Role of capacitor in SD2000

[nick_f]

Hi All,

I was checking the diagram for SD2000 and they use a large value capacitor (100uF/200V) connected via a diode to the hot end of the coil.
Does anybody understand the advantages of using this capacitor?
So far. I've seen this capacitor only on Minelab boards.

Regards,
Nicolae

[Mechanic]

Hi Nicolae,

That capacitor is used to buffer the flyback voltage. You will notice if you hooked up a scope that when you turn the detector on, the peak flyback voltage slowly climbs to 180v over about 5 seconds. A smaller cap the voltage would peak faster. I guess that size is used so it can absorb excess energy if something were to go wrong.

The irf710 is used to clip the flyback to 180v and the tlc271 drives it. Adjusting the values of the 2 110k resistors changes the flyback voltage level.

Cheers

[nick_f]

Hi Nicolae,

That capacitor is used to buffer the flyback voltage. You will notice if you hooked up a scope that when you turn the detector on, the peak flyback voltage slowly climbs to 180v over about 5 seconds. A smaller cap the voltage would peak faster. I guess that size is used so it can absorb excess energy if something were to go wrong.

The irf710 is used to clip the flyback to 180v and the tlc271 drives it. Adjusting the values of the 2 110k resistors changes the flyback voltage level.
Cheers

Thank you Mechanic for your explanation, it makes sense. I think the capacitor + IRF710 is added as a limiting circuit to protect the MOSFETS BUK455. In case the coil would generate a voltage over the limit of the transistors BUK455, they can prevent it. This could be necessary because people try to use all kinds of coils, without doing any measurements.
I had a look and I found BUK455 has a maximum voltage Vds of either 100V or 200V (two different types). They must be using the ones up to 200V, and the limit of 180V you mentioned seems likely.
Based on this, it could be unsafe to change the resistors of 110K.

Best regards and Happy New Year!

Nicolae

[moodz]

Hi ... IMHO the cap is acting as part of a snubber cct. Read all about snubbers on google ... too much to drop here. :-)

[nick_f]

Hi ... IMHO the cap is acting as part of a snubber cct. Read all about snubbers on google ... too much to drop here. :-)

Hi Paul,

Thanks for your feedback. I read about the snubber circuit on wikipedia and I think that circuit bears a close resemblance to the circuit from SD2000. Esentially the role of both circuits is to protect the controlling device. But while the snubber circuit in that image (Wikipedia) doesn't mind at all if the spike is completely killed, in SD2000 they want to control (= stabilize) the level of the spikes to a level as high as possible, but still under the maximum ratings of the mos-fets. It works in a pretty smart way. As long as the DC voltage on the cap can be maintained to let's say 180V, it is guaranteed that the pulses level will be in the upper vicinity of 180V. And it is much easier to control a DC level than the level of a pulse. Perhaps the circuit from Minelab can also be called a snubber circuit.

Best regards,
Nicolae

[moodz]

Hi Nicolae .... I should have looked at the circuit more closely ... notice how the 'cold' end of the 100uf cap connects to the top of the tank caps. If I am not mistaken this is an application of a Bruce Candy / Minelab patent where some of the flyback energy is recycled into the Tx tank caps as well as a snubbing function.One of the main claims in this patent was power saving. The Mosfet switch and associated 68 ohm resistors serve to 'reset' the snubber. It would be interesting to see the waveform on the snubbing switch mosfet.

Paul.

[moodz]

... US Patent 20020053907 claim 22 ... the second method in claim 22 is very interesting ( even more interesting how you can make two distinct claims inside one claim ... slack patent attorney ) this gives a clue to the timing of the switch across the back e.m.f clamping capacitor ( thats what BC calls it )

[Mechanic]

Hi Moodz,

The power recycling feature is used in later models, possibly 2200, but defiantly the extreme onwards. It is not used to recycle power in the 2000.

During the flyback period the voltage to the gate of the irf710 adjusts to limit/keep the voltage to 180v, much like a regulator.

Next time I pull my detector apart I will take some scope shots of the gate voltage for you.

If the irf710 fails, the detector will still run but it will be very unstable and jittery. The flyback voltage will then be limited to the avalanche voltage of the tx mosfets. The tx mosfets will become warm during operation when they are avalanching. In the case of the buk455-200, the flyback voltage will avalanche at 250v. The front end fet that I have(Woody special) seems to be able to handle this higher vlotage. I am unsure if the original tn2410 would handle the 250v.

Cheers Mick

[nick_f]

... US Patent 20020053907 claim 22 ... the second method in claim 22 is very interesting ( even more interesting how you can make two distinct claims inside one claim ... slack patent attorney ) this gives a clue to the timing of the switch across the back e.m.f clamping capacitor ( thats what BC calls it )

[ATTACH]10699[/ATTACH]

Hi Paul,
Thanks for posting that fragment. I an see the 100uF/200V capacitor is connected to the tank of capacitors (2 x 5600uF/10V). During the normal functioning (after the 100uF cap has ben charged to 180V) there would be some amount of current charging the 5600uF caps and that indeed would save some energy (I don't know how much). I don't know how SD2200 model differs from SD2000, but to me this model also saves energy through this connection. I think the designers had a choice to regulate the back EMF pulse by connecting the MOSFET + capacitor either to the point where it is now (and save energy as a bonus), or to the +VB point, and just dissipate that energy as heat. Just by moving one connection point, they accomplish both the regulation of the pulse and also saving some energy. On the other hand, if one doesn't regulate the pulse voltage, there is more kick hitting the target (maximum back EMF obtainable from the coil), so the energy wouldn't be lost anyway. But these circuits don't regulate the pulse voltage. Saying these, all PI circuits without regulation (such as Hammerhead) are still able to detect targets and to me they look stable (this is in reference to what Mechanic just said and I will comment below).

