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which difference

A

Anonymous

Guest
Hello to all:
I wanted to know which is the difference among using the transistor Tip 41C with regard to the MOSFET IRF 740, I don't see it differentiates both since a FEM of 150 volts they take place. Thank you. Carlos
 
Hi Carlos,
My preference would definitely be for the IRF740. Mosfets are closer to an ideal switch than bipolars and I now use them exclusively. The 740 is a 400V device and can pulse up to 40A. It is very rugged and the failure rate is extremely low. Bipolars often have low level tails to their switch off which appear in the receiver as spurious signals. the Vceo of the TIP41C is 100V so for a given current and inductance, its switching time will be longer.
Has anybody tried any other types of semiconductor? IGBT, thyristor etc. I tried a thyristor once and was able to pulse 90A though a coil. I was discharging a capacitor, much like the capacitive discharge auto ignition type circuit, so the pulse was a half sine wave shape.
Eric.
 
Hi Eric Do not let this question delay your book, I need one too. Quistion: Regarding Noise cancellation, why not connect a CMOS mux like 4066 from input of the first amplifier to ground ?. The mux could shortage the amplier to ground after the signal has died away. This would prevent the amplifier from picking up noise in the period until the next pulse. Maybe use a comperator to decide when to enable the mux and som logic to keep the mux on until the next pulse is done. I can not see it is useful to have an open high gain amplifier with a coil sensing everything in the period where we are waiting for the next pulse to arrive. I will try this myself and I will come back with the result, but I would appreciate to hear your comment on this....But do not delay the book! Mark
 
Hi Mark,
It will probably take six months or so to finish the book as I want it to be really comprehensive with lots of pictures and diagrams. However, I have started, so it
 
Hi Eric,
Your comments here essentially mirror areas where I have been dabbling. Although I have been using a separate Rx coil, my input feeds to an approx. 10X non-inverting amplifier. (diode-protected) This stage runs all the time, but is not subject to clipping (low gain) I have added the multiple sample-timings (a la Minelab) together and use this to switch output from pre-amplifier (per 4066) to further gain and inverting stages.
There is the spike you mention after the Rx switching, but it is below signal levels, and is eliminated as you say by the sampling gates.
It is so easy to focus too much upon noise generation/elimination. I'm changing my point of view!! You only have to view target responses after they have passed through the 1st low-pass filter to make this appreciation.
g.
 
Hi Eric
First let me thank you and every contributor to this forum for a most stimulating environment.
Now for my question I notice that you have stated a preference for the IFR740 FET whilst the IFR840 is specified for use in the original EPE/Magnum design is their a reason for rejecting the higher voltage tolerance---perhaps it has higher gate capacitance? I have not been able to unearth a data sheet yet
thanks in anticipation of a reply
Keith
 
Hi Keith,
The only differences between the IRF740 and IRF840 is that the 740 is 400v as against 500v for the 840. On resistances are 0.55 and 0.85ohms respectively. Gate capacitances are the same at 1300pf. Output capacitance is slightly different at 210pf and 180pf but shouldn't make any difference in practice in normal applications. I expect it was availability and cost which made me settle for the 740 many years ago, and I've stuck with it since.
Eric.
 
Hi Keith,
I have a couple of IRF840LC's on order which will arrive tomorrow. It will be interesting to see if there is any measurable difference when I substitute for the 740 in a Deepstar. The LC version is "low gate charge" and it has less input capacitance. This, together with the higher voltage may speed up the switch off; we'll see.
Eric.
 
I would love to hear your results on the comparison Eric. I ordered a couple to try as well but they were back ordered on me. I wish you could go to ONE company and get everything you need. Do you have a way to post waveforms from your scope? I would love to see examples of the signal on the coil and the output of the front end, unless of course there is something proprietary there you don't want to reveal. I'm just not sure of what you are referring to all the time when speaking about shut off of the coil voltage, etc. I understand what you describe, I just don't know what it actually looks like in practice.
Thanks,
Charles
 
Hi Charles,
Tried the IRF840LC's today. There is certainly a difference to the 740, but how significant it is, I am not yet sure. I noticed that on both devices the peak switch off voltage exceeded the typical figure by a large margin. 500V for the 740 and 650V for the 840. A large spike that is present on the 740 at about 5uS, when looking at the receiver waveform, is absent with the 840, Perhaps this is a result of the lower capacitances. The 840 seems to run hotter and I will need to do some temperature measurements. I will record some waveforms on my PC scope and post them in due course.
Eric.
 
Made a mistake in the figures. I wrote down the peak coil voltages at work but trusted to memory when I did the post from home. The 740 peaked at 600V and the 840LC at a whopping 850V. Looking at the data sheet, the BVDss figure quoted is a minimum figure, which in the case of the 840 is 500V. Going 350V above this is remarkable.
Eric.
 
hi Eric
So with this "whopping" availability of 850V, assuming it is repeatable with at least some of the obtainable devices. Could you please comment on what are the pro,s & con,s of producing higher peak values in the coil.
Keith
 
Hi Keith and Charles,
I got a couple of pictures of the coil voltage using a standard scope. I tried to put the pictures together so that I could show them in one post but it didn't work so I'll post them separately. This one is the IRF 740. The scope is triggered by the drive waveform to the gate of the 740. Timebase is 0.5uS per division and vertical scale 100V per division. You can see a 0.5uS delay before the coil voltage starts to rise (gate cap discharging?). The voltage rises to 460V at which point the MosFet avalanches, which causes the flat bit. The voltage then decays exponentially to zero in about 5uS. By the way, I made a measurement error in my previous post when I said the voltage peaked at 600V.
Eric.
 
The 840 plot shows the voltage rising to 600V but no flat portion, so it is not reaching the avalanche point of this sample. Turn off delay seems a bit shorter.
Eric.
 
Hi, Eric!
I know that avalanching has been used to drive such things as LASER diodes, where the high current was useful. Is the avalanche characteristic of any use in this (PI) appliccation?
Louis
 
HI Eric
Thanks for the scope pics very interesting.Also they are useful to a beginner in the PI field such as myself.
Keith
 
Hi Louis,
The fact that this type of MosFet avalanches safely is one way of mopping up excess energy so that you know roughly the maximum voltage that will be flying around this part of the circuit. Also it acts as a protection for the device itself. Aside from that, it does not have any function in the generation of a signal from a piece of metal. I have some new waveforms to post shortly, but will start a new heading at the top of the forum.
Eric.
Eric.
 
Thanks Eric,
Are you sure it decays to zero in 5us?
Thanks,
Charles
 
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