Hero999

I don't think you've given this enough thought. Va will be short circuited to 0V. Attached is a cricuit showing how to bias a depletion mode MOSFET off, on and somewhere in-between. The MOSFET has a threshold voltage of 2.5V. 1) The MOSFET is fully one with 0V between the gate and source. 2) The MOSFET is half way on. With the gate voltage 2V below the source, the voltage across the load is also 2V so the lamp glows very dimly. 3) The MOSFET is fully off. With the gate 3V below the source, no current flows through the drain so there's no voltage accross the load. Note that the only way to completely turn the MOSFET off is to use a negative power supply which in this case is a 3V battery with the +V terminal connected to 0V and the -V to the MOSFET's gate. Why can't you use a monostable such as a 555 timer, a couple of logic gates, a comparator etc? Sorry, about my previous questions, I got astable and monostable back to front.

I'm not sure what you're trying to say. Putting the load in series with the source rather than the drain won't help because as I said before, in order for no current to flow the gate voltage needs to be below the source, not equal to but a couple of volts below the source. Take a look at a datasheet for a typical depletion mode MOSFET. The threshold voltage ranges from -2.1V to -1V, any higher (i.e 0.5V, 0V or 1V) and a significant current will flow. If the gate-source voltage is 0V and the drain is 10V, 140mA will flow. If you connect a load in series with the source and the gate to 0V, when the power is applied, the current through the MOSFET will go down until an equilibrum is formed between the gate voltage and the draind current, much like connecting an enhancement MOSFET's gate to the drain, the source to 0V and the load between the drain and +V. http://www.infineon.com/dgdl/BSP149_Rev1.2.pdf?folderId=db3a304412b407950112b408e8c90004&fileId=db3a304412b407950112b42ef8fa4acf This is one of the reasons why depletion mode MOSFETs are rarely used as switches. As I asked before, why can't you just use an ordinary astable?

The STP2N60FI looks like the closest match. The lower VGS should not be a problem, although it's not possible to be 100% sure without the schematic.

The MOSFET will not fully turn off because the gate voltage ill not be negative with respect to the source, as I said earlier. Why not simply build an astable multivibrator? The only time a depletion mode MOSFET is required for switching is when it's in a solid state relay as a normally open contact and the load is isolated from the switching signal.

As I said previously there's a problem with your substitution before you even look at the electrical specification. The package styles of the transistors you've linked to are different to the FQU1N60C which is designed to use a copper plane on the PCB as a heat sink.

Yes, you're right, I got it the wrong way round but the rest of what I said is still true: a depletion mode MOSFET needs a negative voltage to be biased fully off and you haven't provided enough information.

A depletion mode MOSFET is a normally open switch but it needs a negative voltage to fully turn off. I don't know whether it's suitable or not because you haven't provided enough information.

1) Yes. 2) Good, you couldn't find one so you did your own. All I can say is, providing you've used the correct footprints and allowed enough room on the PCB, there shouldn't be a problem. Please post the Eagle files zipped up.

I won't give my opinion until you give me yours. Have you read both of the datasheets yet?

What are you talking about? What's Electronics Lab apart from this website? I think you've confused this place with somewhere else.

Now I've found the datasheet for you, it should be easy to figure it out. What do you think? I noticed see one obvious problem before I even looked at any of the electrical characteristics.

I've no idea because I can't find a datasheet for the P2N60FI. Finding a datasheet for the FQU1N60C was easy. I suspect you can replace it with a MOSFET with similar or better characteristics but it still might not work because other components might have been damaged. Are you sure it's just the MOSFET which is broken? http://www.fairchildsemi.com/ds/FQ/FQU1N60C.pdf Why not simply replace the adaptor? It's not like they're expensive?

I doubt you'll be able to find such a thing as there isn't much of a market for it, A PLC would certainly do that but it's overkill nd will be very expensive. An Auduinio board is another option but you still need to connect an output transistor to boost the current capacity. Either way, you're going to have to learn a programming language or do some soldering or both.

What are your size constraints?

How are you using the transformer? That topology is only any good for a flyback which produces high voltage negative spikes, not a sine wave or even a very good square wave. The only solution is to drop the single ended approach, use a push-pull driver and reduce the turns ratio of the transformer. This is confirmed by my calculations below: Turns ratio: Np:Ns = sqrt(Lp/Ls) = sqrt(50/200) = sqrt(0.25) = 0.5:1 = 1:2 Reflected impedance of the 20R transducer: Zp = Zs *(Np/Ns)2 = 20*(1/2)2 = 20*0.52 = 20*0.25 = 5R So at 5V, the current will be 1A, Ohm's law. I = V/R = 5/5 = 1A. Why are you using the transformer? Is it for isolation? Do you need to boost the voltage? If it's the latter, you need a transformer with a higher power rating. If it's just isolation the same size transformer will do but it needs to be 1:1.

You have contradictory requirements: programmable and plug and play. A simple PIC microcontroller would be a perfect programmable timer. The PIC12F508 could do what you want with the addition of a transistor, a resistor and a capacitor. It can be programmed with C, BASIC or assembly.

A reactor is just a large inductor. It has many applications, one is to compensate for the capacitance of the cables to minimise losses.

The INA114 looks simialr to the AD521 http://focus.ti.com/lit/ds/symlink/ina114.pdf The OPA128 is recommended to replace the AD515 http://www-micrel.deis.unibo.it/DATA_SHEETS/OPA128.pdf You could also try contacting Analog Devices.

You could try using a single diode half wave rectifier. Failing that, an autotransformer, which is half the sze and cost of a transformer with two windings and a bridge rectifier will do.

Here's an idea for an IR transmitter which uses PWM at 90kHz. The two LM393s should be replaced with a single LM339 to save cost. I just used the LM393 because there's a model for it in LTSpice. I've not tested it. If you want a receiver I'll have a go at designing one.

I was going to say how crappy this project is but decided to leave it to audioguru who I knew would enjoy it more and be more constructive than me. AM is also bad because the response of the photo-diode photo-transistor won't be matched so there will be non-linearities which will distort the signal. To be picky no headphones use FM which would involve changing the optial frequency of the radiation emitted from the LED. The correct term is PFM (Pulse Frequency Modulation) which means the frequency of pulses of infra-red light light is modulated. PFM is better than AM because the LED is either on or off so the non-linearities don't distort the signal. Another simpler method is to use PWM which is easier because the signal can be demodulated using a simple low pass filter but it only works for mono unless you shift one fo the audio channels to a higher frequency band but that's more complicated than using FM.

I doubt there's a problem with the meter. The voltages were unpredictable because they depended on the current drawn by the load which is unpredictable.

You use 24VAC of course. To get 24VAC you need a transformer to reduce the mains voltage to 24VAC.

I don't know and nor will anyone else because you haven't said what sort of servo mechanism you're using.

http://www.lpa-group.com/pdf/downloads/HY-MAG.pdf