Let's start from square one...

[WM6]

I don't see any advantage in the mono coil.

Sorry Tinkerer, but this sounds me like: why simple if we can complicate.

Mono coils are (comparable) still deepest in PI design.

You can build 5 different mono testing coils in time of building only one of others design.

At the end I can not imagine bigger PI coil in other than mono design.

Unshielded mono work very well too and deeper.

If there is some specific drawback by mono coil it does not outweigh advantages. Vote for mono.

[Mechanic]

Hi Guys,

I am wondering, why are we designing the power supply first? IMO this should be done last, as we do not know the requirements of the rest of the circuit yet. For development I would use battery's and ldo's cause they are easy to setup and change to suit the developing requirements. It will also give a good benchmark for the completed supply.

Cheers

[Gimp]

Nice looking switcher

Bottom of page 18.

12V (+/_) supplies at 100ma each.

Simple design.

Change the supply to two LI-ion batteries, tweek the feedback resistors and you could boost the voltage to +/_13V. Post regulate for noise immunity.

Gimp

bunch of crackpots

Hi Steven,

+12 V for TX
+5V for digital
+5V for analog
-5V for analog
+3.3 V for analog

Tinkerer

WHO STARTED this THREAD.........power supply may be good.....but need not be complicated....try isolating the Tx power from Rx using 2 coils ie one coil for Tx one coil or Rx, use an opto isolator for Rx sampling, trigger etc etc....see twin loop treasure seeker thread

[Gimp]

Another possibility is to increase the coil voltage to 13V, then use a straight boost converter. Something along the lines of the TL3577.

[Tinkerer]

Power supply

I'm not sure how much help I can be. My only experience with switchers is using National Semi Simple Switcher (design on WebBench).

One thing I do know is ground layout is critical. Single point ground under the switcher with the feedback resistors kept as far from the inductor flux path as possible.

I've spent a couple of hours this evening on WebBench without any luck. The biggest problem I'm seeing is the low steady state current demand. Most of the switchers don't have good efficiency till you are running over 1A continuous output current. It looks to me like the average current it going to be around 100mA. (My Pulse II is right at 100mA depending on settings.)

I'll keep looking.

Gimp,

thanks for the effort. Maybe we should look at this challenge from a different angle.
We want to design an improved PI detector.
Lets look at ways to improve the HH design.
There are a few decisions to take:

• How much power do we want to use, or, if we have the possibility to chose, what is the minimum power use and what is the maximum power use.
  • How do we want to divide the power use. 50% on the TX and 50% on the signal processing?
    Or 80% on the TX and 20% on the processing? Other ideas?
    Cost considerations:
    Using LI-ION Polymer batteries that are very light, we can use more power, but at higher cost. Higher battery voltage also means higher cost. What is more expensive, a SEPIC power regulator to produce 12V from 8.5V to 12.75V or a 14.5V battery and a BUCK configuration?
    
    Any input is welcome
    
    Tinkerer
    

[Gimp]

I'm not sure how much help I can be. My only experience with switchers is using National Semi Simple Switcher (design on WebBench).

One thing I do know is ground layout is critical. Single point ground under the switcher with the feedback resistors kept as far from the inductor flux path as possible.

I've spent a couple of hours this evening on WebBench without any luck. The biggest problem I'm seeing is the low steady state current demand. Most of the switchers don't have good efficiency till you are running over 1A continuous output current. It looks to me like the average current it going to be around 100mA. (My Pulse II is right at 100mA depending on settings.)

I'll keep looking.

[Tinkerer]

Power supply

If you are concerned about the switcher noise, follow it with an LDO (low dropout linear regulator). Set your switcher voltage at the voltage you need plus the LDO dropout voltage plus some margine (say 0.2V). This will keep your efficency high, and still offer you the low noise of the linear regulator.

Steven

Hi Steven,

do you want to get involved? Here is what we need:

Li-Ion battery has 11.1 V nominal voltage, but peak charge of 12,75V
and a low limit of 8.55V
So we want to use a SEPIC to get regulated:
+12 V for TX
+5V for digital
+5V for analog
-5V for analog
+3.3 V for analog

Analog supply needs to be low noise. Best would be isolated.
All this with low losses and low cost.

Quite a challenge, isn't it?

Tinkerer

[Gimp]

Switcer Plus LDO

If you are concerned about the switcher noise, follow it with an LDO (low dropout linear regulator). Set your switcher voltage at the voltage you need plus the LDO dropout voltage plus some margine (say 0.2V). This will keep your efficency high, and still offer you the low noise of the linear regulator.

