Detector power limits

[Old Town]

I'm really not up on detector electronics beyond putting new batteries in. What I wonder is how much power can detectors use or produce and what is the legal limit? There must be some kind of limit so as not to mess up other common electronic devices used by everyone.

Now if we had no limits, any idea of how deep a detector could be made to detect a common coin? Not large metal things but small coins, etc.

OT

 

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[Kentucky Kache]

I'm not sure about the power limit of detectors, but I know the FCC regulates frequency usage. Otherwise, your detector would be opening your garage door.

 

[U.K. Brian]

If by power you mean the voltage put through the coil most detectors use around 8 volts (even if having say two or four PP3 nine volt batteries in the detector).

My Nautilus puts 44 volts through the coil and an Aquasound TR 52 volts in high power mode. There is a machine being worked on at the moment that will put a maximum 62 volts through the coil.

This doesn't mean you can always use the power but its there when you need it.

 

[Digger]

Not sure about the rest of the world but in the US the FCC not only regulates/limits the frequencies and use, but the output power as well.

 

[SteveP (NH)]

I'm really not up on detector electronics beyond putting new batteries in. What I wonder is how much power can detectors use or produce and what is the legal limit? There must be some kind of limit so as not to mess up other common electronic devices used by everyone.

Now if we had no limits, any idea of how deep a detector could be made to detect a common coin? Not large metal things but small coins, etc.

OT

The FCC doesn't regulate or care about equipment operating below 100 Khz. Since most metal detectors operate from 1.5 Khz to about 20 or so Khz (I know some gold machines go higher) there aren't any FCC regulations limiting output power.

I believe that the problem with increasing output power (measured in watts or in the case of detectors milliwatts) is that the signal from the ground gets stronger faster than the signal from targets and so if you put out too much power than the ground drowns out the target signal. Metal detectors already operate at the sweet spot for the best signal to noise ratio. Plus the more power a detector puts out the faster it will go thru its batteries.

 

[Digger]

Why do you suppose none of the detector manufactures advertise power output? Seems to me they jump on every opportunity to look better based on features. Like frequency(s) and such. Why would they not brag about power output since it would carry more value to most users than any other stat?

 

[Carl-NC]

The FCC doesn't regulate or care about equipment operating below 100 Khz. Since most metal detectors operate from 1.5 Khz to about 20 or so Khz (I know some gold machines go higher) there aren't any FCC regulations limiting output power.

Actually, FCC regs start at 9kHz. Above 9kHz there are radiation limits, but they are spec'd as an electric field gradient and detectors transmit a magnetic field, not an electric field. For all practical purposes, there is no FCC reg that limits detector field strength.

I believe that the problem with increasing output power (measured in watts or in the case of detectors milliwatts) is that the signal from the ground gets stronger faster than the signal from targets and so if you put out too much power than the ground drowns out the target signal. Metal detectors already operate at the sweet spot for the best signal to noise ratio. Plus the more power a detector puts out the faster it will go thru its batteries.

This is exactly right. It's a balance between TGR (target-to-ground ratio) and battery life. Even without the ground signal, it takes a theoretical 64x more transmit signal to get twice the target depth. Instead of a 10 hour battery life, you would have 10 minutes.

Why do you suppose none of the detector manufactures advertise power output? Seems to me they jump on every opportunity to look better based on features. Like frequency(s) and such. Why would they not brag about power output since it would carry more value to most users than any other stat?

Because it's a meaningless number. A metal detector develops an alternating magnetic field by putting an alternating current through a coil of wire. Ideally, there is no "power" because inductors don't dissipate power, and there is no signal "transmitted" as with a radio. There is power consumed by the transmit circuitry, but that depends on the design of the circuit. If I resonate the transmit coil my power consumption drops significantly, but the magnetic field can be the same strength as an inefficient circuit that only sucks down more battery current. Would you pay extra for a less efficient design just because it has a higher "power" output?

- Carl

 

[Digger]

Would you pay extra for a less efficient design just because it has a higher "power" output?

So what does the "boost" option do we're seeing on detectors like the V3 and F75?

A quote from the White's V3 manual say the "TxBoost - Boosts transmit signal"

 

[Carl-NC]

Would you pay extra for a less efficient design just because it has a higher "power" output?

So what does the "boost" option do we're seeing on detectors like the V3 and F75?

A quote from the White's V3 manual say the "TxBoost - Boosts transmit signal"

Yes, it increases the TX signal, at the expense of batteries. That's not what I'm talking about when I asked,

Would you pay extra for a less efficient design just because it has a higher "power" output?

There are different approaches to designing a metal detector transmitter. The most fuel-efficient approach is to use a resonant LC tank where a lot of energy gets recycled. A less efficient approach is to use a brute-force coil driver where energy does not get recycled. Given the same coil and the same transmit drive current, both approaches will achieve identical depth. But the less efficient approach will have a higher "power" output, i.e., it will look better in a comparison chart when in fact it just sucks down batteries faster. That's why output power is not specified... no one would (or should) take it seriously.

I recall back in the 70's manufacturers used to list the number of transistors each metal detector contained. It was a meaningless, and even downright silly, specification that no one should have paid attention to as it was no indicator of the quality of the design. But I'll bet some people did, and bought the detector with the most transistors. I wonder if that encouraged designers to fluff up their circuits with extra transistors?

- Carl

 

[Digger]

Thanks Carl.

 

[Frankn]

Actually the dept of present day detectors is limited by the technology used today. We have reached the point where there is so much background signal return that it would drowned out the wanted signal. In Whites and some other detectors you can adjest the power output. It is the adjustment for coil size.

The pulse induction type of detector has the capability to reach further down deeper because it does not constantly transmit like the TR's do. It transmits, receives,transmite, etc. That leaves less chance of background saturation. The drawback is you can not truly discriminate with a PI.

Now if you want dept, use a two box system. The separation gives a dept advantage , but loss of small object pickup. My old White's 500x two box would go 12' easily on can or larger items.

The big winner in dept is GPR. The military is using units that "exceed 25' ".
The military Sat's with GPS, i would conservatively guess a dept of 200' but you will never know for sure because of classification.


So here's my estimates , Tr-2',PI- 2.5', two box-12',GPR-36'+

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[liftloop]

Don't for get the old analoge 5900..... that monster eats to days detector... serfice to five feet...with the 15" coil....