SD / GP damping resistor

[andre]

With the damping resistor network on the SD2000 can this be modified to use a single Resistor like the GP series ..
I think this is correct as I saw woody video when he was modifying it for a variable Damping .
What is the purpose with the Damping network resisitors .. I would have thought this would have created extra noise…
Can any one explain please maybe ZED you have answer as I think I saw a pic of a modified unit of yours with a single resistor and the network ones taken out.
I have modified mine and took out the 12 resistors and put 2 in parrallel and a variable in series to achieve the same value… it works fine but not on all coils like the small 10 inch eliptical and the bigger 18 inch mono.
Any input welcomed

[ZED]

G`day Andre

The reason for the resistor network who knows ? maybe to reduce inductance,i see no reason why you cant use a single resistor as long as it is of the appropriate wattage and it doesnt get hot.The 47R resistors must remain,they to are part of the damping resistance,
As for coils i cant say although i have used a m/lab 18" mono with a 580R damping resistor with no problems,also the 24" ufo mono did not like the same damping resistor.
Having a scope helps so one can see whats going on, if one mods the damping resistor network i wouldnt expect all the coils that are available for the 2000 to work as intended.

Cheers Zed

[porkluvr]

Here is what your flyback might do if you decide you don't need the damping resistor. These are two views of the same event. Notice the nice high voltage peak, followed by valleys and peaks. I removed the damper for the simulation.

I don't have spice model for the BUK455-600 and my IRF840 model breaks down at 500V. So my spice simulation is not exactly representative of the SD2000. But, if indeed the output transistors are BUK455-600 as shown in the latest schematic offering for the SD2000 - although the flyback pulse won't look exactly like in my pictures, it will still have a high voltage peak and it will have ripples instead of settling quickly.

So, when you go and replace the array of damping resistors, do you take into account that your flyback on the 160us pulse might peak in the neighborhood of 500~600 volts? You might ask yourself: what happens if the damping resistor sustains repeated over-voltage?

Minelab uses an array of resistors to accomodate instantaneous power dissipation AND being subjected to high voltage. Metal film is used for low noise.

If I'm not mistaken, the two 47Ω are part of the blanking gate circuit. Are you going to do away with them?

[ZED]

Your suspicions are correct porkluvr the fets are BUK455200A,they are 200 volt fets.

Zed

[andre]

ye I have left the 47r in place and put 2 x 3 watt metal film in with var pot .. so this gives me good range and I think I have ample wattage ..and works good with my 11 inch dd and 14 inch dd ...
So scratching my head a little bit ..
thanks for your input ... will work with it a bit more
andre

[porkluvr]

Thanks for the feedback, Zed. I should have known that the FETs weren't -600 type. The breakdown voltage of the series diode tells the story. I knew (but then FORGOT) -200 had to be correct and I feel bad about not sticking to my guns. I don't own SD2000 metal detector so I have to filter through (sometimes conflicting) second hand information, without ever knowing. BUK455-x are uncommon devices in my neck of the woods - the IRFxxx series are more common.

You can get a faster turn-off with higher voltage FETs, but with the flyback pulse limited to 200V, the support components don't need to be rated for half a zillion volts.

Even with just 400V on the peak, most oscilloscope probes could be damaged. I don't own a scope or I would probably have already blown up the probe because I only thought of this yesterday.

Glad your detectors working, andre. But I still can't figure why you're replacing the R network. If Minelab advertises coils to work with the SD2000, I wouldn't think that any tweaking would be necessary on the PCB. They are authorized coils, right?? I don't want to pontificate, but if it ain't broke, why fix it? No offense intended, but if you don't understand the purpose of the damping resistor maybe you should leave it alone. If you have an oscilloscope it is not so crazy, but if you don't, aay yeeee- watch out!!!

Maybe if you measure L and R on the coils that don't work so well and compare with the "good" one you'll figure out what's wrong.

[Unregistered]

Thanks for the feedback, Zed. I should have known that the FETs weren't -600 type. The breakdown voltage of the series diode tells the story. I knew (but then FORGOT) -200 had to be correct and I feel bad about not sticking to my guns. I don't own SD2000 metal detector so I have to filter through (sometimes conflicting) second hand information, without ever knowing. BUK455-x are uncommon devices in my neck of the woods - the IRFxxx series are more common.

You can get a faster turn-off with higher voltage FETs, but with the flyback pulse limited to 200V, the support components don't need to be rated for half a zillion volts.

Even with just 400V on the peak, most oscilloscope probes could be damaged. I don't own a scope or I would probably have already blown up the probe because I only thought of this yesterday.

Glad your detectors working, andre. But I still can't figure why you're replacing the R network. If Minelab advertises coils to work with the SD2000, I wouldn't think that any tweaking would be necessary on the PCB. They are authorized coils, right?? I don't want to pontificate, but if it ain't broke, why fix it? No offense intended, but if you don't understand the purpose of the damping resistor maybe you should leave it alone. If you have an oscilloscope it is not so crazy, but if you don't, aay yeeee- watch out!!!

Maybe if you measure L and R on the coils that don't work so well and compare with the "good" one you'll figure out what's wrong.

Ye mate not changine .. am modifying to be better machine... if you variable damp the coil then this improves its detection range .. so that is my aim ..
ML put in single value or resistance for damping ... this is suppose to cover their wide range of coils... but if u varible damp then you can adjust every coil for max response on target
this is my aim
thanks for input
andre