Steven

[gwzd]

Not afraid, just cautious. There are many situations where you want to avoid switchers.

Yes, there are a few, I don't see this particular one being one of them

In this case, the trade-off is between a little more efficient power supply,

Are you serious? a "little"?

versus potential performance degradation.

In which way? noise, radiated EMI, current spikes making miniature magnetic fields? These are not retorical questions, but real ones. You know better the specifics of this kind of NDT instruments and can bring light into the subject.

In a first cut, I would go with linear,

Yes, a possible, but absolutely unnecessary step. Well, I'd spare the time and use it for something else, writing in these forums for instance.

then if switchers can offer a decent improvement in power consumption try them out, and do a direct comparison.

- Carl

Yes, well, as you said that would be the cautious way of doing things, and probably you'd be right. However, putting down the specs and sticking to them is another way and you slip the making of two boards.

I saw you used in your HH a 7660 with a 10kHz switching frequency, I don't see how that is better than using switchers with 1MHz frequencies that your slow opamps aren't even able to see. The only thing that comes to mind is price, but in no way performance. Are there any subtle reasons for that?

I most agree with you that doing a reasonably good compact design on a two layer board using switcher is a challenge, but I thought that is the beauty of these forums. You find here people with very unorthodox way of thinking and I was expecting something like that. By any chase have you look inside a Keithley 2502 picoamperemeter? I have and there are a few switchers there, that's to say the least. Well, they go down to 15fA without any problems.

Regards,

[Carl-NC]

There are advantages to a 2-battery system. For sure less crosstalk. For sure better tailored batteries for each function. But I've never liked multi-battery systems because of inconvenience and weight. And I think it's not difficult to design around a single battery. However, if there is a clear case for a second (i.e. a usable performance improvement) then I will certainly reconsider.

- Carl

[GT Blocker]

Q, Carl,

Hey there Carl,
I have some thoughts. You said above,"While it might be more efficient, I don't care for this approach. I prefer a single battery (pack)."

What is the difference between the coil's needs and the circuit's needs?
Without the coil's needs, how long could the circuit run on a smaller Li?
And would there really be an advantage to a smaller Li for the circuit and a larger one for the coil?


I wasn't thinking all that much about pwr efficiency as much as cross talk, et al.

Thoughts..........

[Carl-NC]

Although not necessarily, but not excluding neither.
Why are you people so afraid of switchers? Just use good ones not 20 years old stuff, put some thought into the layout, filtering, synchronization, and screening and you'll have something much more efficient, lighter, nicer than the lame linear regulators alone. But, yes, it takes a little bit more refinement and ingenuity to do those.
Regards,

Not afraid, just cautious. There are many situations where you want to avoid switchers. In this case, the trade-off is between a little more efficient power supply, versus potential performance degradation. In a first cut, I would go with linear, then if switchers can offer a decent improvement in power consumption try them out, and do a direct comparison.

- Carl

[gwzd]

Do you mean for the coil TX circuitry, or all the circuitry? The TX circuitry can tolerate a lot of variation. Everything else needs to be regulated, but not necessarily using switching regs.

- Carl

Although not necessarily, but not excluding neither.
Why are you people so afraid of switchers? Just use good ones not 20 years old stuff, put some thought into the layout, filtering, synchronization, and screening and you'll have something much more efficient, lighter, nicer than the lame linear regulators alone. But, yes, it takes a little bit more refinement and ingenuity to do those.
Regards,

[Tinkerer]

Coil weight?

Maybe, but I don't see it off-hand.



I agree, don't put off the power supply until the end, but consider it up front, and make changes as necessary.

While it might be more efficient, I don't care for this approach. I prefer a single battery (pack).



Certainly if you use a switching reg, it should be synchronous. But it can still cause problems because, as the load current varies, the impact of the switching reg will vary.



Do you mean for the coil TX circuitry, or all the circuitry? The TX circuitry can tolerate a lot of variation. Everything else needs to be regulated, but not necessarily using switching regs.

- Carl

With a single coil configuration the voltage difference makes a difference in the TX coil current. This in turn demands a change in the damping resistor. It is not critical, but will reduce sensitivity to small fast targets.
With IB coils the change in coil current is VERY critical.

One way out could be a coil current control or constant current control.

Using Nicads or NImh batteries, the voltage range is less, but the future seems to be in LI-Ion.
I agree that everything should be regulated.
To get high performance the noise factor is probably the most important one.

Tinkerer