Delta Pulse

[porkluvr]

how long was PULSE WIDTH in your interpetation? I ask because there has not deep sense to use the diode at range before 150us, but has after, near to 500us.

about 73us. Hmm, so I should redo the sim using a much longer pulse (allowing the coil current to rise higher towards the "max" value - battery voltage divided by resistance in coil and related components)?

OK


edit: KT315, what would be your choice of inductance and resistance for a DP coil?

[kt315]

73us is low... no need to use the diode for this case. yes you should redo the sim for 500us, this is maximum PULSE WIDTH for DP.
let be inductance 400 uH and resistanse 680 Ohm.

thanks

[Tepco]

Possible diode avalanche mode?

This may be important. Following this discussion, (all sides apparently have something to say or claim) I get one interesting idea do explain discrepancy.



I never tried to add diode in Delta, especially not specified type, but did tried on many other designs, getting results exactly opposite of what is claimed here. Namely, on long pulse detector, something like 100-120uS or more, there is practically no difference, during MOSFET breakdown period diode is forward biased, coil voltage will never exceed this level, so there is no 1500V peak present (but KT well may be right in it's claim?). Some small advantage can be expected in short pulse, high repetition rate detector, having less energy stored per pulse and smaller flyback, below breakdown level. Then diode will isolate MOSFET from circuit faster but improvement is minute. Exactly opposite is claimed here, and one particular diode type recommended, giving me some idea:


Under very long pulse condition, large amount of energy released, diode may enter so called “avalanche mode”, at least this particular type. Normally, computer simulation will fail to predict this. Under this condition, circuit behavior can change slightly, producing that famous 1500V peak and significantly influencing detector behavior. Someone with suitable hardware should provide scope pictures, eventually experiment with other diode types under various conditions. Some modern fast diodes are specifically designed to avoid avalanche, ordinary slow types can reach this mode easily, recommended diode is probably somewhere between. It is less known that ordinary 1N5408 rectifier, otherwise too slow to rectify even at audio frequencies can produce very high power nS pulses in avalanche mode. Experiment with different types can be interesting, at this point ignore capacitance and switching speed, try some ordinary ones too.


If avalanche mode behavior can be verified , along with it's impact on detector performance, this may have impact to overall detector design, replicating similar effect in more controllable fashion.

[calebsone]

So I know this is off current topic of the thread but by tune on the 680ohm resistor and the 560ohm do you mean exact to that number or find something close for the coil resistance and meter type. Or is it the number or nothing I'm all most done just needed this last bit.

[porkluvr]

73us is low... no need to use the diode for this case. yes you should redo the sim for 500us, this is maximum PULSE WIDTH for DP.
let be inductance 400 uH and resistanse 680 Ohm.

thanks

Thanks for the insight, Tepco.

I previously was of the mind that because the DP seems more like a relic hunter than a gold machine, an added diode in series with the coil would be superfluous. I no longer hold that opinion because I do plainly see what looks like a faster flyback with the diode in place. But there is more to it than what appears on the surface...

Here is a point I would make. Adding the diode by itself may not be magic, and perhaps only adding the diode is not going to make much difference, but if you can increase the value of the damping resistor and still finish with a slightly faster end to the flyback period, that is something worth investigating.

KT315, I was actually inquiring about your choice for coil winding resistance, not the damper. But I have to admit my question was odd. Still, damping R should be adjusted according to coil parameters, and not the other way around, right? I wanted to have a concrete number for coil resistance to start out with. Damping R would be a variable after selecting coil wire.

Here's my "design" process. I am not intending to belabor any particular issue but only trying to make it easier for somebody to follow my (often derailed or just plain wrong) train of thought:

Using MiscEl's rectangular coil calculator: basing an imaginary coil on a 0.47m x 0.47m form, MiscEl tells me that 14t will get me about 400uH. With 0.4mm cross section wire (~26AWG) the resistance will be ~3.5Ω.

