irf740 gate switch

[kosacid]

what's the best timing to switch the gate on a irf740 im using a 350mh coil, so what will be optimal timing im using 80us is there a way to work it out ?

[homefire]

What Detector are you working on ? What are you trying to optimize it to find? Shorter or longer long times can come to play. Larger or smaller coils can come to play.

Copied from the MiniPulse Document:

""The technical specification states that the transmit pulse rate is 86pps, but the one I have was
measured as 80pps, and the transmit pulse width is a whopping 234us. Normally such a low pulse
rate is used by PI detectors with large coil frames for finding huge buried targets. ""
HF

[kosacid]

i was testing the gate and sampling the signal returned when the gate is switched off, tested 1us 25us 50us then 100us each one gave me more depth its a 6" coil at 350mh, im not talking about pps or the length of the signal im talking about holding the gate open closing it then sampling, there must be a way in working out how long the gate needs to be on for optimal settings, im just trying to work it out

[Qiaozhi]

i was testing the gate and sampling the signal returned when the gate is switched off, tested 1us 25us 50us then 100us each one gave me more depth its a 6" coil at 350mh, im not talking about pps or the length of the signal im talking about holding the gate open closing it then sampling, there must be a way in working out how long the gate needs to be on for optimal settings, im just trying to work it out

It depends on the TC of the target you're using to test the response, and the TC of the coil. The TX pulse width should ideally be 5x (or at least a minimum of 3x) the coil TC to allow the coil current to flat-top, and long enough to allow turn-on eddy currents generated in the target to dissipate before the main sample starts (i.e. 5x target TC).

[kosacid]

so how do you work it out ?

[Qiaozhi]

so how do you work it out ?

The coil TC is:

where L = inductance of coil in henries
R = coil resistance + any series resistance

Depending on the actual circuit, the TC may be different during charging and discharging.

If you struggle with the mathematics, then you can experiment with a real circuit (which will require access to an oscilloscope) or you can do the same thing in a SPICE simulation.

[kosacid]

used the scope also used spice, spice is pretty accurate thats a good tool but to be honest i don't know what to look for am i looking for the point where the wave form dosent change and set the gate timing to that point ?

[Qiaozhi]

used the scope also used spice, spice is pretty accurate thats a good tool but to be honest i don't know what to look for am i looking for the point where the wave form dosent change and set the gate timing to that point ?

After re-reading your posts, I'm now a little confused. Are you trying to find the optimum TX pulse width, or the main sample delay and pulse width?
All of these factors are dependent on the actual design, the coil parameters, and whatever target you want to optimize for.

[kosacid]

ok im using a copper wound 350mh 0.4mm coil, im controlling the time the fet is switched on when, i switch the fet off im reading the results, basicly im asking how long is optimal to hold the gate open before i drop it and read the results, it must have been discuss before it effects depth

[homefire]

No Effect on Depth that I know of. The effect is on Target Response. As stated above you need the ON TIME long enough for there to be no more Voltage / Current Gain in the coil and Target to Go into Saturation. Then Turn it off . How fast you can do your sampling is determined by Coil Speed (TC) getting back to Zero after the Fly back Voltages hits. The faster you can sample the better the Target Response on low conductive targets. Using a scope you would be able to figure out how long your coil is taking to reach that point. The Voltage will be seen to reach the Apex of the spike. From Switch on to just after that point is your optimum On Time. Less will not use all the potential power to energize the target. Longer and the Target Eddy Currents may be done responding before you sample.

Any Math to calculate this happening with any set wire size, Number of turns ect ect is WAY over my pay scale and I've been at Electronics a Long Long time.

Did that make sense ?

HF

[kosacid]

ok got it now so when it reaches max close the gate holding it longer might have the opposite effect. i might have a look at what my surf pi doing or my barra take the 555 out and test opening the gate at different times