For this application you should be using a bench power supply with current limiting, not a "serial bulb" (I assume you mean a fuse, which is designed to break at a low current, however these are most typically rated for several amps, not typically in the mA range). You can set the voltage and a current limit. If the current goes beyond the limit, then the power supply will drop the voltage to keep the current below the limit or latch off. You can get a fairly cheap one for about $50-60 off of eBay, which won't be the best but is sufficient for hobby use
Hey, thanks for your reply. By serial bulb I mean a incandescent lamp in series with the circuit. I was looking for a cheap and diy option, but I'll take a look on a bench power supply. I still need to get me a decent one anyways.
Ah. It's not going to be possible to size it because the bulb is then acting as a resistor essentially. Unless you know what the equivalent resistance of the circuit you're testing is, and it draws a fixed current, you aren't going to be able to cap the current; Adding a resistor (or bulb) is just going to drop the input voltage and you will probably end up having other issues
It has the high current because it's cold, it only needs a short time to heat up and light up and the majority of circuits can handle very short overcurrent really well because the connections need to heat up before they break. Using a lightbulb for current limiting works pretty well.
I've successfully used incandescent Bulbs in the past. They have barely any resistance when cold, which is the reason why they usually blow up when you turn them on.
For their size, usually I use more than the power the device will use but less than the wires can handle for a while. In your case id get a lamp with a E10 socket, for example 3.5V 0,7W.
Alternatively you can use a PSU with current limiting features.
You could try connecting a current source to the source of the FET, connect the gate to a voltage reference, and connect the drain to a supply. Then the gate source voltage will be kept at threshold at all times and can be measured with a difference amplifier.
This could be done with a couple of op-amps provided they aren't adversely affected by the radiation.
You can make your own current-limited power supply, probably from bits and pieces you already have. Let's say that you have a 5v dc power supply and a hand full of rectifier diodes and resistors (various values and sizes).
Put a series chain of forward biased rectifier diodes and resistor(s) across your 6v supply. Choose enough diodes to give you a 3v output. Now choose a combination of series/parallel resistors to give you a 2v drop with a current of, say 100mA. You need 20 ohms - so that could be 5 x 100 ohm resistors in parallel.
The most current that can put out is the full 5v across 20 ohms - but at that point the output voltage will be near zero.
Bench supplies, well reasonable ones, allow you to set a current limit as well as an output voltage. At loads below that current limit - it operates as a constant voltage supply. At loads above - it operates as a "constant current" supply. You would set the output current limit to 100mA and that's the most that it will output.
Now the rectifier diodes plus resistor would allow the current to increase above 100mA, up to 250mA when the output voltage will be near zero (short circuited) - if you want better than that, then you can add a transistor and a few other components.
Connect the gate to a function generator via a series resistor. (Gnd on the source pin) Drive it with a square wave 0v low 5+v high and observe the gate voltage on the oscilloscope. You will see a little plateau spot in the waveform, this is at the gate threshold voltage.
Interesting, I've never thought of doing it exactly this way. Usually I see high surface area PIN junctions used to detect particle impact either by reverse biasing the junction or by directly measuring induced voltage. The amplification stage is not so easy. This is for particle counting and energy measurement though.
What kind of radiation are you measuring? Gamma I guess?
I guess the first thing that comes to mind is that for a given signal, as Vgs increases perhaps the on-resistance at a given voltage does too? If so, it might be easy to measure the voltage drop across the MOSFET on resistance and how it changes with dose.
If I think of anything else though, I'll let you know!
Edit: I suppose you could also use an R/2R network to provide an increasing voltage to the MOSFET base, and measure the point where the output reaches some threshold, a direct measurement of Vgs. That should be pretty easy using one full output port of an Arduino and one of the ADCs.
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