Power supply for voltage references

[antintedo]

If relays like the AGQ200 (we want very low capacitive coupling)

Pickering 103 series might be more suitable - they have guard shield and the capacitance is specified in the datasheet, it's on the order of 0.1pF. They have much higher lifetime as well. Some small signal relays have high frequency isolation specs. I've seen this in G6K series, capacitance derived from the graph was <0.2pF.

[Echo88]

Unfortunately the Pickerings arent easily available from Mouser/Digikey. But the general idea to use Guarded Relays is nice. Can you explain how you derived the coupling capacitance via hf-isolation spec, since im not a hf-guy?

[antintedo]

Can you explain how you derived the coupling capacitance via hf-isolation spec, since im not a hf-guy?

Me neither, but my understanding is the isolation test can be modeled as a voltage divider with a capacitor in one leg and 50 ohm termination in the other. Capacitor impedance will be 1/(2pi*f*c), so substituting numbers, for G6K -86dB at 1MHz the capacitance turns out to be 0.16pF.

[dietert1]

A small reed contact from my drawer wasn't more than 0.1 pF (a bit difficult to measure). Another, larger one had less contact distance and it gave about 1 pF. Then with a reed relay there will be additional coupling along the coil, so to keep coupling low a metal foil gets inserted between contact and coil. The Pickering relay has that shield and researching the web i saw others like that. I also measured an Aromat RF relay in a VXI E1473A multiplexer, they are below 0.1 pF as well.
The same was true for a Teledyne 732-12, once i connected Guard to the metal can. That one is DPDT, so it replaces four of the Pickerings. But it's small and the wires close and difficult to guard from each other.

General purpose DIL type relays in plastic enclosure had about 0.4 pF. There are special RF relays like Axicom HF3 with 60 dB isolation at 2 GHz. -60 dB in a 50 Ohm system indicates an isolation impedance of 50 KOhm at 2 GHz. That is a capacitor of 0.0016 pF. Strange. Have seen some RF relays where they create shorts on the inactive ports to improve isolation.

If this discussion is about building a multiplexer for two rechargeable batteries, then there will be eight contacts, with four of them open at a time. Then you put the batteries into two separate guarded compartments and you may want to switch those guards between primary and secondary, so maybe five open contacts at a time. Plus there will be stray capacitance in the wiring near the relays. If you get down to about 1 or 2 pF coupling in the end that would be a nice result, similar to the DMM6500 number given above.

Regards, Dieter

[antintedo]

Maybe this image and an old version of the datasheet will shed some light on the construction of HF3 relay:
http://mt-system.ru/sites/default/files/docs/documents/File/TYCO_PDF/HF3.pdf

It might be that there is no direct line of sight between common and switchable contacts. There are some recessed spaces in the base of the relay where signal contacts are supposed to be. When the armature is not connected, it might be shorted to guard - big yellow contact on the left of the image. I could not find any relevant patents, so the best course of action would be to buy one and disassemble it.

[Kleinstein]

The relay looks very much like using 2 contacts in series and maybe even connecting he movable part to ground when open. This can explain very good RF isolation. However the good RF isolation does not mean there is low capacitance of the additional ground (would be an extra shield potential) is not there.

Even when not using the extra shield / ground it can still be a good relais.

[branadic]

Some progress to share:
- boards designed, ordered and received
- cores for primary and secondary side wound
- CAD design of the shield and arrangement drawn
- first samples of the shield fabricated and checked for fitting into each other
- finalized the CAD drawings (dimensions slightly adjusted to tighter tolerances)

Now the final parts are being fabricated. Components on order, so hopefully a first version can be assembled and tested soon.

-branadic-

[EmmanuelFaure]

Nice job branadic! Is the plastic shield made of some conductive plastic for EMI shielding?

[branadic]

Exactly, the shield is made from a conductive material.
Meanwhile I got all parts done to complete at least the transformer.

With finally installed transformer cables to the board, but rest of the PCB unpopulated, I can measure some 17 - 18pF between primary and secondary ground. However, on the original 7000 board some 1nF/1kV cap (blue) can be seen. So not yet sure what this number means by itself.

-branadic-

[dietert1]

In my opinion that 1 nF capacitor is a mistake unless it is necessary to protect the optocoupler from malfunction. Are you using an optocoupler with built in guard?

Like Andreas proposed above, with two shields of the transformer you have several options how to wire them. When making a guarded solution you can connect the coupling wire to guard - it doesn't have voltage on it. Then the optocoupler should move next to the transformer and maybe get a guard trace as well.

Regards, Dieter

[fcb]

17-18pF seems quite high, is the complexity worth it? I can see that the isolation voltage would be reasonable and noise shielding possible.

[Kleinstein]

17-18 pF looks quite high. This should be capacitance between 2 shields - though I somewhat don't see the connection in the pictures.
So the injected current can still be small, as there should be essentially no AC voltage across the capacitor.
The shown transformer could be relatively high power - from the sizes I would consider 20-100 W possible.

If needed one could probably reduce the capacitance with a little more space between the coupling windings and the inner part. For lower  power thinner and less (e..g. 4-5 loops at 0.6 mm²) wires should be Ok too.

[branadic]

Just to clarify, the transformer shown above is a direct replica of the Pickering in F7000 with LT1533, so same dimensions, same amount of copper etc.
With respect to the report linked in the first post, this circuit and transformer was designed for 7W.
The capacitance was measured between primary GND/shield and secondary GND/shield.

-branadic-

[ignilux]

Hi, all-

I just want to clarify my understanding of the concern over transformer capacitance. The principal issue seems to be the coupling of noise between the outside world (primary side) and the circuit (secondary side), correct? Magnetic coupling is largely mitigated with a toroidal core, but that does nothing for electrical coupling through the parasitic capacitance. Hence, trying to minimize that quantity. Am I all good so far?