Regards,
Nicolae

PS - All my comments are more or less educated guesses and attempts to understand the circuitry. If I had more knowledge, I'd probably come up with my own ideas in building a metal detector. At the moment, all I can do is to pick bits and pieces from existent designs (not that I'd implement this circuit anyway).

[nick_f]

Hi Moodz,

The power recycling feature is used in later models, possibly 2200, but defiantly the extreme onwards. It is not used to recycle power in the 2000.

During the flyback period the voltage to the gate of the irf710 adjusts to limit/keep the voltage to 180v, much like a regulator.

Next time I pull my detector apart I will take some scope shots of the gate voltage for you.

If the irf710 fails, the detector will still run but it will be very unstable and jittery. The flyback voltage will then be limited to the avalanche voltage of the tx mosfets. The tx mosfets will become warm during operation when they are avalanching. In the case of the buk455-200, the flyback voltage will avalanche at 250v. The front end fet that I have(Woody special) seems to be able to handle this higher vlotage. I am unsure if the original tn2410 would handle the 250v.

Cheers Mick

Hi Mechanic,

To me the SC2000 is still able to save energy (see my comment to Moodz). We both agree that IRF 710 has a regulating role.

Now, when you say "if IRF710 fails", the detector is unstable and jittery.
There can be all kind of faults with a mos-fet. If we would remove the BYV28-200 diode, that would disconnect the whole regulating circuit.
Depending on the width of the Tx pulse, the back EMF can reach (and most likely will reach in this case) the max voltage for the MOS-FETS (BUK 455), which may cause them to get warm (actually one of them more than the other). My guess is that the avalanche voltage for a transistor, will act as a zener diode and I don't see why the detector would be unstable and jittery (assuming the temperature for the BUK455 won't be too high).

As far as I know, any particular MOSFET, rated for example 200V will have a specific avalanche point (a constant voltage, pretty much like a zener diode), and the value of this voltage will differ from one transistor to another.

Mechanic, with your modified SD2000, do you know (or can you measure), what is the detecting range in air, for a 1 inch (and a 0.5 inch) piece of thin aluminium foil? If you do the test, could you please specify if you are using a mono coil or a DD coil? This could act as a benchmark point for us, building other detectors. I would be curious to know what are the values for GPX4500 as well. Many people comment that the depth in air is not interesting, all it matters is how it performs in soil. But what I say is, if a MD can only detect a target in air 20 cm, for sure we can't find the same target in ground at higher depths than 20cm...

Regards,
Nicolae

[Mechanic]

Hi Nicolae, Paul,

I'm still not convinced that the circuit is recycling power. It takes over the role of the avalanche function of the buk455. If this cct was not connected, the avalanche would still go to the batt- rail(-side of the 5600 caps.) as it does now through the irf710. But I could be wrong, this will puzzle me for a while!

When I said about the irf710 failing, I was referring to the pins breaking off at the base of the mosfet. This is a common problem because the mosfet is attached to the case, and has happened to me. I am unsure why this makes the detector become jittery, but probably does have something to do with the excess heat in the byv28-200 diode, hot buk455-200 and hot dampening resistors. The detection depths for small targets also seems to dramatically increase in air, but I have not tried in ground for a comparison. I think that also the sample timings would have to be changed a little too to be able to set the GB properly, as from memory last time it happened to me I ended up at the extremes of the GB pots in mild ground.

If the tlc271 fails, this causes the flyback voltage to be clipped at 12v and instead of lasting 4us(after 260us on time) the flyback lasts about 200us!
I will do the foil test for you next time I'm out bush.

You said "if a MD can only detect a target in air 20 cm, for sure we can't find the same target in ground at higher depths than 20cm..."
I couldn't agree more!


Paul, if I remember correctly, that patent was filed in 97 and at the start it says,
United States Patent 6686742
"In U.S. Pat. No. 5,576,624 I disclosed a metal detector apparatus which is capable of substantially reducing signals arising from ground mineralisation containing magnetic soils. Several models utilizing these principles have been commercialised and used highly successfully by gold prospectors and for land mine detection. Both land-mine de-miners and prospectors desire low power consumption so that battery weight and costs are low and operating life is extended. The aim of this invention is to improve upon power consumption."

Cheers Mick

[nick_f]

Hi Nicolae, Paul,

I'm still not convinced that the circuit is recycling power. It takes over the role of the avalanche function of the buk455. If this cct was not connected, the avalanche would still go to the batt- rail(-side of the 5600 caps.) as it does now through the irf710. But I could be wrong, this will puzzle me for a while!

Cheers Mick

Hi Mick,

Now I am completely confused. I only realised now that the back EMF from the coil generates a positive voltage applied at the negative pin the 2 x 5600uF capacitors (via either the cap 100uF/200V or the avalanche of BUK455). This would only discharge the capacitors!? I must be drunk despite I haven't had any booz (today). Maybe I should drink something in order to think straight?
Anyway, if I got it right this time, it means you were right, the circuit only regulates the 180V and it doesn't save any energy.

Regards,
Nicolae