With PW set to 500us (12V supply and 3.5Ω coil resistance) a 500us pulse-width may be too long. LTspice showed a flat-top on the current waveform which I believe represents wasted power. (So, it's "back to the drawing board".) There are no pictures of this shown here.

To eliminate wasted power I could either reduce PW by about 70us~100us, or else I could use a larger gauge wire. Since "virtual" wire is very inexpensive, I will increase wire size.

Next I'll try 0.5mm. MiscEl tells me that the same 14t of wire should exhibit 2.3Ω resistance.

After making the necessary change in the simulation file, it looks like 0.5mm wire (which is only slightly smaller than 24AWG) might work very well for a 400uH coil, with PW set to 500us. As I am comparing one circuit with a diode and one without, I am having to split the difference when figuring out where I want to be... it is not a huge difference in peak coil current when using a diode as compared to not using it, but the difference is measurable.

Now to run the sim and take some pictures. Note that the 40us mark in the pictures represents about 14.5us from end of tx pulse.

After comparing things, it seems apparent to me that using a diode can definitely allow you to use a higher value damping resistor and still sample earlier than without. In retrospect, a series diode will affect the coil current final value and perhaps adding a small resistance in the test circuit on the left side would have equaled things out, making the comparison more valid, but I have no time for that particular exercise.

I want to understand this because my design in progress uses two transmit channels, alternately. A diode in series with each coil is required for the two channels to have some degree of isolation. I hope to see it come together before the end of summer.

Sorry about the long post.

[green]

Recorded some data. No diode, MUR460. 1N4937, 1N4004. The data was recorded with R damping disconnected. 280uh coil, the diode was connected in series with a IRF740 fet. No coil and 1N4004 had a resonance around 340khz, Mur460 and 1N4937 doides had a resonance around 1.25Mhz. The top traces had the scope probe connected to the coil. The bottom traces had the scope probe disconnected laying near the IRF740. Planning on recording amplifier out with no diode and a MUR460 diode with the coil critically damped to see how much faster the signal could be sampled. Those are the only high voltage diodes I have.

[porkluvr]

Here are some models and symbols that might come in handy for somebody wanting to play with the DP tx timer. I won't post my circuit, but it should be easy enough to reproduce for somebody who gives it half a try. Of course you'll need to have LTspice (or some other other compatible spice application) installed on your computer.

Even though LTspice comes with an LM555 symbol, mine is modified from the original, and I've included it here. Nothing is wrong with the original symbol, but mine has more information.

Find the LTspice\lib\sub folder and put the .sub files there.

My standard.dio file has more diode models than the original. Find the cmp folder. Either simply replace the original standard.dio file with mine, or you can open it with a text editor and extract the STTH1210 model or whatever else you want, and then paste into your standard.dio file.

Make a new folder in the ltspice/lib/sym folder. I call mine "3rd party" and I have nested subfolders for MOSFETs, COMPARATORS, opamps, miscellaneous, and etc.

Place IRF740 symbol somewhere in the sym folder. Perhaps like so: LTspice/lib/sym/3rd party/MOSFET/IRF740.asy. Put the potentiometer symbol wherever you want. If you don't know how to use it, you'll figure it out. or ask for help.

You can drag and drop any symbol file (.asy) into an open LTspice window for editing, or maybe if you are just curious to see what makes it tick. CTL-A brings up an attribute editor window. See how I've construct 3rd party symbols so that when they are inserted into a simulation LTspice will locate and use the appropriate model file automatically (usually a .sub or a .mod file).

When you restart LTspice, the symbols should be available by clicking the icon that looks like an "and gate".

Whew, it's been a long day. I've gotta go.

[porkluvr]

So I know this is off current topic of the thread but by tune on the 680ohm resistor and the 560ohm do you mean exact to that number or find something close for the coil resistance and meter type. Or is it the number or nothing I'm all most done just needed this last bit.

Just so long as you are in the ballpark with the damping resistor you should be OK. If you don't have a scope, proper selection can only be trial and error - unless you already have a tested and true coil formulation procedure.

