Is Lindsay?s ?SM? a fraud?

[pauldude000]

@all

Below may be old hat info for most, but not for all.

@nickc44

Depending upon your mosfet (IRF840, Buz11, etc..) you may be hooking up the mosfet wrong. Check the spec sheet. Don't feel bad if this is the case, as I had the same "on all the time" problem when I first started using mosfets as well. One can tend to think transistor when hooking them up........

______
|__o__|
|         |
|_____|
|   |   |
|   |   |
1  2  3

Pin 1 = Gate
Pin 2 = Drain
Pin 3 = Source

If you hook the mosfet up wrong, it will indeed "stay on", and will not shut off, either if you confuse the Gate and the Drain. (The base will be high all the time.) If you hook up source and drain backwards, it will be conducting all of the time, due to the built in diode.
  _
_| |_ Pulse to Pin 1 (Gate)

+ supply hot to Pin 2 (Drain)

- ground at Pin 3 (Source)

Put the load in series EITHER between power supply + and Drain, OR between Source and - Ground. (different voltage/current/signal effects)

Usually the load is put between source and ground with a current limiting resistor of the needed resistivity.

If this is not the case, then hook up your scope with the circuit running,  from 555 pin 3 (Sqaure Wave Output) and ground to make sure it is still oscillating. If it is, it is not the 555 circuit, it is then the mosfet connections or the mosfet itself. The 555 will not handle a greater voltage than the mosfet's Gate/Source or Gate/Drain breakdown voltages, which the lowest tends to be +-20V, unless you are using the wrong mosfet for the job.  Again, check the spec sheets.

Paul Andrulis

[otto]

Hello all,

@Paul

thanks a lot. This is what I need to have good SS oscillators. A stable signal in voltage at all frequencies. My voltage drops with higher frequencies and thats really not good.
I dont know if I can buy such type of 555s so I have the next question: what about an oridinary 555 timer IC? I think you wrote that already but I cant find it. Lower frequency?? My memory is not the best.

@Nickc44

the only problem is heat. The saturation is needed!! because the bulb lights very near the saturation point. If I have saturation for 100% then the current rises dramatically and so I have to move a little my frequency mix and everything is OK.

We know or should know that a TPU doesnt work in oil. Yes, another vacation would be great!!

@wattsup

thanks.

Otto

[pauldude000]

@nickc44

Actually, I have to thank you! I was using NE555 in the test circuit, which source 500ma. I did some more research, just to make sure I was sourcing enough current to saturate the IRF840's properly, and using the TS555CN at 200ma I do not think so, at higher frequencies.

It would be a good idea to put a complimentary emitter-follower gate driver circuit as a higher current buffer for complete gate saturation at higher frequencies.

Here is a pdf for more information on the subject.

http://www.zetex.com/3.0/appnotes/apps/an18.pdf

Again, thanks.

Paul Andrulis

[otto]

Hello all,

this is exactly what I have. I mean the pictures in the pdf. Im my oscillators are the BC 140-16 and BC141-16 as complimentary transistors as outputs.
An IC, dont remember the type, is driving this transistors and this IC gets the signals from a ICL 8038 IC.

Not bad signals but as said, the higher the frequency the lover the signal.

Otto

[pauldude000]

@otto

Use figure 2 in the pdf. Use the 555's pin 3 output to the transistors bases, with a current limiting resistor suitable for drive current according to the transistor(s) needs, and your applied source voltage.

Make sure you are using transistors capable of the frequency. (IE use transistors with some frequency "elbow room" compared to your application. If you are desiring up to 4 Mhz, then make sure your transistors can go to at least 10Mhz or more, for example.

Paul Andrulis