Here's where it gets a bit fuzzier for me. On first glance, I would assume that most of the electrically coupled noise is common mode, since the noise would have coupled equally to both the hot and neutral conductor in unshielded mains wiring (Romex, in the US). If that's the case, why couldn't the noise be reduced with the use of a common mode choke on the primary and/or secondary side of the transformer? There must be more to it than that, otherwise all of the knowledgeable people here would have suggested it by now.

Anyway, I'm in the process of building a power supply for my AD587JQ and was thinking of starting a thread on the subject, so I'm glad to have found this discussion. Branadic, what type of toroid core are you using in your DIY Pickering? Sorry if I missed that.

Cheers!

[Kleinstein]

The main problem with a simple transformer is common mode injection due to point with different voltage in the coils.  The shield can reduce the voltage to less than the votlage of one turn - below that one has to balance and chose the grounding point right.  So even with a shield, low capacitoance is good.
A common mode choke is of limited use, when at a very good isolation level. It is only am additional seires impedance and this may not be much compared to some 20 pF of capacitance at the transformer.
It mainly helps to the the higher frequency part. For the lower frequency part a low capacitance transformer is more effective.
I would still consider a CM choke worth while, but the transformer should still be low capacitance, especially if the shield is not good.

There may be also the case that the rest of the circuit has some CM voltage to ground - so good isolation is definitely helping and even just capacitance to ground can be negative.
 

[Andreas]

If that's the case, why couldn't the noise be reduced with the use of a common mode choke on the primary and/or secondary side of the transformer? There must be more to it than that, otherwise all of the knowledgeable people here would have suggested it by now.

Hello,

the frequency ranges where a shielded transformer (< 1 MHz) and a common mode choke (> 1MHz) work effectively is different.
So usually you have to use both.

Anyway, I'm in the process of building a power supply for my AD587JQ and was thinking of starting a thread on the subject, so I'm glad to have found this discussion.

Mhm,

for a AD587 reference (< 5mA) I would always use batteries. 12 AAA cells last 7-10 days before you need to recharge.
See also my AD587LW design:

https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1449494/#msg1449494

with best regards

Andreas

[dietert1]

Yes, a common mode choke helps, too. And it isn't easy to make one with low parasitic capacitance, either. Sometimes people combine common mode chokes with coils and sheath current filter (ferrite on cable). A primary side common mode choke works best when combined with a guard. Then you get an inductive divider between the choke inductance and the effective inductance of the guard net.
As always some people try to avoid own studies by "best practices" approach, others try to get own hands-on experience. When i made a short on the probe tip (with its own ground clip), put the probe onto the table without any other connection, adjusted the trigger for mV and turned on/off the microscope illumination (transformer inside!), i captured how that mains switch sparked. For precision measurements you don't want to have spikes of several mV or tens of mV in your circuit, so it will have to be shielded/guarded, like "best practice" tells you for any other audio or RF setup. Some people even connect an amplifier/headphone to critical signals and listen to them..

Regards, Dieter

[ignilux]

Thanks for the replies. I hadn't really considered the orders of magnitude difference between frequencies where each solution is effective, but that definitely makes sense.

Andreas, I'm aware of that thread and am basing much of the project on your design. For the time being my most precise voltmeter is a 5.5 digit Fluke 8840A, so I don't feel the need to go all the way with ppm tempco compensation, etc. It's more for the learning process, and to have a accurate reference on hand for other uses. To that end, I'd prefer to have a mains-powered device that I can leave powered on 24/7. Sealed lead acid might be nice, too, if I go for a battery powered solution. That would extend the run time quite a bit, eh?

Anyway, thanks again!

[KT88]

@Dietert1: Common-mode chokes won't help if one side is floating as a filter consits of a passing- and a shunting element. The shunting element is missing here. There would be some capacitance against ambient though - this would unfortunately resonate at some frequency...
The most viable measures to mitigate CM noise in this case is reducition of coupling capacitance, shielding and symmetry.

Cheers

Andreas

[dietert1]

No, sorry, a primary side common mode filter is an established method to keep the guard/shield clean.

As i wrote, a common mode choke helps in combination with guard. In that case it reduces excitation of the guard net by spikes from outside. Ideally guard would be a Faraday cage, with a very high resonance frequency depending on its dimensions. Below that frequency the inner of the cage would be well protected against excitation from outside. But in general it isn't. It consists of many partial guards in many instruments, connected to each other by (inductive) wiring. So the resonant frequency of the guard net will be somewhat lower. By the way introducing some 50 to 100 Ohm resistors into the guard net allows you to make measurements.

Regards, Dieter

[KT88]

I agree on the placement at the primary side. I was referring to your first, more genral statement. I have seen a lot of examples where CMCs were placed on floating parts of a setup without the desired result of reducing EMI. I some cases during EMC tests some frequencies were even exaggerated depending on cable length...

Cheers

Andreas

[d-smes]

Exactly, the shield is made from a conductive material.

If you used conductive plastic, beware that this creates a shorted turn on the torroid.  I suspect resistivity of the conductive plastic is high enough, or that contact resistance between the two halves is high enough, that the current in the shorted turn becomes meaningless.

[Echo88]

@d-smes: The shields are designed to be just conductive enough to fulfill the shielding while only dissipating neglegible power.
Please read https://eprintspublications.npl.co.uk/1888/1/bemc99-7.pdf page 3 Figure 6 discription.

[exe]

@branadic what plastic did you use for the shield?

[ignilux]

What's the consensus on so-called "semi-toroidal" transformers like this series from Bel Signal? It looks to me like an R-core, but made with a laminated core rather than a secret sauce composite.