Even if you do have a scope, it takes a little bit of know-how to be able to interpret the results to know which way to go.

Tepco wrote a post not too long ago that has some nice photos and a good description.

Understand that coil resistance and inductance are not the only factors that affect your proper choice of a damping resistor. You also need to control the various capacitances of the coil windings, the cable, and MOSFET (hmmm, where did I see that before).

Here is a link to Tepco's post.


NOW it's time to go

[green]

Recorded more data. No diode vs MUR460. Didn't get what I expected. Thought the amplitudes would be the same at longer delay times. The no diode recordings are higher. Using pi*L uh*circuit resonance Mhz(no R damp) to calculate R damp. No diode=303 ohms, MUR460=1100 ohms. I used 286 and 1000, a little over damped. I already had a 1k resistor for R damp, paralleled 1k with 400 for 286 ohms The diode in series reduces the current at turn off, might be the cause for lower amplitude. Used a 6 volt supply, 12 volt supply might have less difference. Any other thoughts would be appreciated.

[calebsone]

I think I get it thanks porkluvr so with whatever coil,cable,and winding inductance and capacitances KT used needed the 680ohm and whatever my set up equals is how I need to set my dampening am I close to right? Or way off with how I understood what you said? And the easiest way to find that is through a pot to adjust till I find the right resistance for my coil set up

[mohandes]

Here is a virtual comparison of using a diode and not having one. I see some apparent difference between the two circuits' performance, but how or if that equates to improved target detection can only be determined in the field .

It looks to me like adding the diode will cause flyback to decay much faster in the first few microseconds. If adding a diode works out for builders being able to sample earlier than they could have otherwise, that is a good thing. And if you can use a higher damping resistance than before, that should also allow a stronger signal to be seen at the receiver input.

I don't think the diode would need to have a 1500V breakdown voltage (although for all I know, the STTH1210 might still be the best choice for some reason or another), but it would be a bad idea to use something with a lower breakdown voltage as compared to the zener voltage of whatever MOSFET is being used.

In case it is not obvious, what I did with the progression in the photo was start with the "big" picture, and then zoom in sequentially towards the last few microseconds of the flyback's settling. Note that the 176us point is about 14us after the end of the transmit pulse.

hi
dear friend i use 450ohm for damping res and my coil near 460mkh and 2.2ohm ( plus coaxial cable near 3.5ohm )for 28cm coil. and this value is best result for my DP.
i test with 680 , 750 and ...1.2k but best result in 450ohm in AIR testing.
i can not find stth1210D in my country so i buy stth12R06D , Do you thing it is work or burn ?
and in finally , Diode help for faster respond and i think can better for Depth , i say OK ?or better say diode cause DP work better ?
can you simulation with my value i say you above and post here.i want know 450 ohm ok or 680 ohm.
regards

[mohandes]

hi
i see pis of dear Tepco , and this pic of my coil : 28cm , 460mkh and 2.1ohm (without coaxial cable ) and damping R is 820ohm

[Tepco]

This looks reasonable, actually surprisingly fast. If you measured this with coax cable connected leave it as is, some ringing on waveform wont hurt, eventually reduce Rd to 680 ohm.

[mohandes]

This looks reasonable, actually surprisingly fast. If you measured this with coax cable connected leave it as is, some ringing on waveform wont hurt, eventually reduce Rd to 680 ohm.

yes , you learn me before , i change it with 680 ohm and next 450ohm , and next see in scope , both of them have a same respond in scope ( less than 15mks ) but why 450ohm work better ?
regards

[kt315]

but why 450ohm work better ?

that is only your subjective perception. fantazy. in original schematic Rd value is 820 Ohm. for DP working on long MAIN SAMPLE DELAY the ringing is not so actual
like may be for Surf PI, Bara, HH etc. from your pic BMP clearly seen that if you set MSD on 40us (really practically I set on 100-120us) the ringing is leaving at left on timing and not effecting.