz into the ground line, so with reasonable wiring errors will be a fraction of a microvolt.

I found that our LTFLU references shifted by about 0.15 ppm when i replaced the previous LM7815 based power supplies (e.g. from 14.79 to 15.000 V). That means even a well designed reference will change when you change the supply voltage by such an amount.

Regards, Dieter

PS: I made six of those and have parts for more. Open two chamber transformers where you can ground the core are superior, but the minimum size i know of is 12 VA (much bigger) and the difference in performance is small.

[babysitter]

I digged trough a old Siemens patient monitor with exchangeable modules, they used a IR communication and two pot-core ferrites for power transmission, mounted like "E  3" . Think inductive charger. They did it for low leakage and cleanable surface, but should give low capacitance too. Maybe a royer circuit would be a good start for the primary part. Load the secondary to prevent high output voltage.

[SilverSolder]

The construction details of the GR shielded bridge transformers are interesting- https://www.ietlabs.com/pdf/GR_Experimenters/1935/GenRad_Experimenter_Oct_1935.pdf
You can also shield a toroid with copper tape.
Me, I'd probably use a couple solar cells, lit up with some big incandescent lamp! Can't get much better isolation than that.

I have a couple of double shielded tranformers made by Singer/Gertsch and they are also 1:4.

What is the reason for these double-shielded transformers often being made with a 1:4 turns ratio?

[branadic]

I found that our LTFLU references shifted by about 0.15 ppm when i replaced the previous LM7815 based power supplies (e.g. from 14.79 to 15.000 V). That means even a well designed reference will change when you change the supply voltage by such an amount.

I must add that W/F7000, F7001, F7004 and F7010 are powered by a Hitron Model HER-57-312 wall adapter with 12.88V and 1.2A. This powers the DC-DC converter, so it's already kind of a two stage concept.
Attached is a measurement of the averaged output on F7000T, running the modules from their internal batteries in the first half and from the wall adapter in the second half. As far as I can see there is absolutely no difference. One can repeat that of coarse on the individual units too, just in case there was some cancelling effect going on.

-branadic-

[niner_007]

I was thinking of starting a thread on it the other day

My thoughts on this are to use rechargeable batteries, stored in the same heavily shielded metal enclosure, alongside the reference, like in a Fluke 732. For the batteries, something practical with lots of energy and long life, like LiFePo or LiIon. I do not know what kind of noise these have, if lead acid has lower noise, that can be an option too. Keep two independent battery banks, one that is being charged, and one that is used to power the reference. As one bank is completely discharged, you switch over to the other bank. That way you remove any noise from ever entering the system from the outside, with runs with the assumption that the batteries are relatively low noise.

For regulation or step down to a practical range, I was thinking of a simple npn - zener regulator, using a low noise zener diode (1N5518, 1N5519, 1N5520, 1N5521, 1N5522, 1N5523). Another option that could be great to try is using the LTC6655 low noise reference. It could also be used to reduce the noise of the LT3042, which on its own has something like 10uV of noise 0.1 to 10Hz, and 2uV of noise 10Hz to 100kHz.

[TiN]

For the batteries, something practical with lots of energy and long life, like LiFePo or LiIon.

Ouch, that means good bye shipping for calibration without paperwork/etc.

[dietert1]

I liked the Cleverscope transformer, thanks. Using a twisted pair bifilar transformer with symmetrical drive in resonant mode (like in the art of electronics proposal) may also remain below 1 uArms of current injection. It's just a matter of designing a 3D printed part to hold off the secondary. That's something to try. I still have the circuit somewhere on the bench.

The AD paper AN29 mentions 30 uV somewhere, that's a lot. And high frequency is more difficult to absorb into wiring due to cable inductance.

How to measure the injected signal? Use two scope channels, one to trigger on the converter frequency (proximity, ground lead remains open), the other one to measure injected signal over a 1 KOhm resistor. One side of the resistor gets connected to converter output ground and probe tip, the other side gets connected to the ground lead of that probe. Then use averaging to raise injected signal above noise. With a modern DSO this will get you down to some 100 or 500 uV sensitivity, which then corresponds to about 0.1 to 0.5 uA injected current. Need to avoid clipping though: The scope power supply also injects its own asynchronous noise. Clipping will screw up averaging, so you may have to adjust scope sensitivity before.

Regards, Dieter

[notfaded1]

@niner_007  Another nice thing about using two sets of rechargeable batteries is you can change out the one side with new batteries while the other side is running.  You'd never have to come off of battery power that way ever and always have good fresh batteries.

Bill

[branadic]

Repeated the measurement battery vs. wall adapter this time on cell1 only, I leave it up to you what you can spot.

-branadic-

[notfaded1]

It's definitely doesn't look as stable as before.  Downward slope.

[Conrad Hoffman]

The construction details of the GR shielded bridge transformers are interesting- https://www.ietlabs.com/pdf/GR_Experimenters/1935/GenRad_Experimenter_Oct_1935.pdf
You can also shield a toroid with copper tape.
Me, I'd probably use a couple solar cells, lit up with some big incandescent lamp! Can't get much better isolation than that.

I have a couple of double shielded tranformers made by Singer/Gertsch and they are also 1:4.

What is the reason for these double-shielded transformers often being made with a 1:4 turns ratio?

Not a clue, but no doubt there was some sensible reason, possibly a desirable impedance conversion.

[niner_007]

For the batteries, something practical with lots of energy and long life, like LiFePo or LiIon.

Ouch, that means good bye shipping for calibration without paperwork/etc.

Can't have everything in life

You can build it modular, if you don't want batteries, it can be mains power. You can enable any chemistries also, and any capacities. Volumetric energy storage is what I mean, the batteries can be as low as 0.5Ah, it all depends on how long you want the system to run without external power.

[Jay_Diddy_B]

Hi,

The LT1533 is well suited to these applications. It has controlled slew rate for both current and voltage. This allows you to trade efficiency for low noise.

Link: https://www.analog.com/en/products/lt1533.html

If you don't need isolation, you can look at the very similar LT1534.

Regards,
Jay_Diddy_B

[dietert1]

The LT1533 specification of the injected signal seems to be 100 uVpp, including high frequency transients as visible on the scope trace, so once more a lot. That is because they are using switches, not linear drive as in the art of electronics proposal. Of course it also depends on the transformer geometry.

I remember from my experiments with the art of electronics circuit, that depending on the transformer the injected signal at the secondary ground could be between 300 uArms and 1 uArms. It got lower with lower frequency e.g. 20 KHz instead of 50 KHz and when using a slotted copper foil separating primary and secondary coils. I also used plastic insulator to hold both coils away from the core. This was with a resonant circuit and with primary side amplitude regulation and after a 100 uH common mode choke on the secondary side. That circuit gets tricky if you want to reach 1uA rms or less.

Regards, Dieter

[fcb]

Interesting subject. We've used a hugely oversized E3 transformer running at 100KHz in one of products to lower capacitance between two circuits.

I liked the Cleverscope arrangement, but my favourite is the Keithley DMM7510 teardown Dave did that uses coax windings.  Newtons 4th (voltech) have an interesting method using single turns on ferrite rings with copper tape guards.

To get round capacitively coupled noise on something i'm finishing at the moment I've taken to shutting off the PSU, taking a measurement (80us at 30Hz), and then restarting the supply (100KHz).

[fcb]

This I've seen done with a single winding (like the joined secondaries) and with a copper tape screen on the primary.

I'd probably do it with a 3D printed bracket to guide the windings off the core (reduce capacitance further).  This is also used for data transmission in some power-analysers.