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pulsed current circuit

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unlv007

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Hi
We are performing an experiment where we need a circuit to produce a constant current of 10mA for 30s followed by a sequence of 10 pulses of 5sec duration, each pulse being 400mA. The pulse separation is arbitrary.The pulses are applied to an experimental resistive device, which has a nonlinear time dependent resistance. We expect that the voltage drive needed might be about 100V but the higher we can design, the better. However we expect the voltage drive will be maximum 200V.
Please give me some tips on how to proceed on this problem
Regards
 
One way would be to design a circuit as follows:

1.) Use a PIC or other microcontroller device capable of operating the 555 timers spoken of below. Use 2 power supplies, one for each desired current output.

2.) The 1st 555 timer will be set up in monostable mode configured to provide a 30 second voltage on pin 3 when a voltage from the PIC is applied to pin 2 of the 555. A relay is set up on pin 3 which, when it goes high (voltage is applied to it) allows the 10 mA from your supply to flow to the experimental load. After the 555 counts 30 seconds the voltage will cease to be applied to the relay via pin 3, cutting off current flow into the load. The PIC will then apply ground to pin 4 momentarily ensuring this happens.

3.) The second 555 timer which is hooked up in ASTABLE mode to provide the pulses of 5 second duration. This is also hooked up to a relay in the same manner (you would make sure this relay would be rated above 400mA, of course) Program the PIC to shut this down after the 10 pulses ends. It will also be supplied with a separate supply only this one regulated to the desired 400mA

You can also accomplish the above (without having to program a PIC) by setting up switches to actuate the timers and shut them off again. On the 555 pin 2 is the trigger which turns them on, pin 4 is reset which stops them, and pin 3 is the output pin. Note that with the 555 in astable mode you are basically using it as an oscillator whose frequency is determined by the values of the supporting components. Have a look at the data sheet for more info.
 
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you dont need more current, the relays are what is carrying the current to your load, the 555's merely actuate them..

The relays you need are going to be normally open (NO) and will have 4 pins on them. Two of those pins are used to operate the relay (turn it on when power is applied, off when power ceases), and the other 2 are the ones which will allow current to flow to your load. You would hook up one pin from the actuator side to ground, the other pin will go to pin 3 of the 555 timer, and on the other side hook up one of the wires from the power supply and the other to the experimental load..
 
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Thanks for your tips.
I will do some search on what specific Relay to use and get back with a finalized version.
 
Relays would probably be fine, except unlv007 stated that he needs constant current into a nonlinear, time-varying load. A relay and a resistor are not going to provide constant current.
 
**broken link removed**

Thanks everbody for your tips. I was thinking about what Roff just now wrote. So in the diagram attached by 0x00x0 above, the power supply should be a constant current source of 400mA current generation capacity and compliance voltage 100-200V as per the load demand. Is that correct?
If so, i will start working on designing a current source of 400mA and 100-200V compliance voltage.
 
Yes, you would design your power supplies to provide your required voltage and current requirements. That part I cannot help you with. I am no good at power circuit design yet..
 
Why on earth would you use 555 timers as well as a PIC?

The PIC can emulate the 555 timers pretty easilly.

The constant current source is the only really tricky thing, what power supply do you have?

The only sensible way of doing it is to build a switching regulator, which is an entire project in itself.
 
As per the design above, i need two power supplies- one to produce 10mA and another to produce 400mA. The compliance voltage is likely to be between 100-200V.
I have a power supply that can provide 80mA and source up to 500V.
However i do not have a power supply that can generate 400mA constant current and provide the driving voltage= 200V. How to build this power supply?
 
I meant what's your primary power supply?

If it's the 500V 80mA power supply then it's hard luck because it can only provide you with 40W and you require 200V @ 400mA which is 80W.
 
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I have one more power supply that can produce 1A and upto 60V- that is 60W power available. How do i convert it into 300mA constant current source with 200V compliance?
 
You can't, no converter is 100% efficient.
 
My requirement is 300mA constant current at 200V. I can borrow from another lab a power supply with 60V and several amperes say 5-6Amps rating, Now how do i design my const current circuit?
 
With great difficulty.

Firstly, is it possible that the load could cause the output to drop below the input voltage?

If so you can forget using a single inductor flyback circuit, you need either a SEPIC or a transformer, the latter is probably a good idea anyway given the voltage step up.

Does the output need to be isolated from the input?

Can the output 0V rail be different to the input 0V rail?

I did a search on National Semiconductor's site for a switching IC that can accept 60V and put out 200V and it came up with this:
**broken link removed**

You'll need to modify the circuit so it limits the current. My post linked below shows how you can modify an existing switching regulator to make it a constant current source. Beware that in this case you need to either do low side current sensing or use a hall effect sensor for high side sensing.
 
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