Surf PI Pro - 10 AA NiMH Cells?

[hobbes_lives]

I was thinking of running my Surf PI Pro with 10 AA NiMH cells. I currently use 8 NiMH cells in the standard pack, but think that I may get better performance if the supply voltage were closer to what it should be, 12 volts. Since NiMH cells average around 1.2 volts, 10 NiMH cells would be closer to the 12 volts that would be supplied by the standard 8 AA alkaline cells.

My concern is that when the NiMH cells are fully charged, they may be a little over 1.2 volts for a period of time. I don't see any components supplied by the 12 volts that should get damage, but want to be safe instead of sorry.

Has anyone tried this before with a Surf PI Pro or Surfmaster PI?


BTW, this is my first post, but I've been a lurker for quite a while.

[Geo]

Hi.
Surf PI has a voltage regulator, so the voltage is the same. The only components that has the battery voltage (12V) are the transistor output that drives the coil and the oscilator (tlc555).

[hobbes_lives]

Hi.
Surf PI has a voltage regulator, so the voltage is the same. The only components that has the battery voltage (12V) are the transistor output that drives the coil and the oscilator (tlc555).

Hi Geo,

Thanks for your reply.

The driver stage for the headphones is supplied by the battery voltage too, not that it really matters so much there.

The fact that the coil driver is supplied by the battery voltage is why I think the additional voltage will improve the performance of the unit. Driving the coil with the appropriate voltage, or perhaps a little more, should make the unit more sensitive, or at least as sensitive as it would be with 8 AA alkaline cells.

I'm just concerned about damaging any components if the NiMH batteries are over 1.2 volts each, which they may be right after a fresh charge. I think it should be safe, but I figured I'd ask first to get other opinions, and to see if perhaps someone has already done this.

I think the 555, coil driver stage and headphone stage should be OK. What do you think?

[Carl-NC]

Most likely, even up to 15V should be fine. Double check the data sheets for any parts (e.g. 555) that see the full voltage. Also, since the SMPI was designed for 12V, it's reasonable to assume there is some design margin for overcharged batteries.

- Carl

[Geo]

Hi Geo,

Thanks for your reply.

The driver stage for the headphones is supplied by the battery voltage too, not that it really matters so much there.

The fact that the coil driver is supplied by the battery voltage is why I think the additional voltage will improve the performance of the unit. Driving the coil with the appropriate voltage, or perhaps a little more, should make the unit more sensitive, or at least as sensitive as it would be with 8 AA alkaline cells.

I'm just concerned about damaging any components if the NiMH batteries are over 1.2 volts each, which they may be right after a fresh charge. I think it should be safe, but I figured I'd ask first to get other opinions, and to see if perhaps someone has already done this.

I think the 555, coil driver stage and headphone stage should be OK. What do you think?

Yes as i saw the audio stage is supplying with 12V.
As Carl said don't afraid until 15V. But i believe that you will not improve the performance.From my experiments i saw improving of the performance, when i supplied the unit with more voltage, only when used large coils (1x1, 2x2 m).

[Mr.Bill]

I have not seen any noticeable performance change by increasing the voltage.

On the bench with a power supply, I didn’t notice any change in its sensitivity range by raising the drive voltage over 12 volts. It would take more than the voltage of a couple of cells to make a noticeable difference. I have seen many of the circuit copies do real well with 9.6 volts by using re-chargeable cells.

Mr. Bill

[hobbes_lives]

Thanks all for replying.

Mr. Bill, have you tried reducing the voltage to see the result? I did a crude test with my Surf PI Pro, and here's the results that I found.

I don't currently have a working variable power supply at home, so I had to use batteries for the various voltages used in the test. I did the test with 10.6 volts (6 charged NiMH cells), 11.6 volts (6 slightly used alkaline cell) and 13.55 volts (6 NiMH cells plus 2 alkaline cells).

I found that at 10.6 volts, I could detect a nickel at a little over 12 inches. At 11.6 volts, I could detect the nickel at close to 15 inches. At 13.55 volts, the results were similar to the results for 11.6 volts, close to 15 inches. I retested at 10.6 volts and the results were the same, just a little over 12 inches, so it seems that the reduction in voltage does have quite an impact on performance.

Seeing as how six NiMH batteries will normalize at 9.6 volts after some discharge, this sure seems like a problem to me.

I'm going to try to get a variable power supply working, or borrow one from work, to perform a more controlled test.

