Question for advanced guys !

[khelz]

Wassup guys,

Here is what I'm trying to do : I want to control 300 V with a switching diode to create a square wave signal of 20 % duty cycle with a 200 Hz cycle. But I don't know how to control the cycle of a switching diode. I don't know if it works like a relay with a cross signal passing through it. So PLEASE if you know how we can control a switching diode please give me a wack !

A reward of a million dollars will be given to the one !

Thank you
-khelz-

 

[PCBWING]

This is a lamp circuit, you can refer to it.

 

[khelz]

Okay so I get the idea of controlling the conduction angle, but I still don't understand how to control it or adjust it physically. Can somebody help me up with that ?

 

[tvtech]

This is a lamp circuit, you can refer to it.

LOL. You're funny.

No Spamming now or ever. You hear??? You are a PCB manufacturer. Stick at what you know buddy.

Cheers Mate

 

[Sceadwian]

khelz, we can't read your mind and determine what you're talking about, your post was less than plentiful on detailed information about what you're trying to do. What you've said doesn't actually make any sense that I can determine.

 

[PCBWING]

LOL. You're funny.

No Spamming now or ever. You hear??? You are a PCB manufacturer. Stick at what you know buddy.

Cheers Mate

My circuit knowledge is very limited, only scattered memories of long ago from a teacher point of knowledge in class terms. I am very grateful for your suggestion, we don't send spam, this is our business principle. My remarks at the forum also follow the principle of non-chaotic.

 

[khelz]

khelz, we can't read your mind and determine what you're talking about, your post was less than plentiful on detailed information about what you're trying to do. What you've said doesn't actually make any sense that I can determine.

Okay I'll try to be more general. To give you a big picture I'm working on a biological project that needs a very specific electrical field : The project needs big electrical pulses (300 V) really shorts in time (1 milisecond). In my lab, I can get easily 300 V so what I tried is to get it on and off at the source itself. But the response time of the power supply was too long and I didn't get a pulse form anymore(it was more a big parabole).

So what I'm thinking is to put a DC constant voltage of 300 V and to control the pulse by a high voltage diode, which would act like a switch and which the turn-on and turn-off time are quick (50 nanoseconds). This could give me a good pulse form(almost perfect square wave) but the thing is that I don't know how to control the diode to adjust it to my desired turn-on time (1 millisecond). So basically the square waveform cycle got an amplitude of 300 V, a period of 5 millisecond, which during the first fifth, the diode let pass the current and turn off after for the 4 milliseconds left before turning on again. The thing is just to know how to control the on and off time of the switching diode.

Don't hesitate to ask me any questions, I really need help on this one.

 

[Sceadwian]

You can't control a switching diode like that. A diode conducts when it's forward voltage is above about .5 volts and stops when the voltage drops bellow .5volts you can't control when it switches. What you need is a transistor. I can understand why you would have trouble trying to turn it on and off from the power supply the filter capacitors would give the curve you describe. The first thing you have to answer is how much current this pulse is going to have to deliver because that will determine how you can switch it, from there we should be able to recommend a transistor. A simple 555 timer circuit would easily produce the pulse you're talking about and be easily controllable which would be fed to a transistor to control the actual pulse from the main power supply, it all depends on the current as to what should be used.

I'm not sure such a circuit could be done with a switching diode alone, it would have to be biased very carefully. Also in your first post you said you need 200hz, but later you said you need simple pulses? Which is it? A constant pulse train or a single isolated pulse?

 

[khelz]

I want single pulses but I will repeat them at each 5 millisecond which is 200 Hz if you calculate.

But for the current, the less the better. There's a minimum, but I really don't want power at all, just voltage. So you think that a 555 timer would be more what I want here...is the timer just act like a switch ? and why I have to fed connect it with a transistor ? is it just to get the voltage up ?

Thank you

 

[Sceadwian]

Supplying a voltage will draw current, you have to know how much or you might not get the voltage you desire it's something and you need to know because it will effect the choice or transistors you use.

Yes, the transistors are just to pass the higher voltage, 555s are low voltage devices only, I think the highest voltage you can run a typical 555 timer at is 15 volts, but they can supply enough current to drive the base of high voltage transistors. The choice of transistors depends on the current that this 300 volts will drawing. Could you describe how this is supposed to work a little better? Are you basically just charging up a plate that your experiment is sitting on? If that's the case you're right it won't be using much current at all

 

[Sceadwian]

By the way Khelz, it's generally not required to private message a specific person when responding like that, most people keep tabs on any thread they post in, at least the ones that you want to hear from.

 

[khelz]

Okay so the circuit will just be used to charge a capacitor. That's it. But How can I know how much current will be drawn ? My first thought is to calculate the impedance of the circuit(by summing each impedance of each components that will be written in the specs sheet) and then by V = RI find the current. Is that a good direction ?

 

[Sceadwian]

200hz is relativly slow, but charging and discharging a capacitor will cause AC current to flow so yeah you'd need to find the impedance of the circuit with the waveform you'll be driving it at, if you know the capacitance it's pretty easy to use something like LTSpice to simulate the current draw with a square wave.

 

[khelz]

Well I didn't choose any capacitor yet. Basically I can take anyone that can handle 300 V. But I guess that the lowest capacitance will provide the higher impedance which will provide the less current. Do you have any suggestion ?

 

[Sceadwian]

Wait now I'm confused, you said this is for a biological project where you need to charge a sample to 300 volts, now you're talking about charging a capacitor? What exactly are you trying to do? The only thing I've been able to think you're talking about is charging a piece of foil that's near a biological sample container to 300 volts to see what happens to the critters when they're in the presence of an electric field.

 

[cowana]

You also said that the lower the current is, the better. Are you aware that a very low current will equal a very long time to charge the capacitor? What is this specifically for?

 

[Sceadwian]

Only if the capacitance is high cowana, if all he's talking about is charging a plate to a voltage the capacitance will be very low at 200hz there will be little leakage current so the demand on the power supply will be low, I'm just confused as to why they're now talking about adding a capacitor to charge. Specifics are definitely needed.

 

[khelz]

I took a look back at my project....and there is no capacitor at ALL....I was way in the clouds. So Basically the voltage will be applied to an electrode about 1 mm width and 2 mm long. So we got the voltage source which is DC and around 10V or whatever how much the 555 timer can handle, and we'll have the 555 timer, after that we'll have the transistor and finally the electrode. This is the circuit if I understand well.

 

[Sceadwian]

What is this electrode connected to? It's a very different story if it's submerged in a liquid as opposed to isolated from the medium and just there to produce an electric field.

 

[khelz]

it will be in the air basically...but no submerged in any other fluid