Mr. Bill (or anyone else, for that matter), if you are able to perform a similar test, that'd be great! I'd like to know if you get similar results.

[Mr.Bill]

Thanks all for replying.


I found that at 10.6 volts, I could detect a nickel at a little over 12 inches. At 11.6 volts, I could detect the nickel at close to 15 inches. At 13.55 volts, the results were similar to the results for 11.6 volts, close to 15 inches. I retested at 10.6 volts and the results were the same, just a little over 12 inches, so it seems that the reduction in voltage does have quite an impact on performance.

I will check it again.

I will say if your geting 15" on a nickel with a Surfmaster, you best leave things the way they are. You have one super Surfmaster.

Mr. Bill

[hobbes_lives]

OK, I was able to set up a better test rig. I used a variable power supply and mounted the coil on a 13" long box.

The results were as follows:

At 14.5V, I could detect a nickel at 12.75".
At 12V, I could detect a nickel at 12.5".
At 9.6V, I could detect a nickel at 12".

So, it appears that my initial test results were skewed, and the supply voltage doesn't have as much of an impact on the detection depth as I initially thought.

Out of curiosity, I checked the output of the NE5534 at the different voltages. One thing that was odd, as the supply voltage was decreased, a slight ringing could be seen on the signal rise. The signal was still dampened fine and didn't overshoot 0V, plus it was prior to the 15us sample point, so it shouldn't affect the performance. I just found it kind of odd.

Perhaps this is more in line with your findings Mr. Bill?

[Mr.Bill]

I just tried it out yesterday, as I had a repair to do.

Going from 12 v to 15v, I noticed an increase using a US Nickel of about 1". This was on an older Surfmaster that I modified to 15 uS. pulse delay.

It's questionable if I would avacate hacking up the case to fit the extra cells. Certainly would be nice to find a "pack" that would fit in there.

Mr. Bill

[hobbes_lives]

Hi Mr. Bill,

Going from 12 v to 15v, I noticed an increase using a US Nickel of about 1". This was on an older Surfmaster that I modified to 15 uS. pulse delay.

What was the detection distance at 15V? Was it around 13"?

It's questionable if I would avacate hacking up the case to fit the extra cells. Certainly would be nice to find a "pack" that would fit in there.

The case would definitely have to be modified to fit a 10 cell pack. The two plastic pieces that hold the stock battery pack in place would have to be cut down. Even then, it looks like a 10 cell pack with the 9V snaps would probably not fit, or at least it would be a tight fit. A 10 cell pack with wires would need to be used instead, with a 9V clip soldered onto the wires. A little foam around the pack could be used to keep it all in place.

It's too bad a 4-series LiPo pack could not be used. The nominal voltage would be 14.8V. Unfortunately, the pack would exceed a safe operating voltage when fully charged, unless some sort of regulation were included, which would probably be more trouble than it's worth.


Kyle

[Mr.Bill]

This wasn’t a very scientific test I did. I just ran a nickel by the coil with the supply voltage at 12, and turned it up to 15 volts.

I was looking at a few marks on the side of the divider wall, and it went out about 1 mark, and that’s about a inch. I have no reference as to the distance it was from the coil. Sorry about that but I was busy at work, and I quickly did it.

13” would sound about right at maximum range in the shop.

Mr. Bill

[Matt_Rowe]

been checking on battery packs and really cant find an answer but here is close, i want to run 4-3.6 packs, but fully charged would be 16volts, is that to much for our PI detectors, or should i only use 3,at full charge would be 12 volts, but at 3.6 volts 3 would only be 10.8 volts, just trying to keep voltage up for as long as possible with the best performance.

[waltr]

been checking on battery packs and really cant find an answer but here is close, i want to run 4-3.6 packs, but fully charged would be 16volts, is that to much for our PI detectors, or should i only use 3,at full charge would be 12 volts, but at 3.6 volts 3 would only be 10.8 volts, just trying to keep voltage up for as long as possible with the best performance.

It depends on what parts in in your SurfPI detector. What Voltage are the caps, C7, rated for?
Will the TLC555, U5, take 16V?
Does the 78L05, U3, have enough heat sinking to dissipate the extra power?
With higher Voltage to coil (higher current and magnetic filed) there will be high 'kicj back' Voltage. Will the MOSFET, Q2) handle it?
Same with the other circuits that are connected to VBATT.

[surfdetector]

TLC555 is good for 18V max supply