Charging-capacitor Circuit

Discussion in 'The Projects Forum' started by jpborunda, Jun 2, 2014.

  1. jpborunda

    Thread Starter Member

    Apr 9, 2014
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    Hello everyone.
    I could really use some guidance right now...
    I am designing a circuit, its a voltage controlled current sink. As shown in attachment 1. The circuit is pretty simple. It will provide current pulses to a Load of 1kΩ. Those pulses, need to be: current amplitude = 10-100mA, pulse width = 200-500µsec, frequency of 20-100Hz. To apply this current amplitude to a 1kΩ, a high voltage is needed (100V at the load).

    Here is where I need your help.
    To obtain this voltage, I bought a "miniature DC converter". Here is the datasheet: (http://www.datasheets360.com/pdf/5050517326450757206) It is a 5 to 200V converter. It's limitations are that it can only source 6.25mA at max load. Attachment 2 shows a schematic from an article that used this converter. The converter's datasheet says that a 0.1µF, 1000V DC capacitor is needed at the output, and the article says "R10(220kΩ) ensures minimum load for the converter, and R11(47kΩ) ensures the current it sources is always below maximum." That leaves the other 2 capacitors, which are charged up to 200µF total and used for delivering the voltage/current needed.

    However, if the math is right, this might fulfill my requirements.
    I have two parallel 100µF caps, = 200µF = 200(µA·sec)/Volt
    The load is 500µsec times 0.1A = 50µA·sec
    C = Q/V; => V = (50µA·sec)/200µF = 0.25 V drained from caps EACH PULSE.
    Now say for 1 second, I would have 500µsec· 100 pulses = 0.5 sec of ON time, now 0.5sec · 0.1A = .005A·sec per second = Thats 5000µA·sec per second.
    Since the caps have 200(µA·sec)/Volt, thats 5000/200 = 25V loss from the capacitors EACH SECOND.
    If the capacitors ran continuously after this point at 175 volts instead of 200 volts, the replenishment current through R11 (47k) would be 25V/47k = 532µa sec per second. As you can see this is not enough to keep the caps fully charged because the full load will require 5000µA·sec per second!

    I was hoping you guys could help me in figuring out what changes I should make to this design so that I can deliver the current/voltage I need, considering that the converter sources only 6.25mA MAX. I see that the capacitors are key here, but I dont have a lot of experience with them so I'm having a hard time figuring things out.

    If anything is not clear just let me know. I'm a beginner so any help at all is appreciated.
     
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  2. jpborunda

    Thread Starter Member

    Apr 9, 2014
    55
    0
    By the way, I'm having trouble posting a thread with a tittle longer than 25 characters, so I'm sorry if the tittle is a little vague.
     
  3. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    The capacitors are not the problem or the solution. They are just cans full of energy.
    1K at .1 amp will require 100 volts minimum on the capacitors (plus a few volts for the transistor and a few volts for the sense resistor). The ultimate load is 5 ma, so you can't afford to lose more than about 95 volts in R11 when 5 ma flows.

    If your square wave is 1 volt high, the sense resistor must arrive at 1 volt when .1 amp flows through it. That's 10 ohms. If you can drive 10 ma into the base of the bipolar transistor, you can get 100 ma to flow with a loss of a few tenths of a volt. So, you need less than 2 volts for the driving and sensing components. You have 200V to start with so you can lose 98 volts in R11.

    The highest resistance R11 can be is 19,600 ohms.
    A standard size is 18,000 ohms.
    If 5 ma flows through 18k, the power it causes is .45 watts. Buy a 1 watt resistor.

    Can you get into the power supply to change R11?
     
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  4. jpborunda

    Thread Starter Member

    Apr 9, 2014
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    I see.
    I CAN change anything on the supply except for the 1000V-0.1µF Capacitor.
    When referring to the ultimate load being 5mA, that's the current needed when delivering the "worst case scenario" values right? Which corresponds to the 5000µAsec per sec for the capacitors to maintain a steady voltage correct?

    Just to check if Im understanding this correctly: If I change R11 to 18k, the resistor would be consuming V=(5mA)(18k)= 90V, then I should have about 110V on the caps, but those 5 mA (which is the highest current the circuit demands) would also be maintaining the voltage to keep them charged correct?
    Plus having 110 V would leave enough room for the voltage at the sense resistor, and the transistor.
    Thank you very much again!
     
    Last edited: Jun 3, 2014
  5. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Sounds right to me.:)

    You could even lower R11 to 15k without smoking the power supply but you're pushing the heat up to using a 2 watt resistor.
    Of course, you can oversize an 18k to 2 watts power rating and the only result is that it will run cooler under load.
     
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  6. jpborunda

    Thread Starter Member

    Apr 9, 2014
    55
    0
    Thanks a lot for the help! Really appreciate it.
    Sorry it took me so long to answer back!
    I will be implementing this circuit shortly (I hope). We'll see how it goes and let you know hehe (just for fun)!
    Thanks.
     
  7. jpborunda

    Thread Starter Member

    Apr 9, 2014
    55
    0
    Hi again, I got caught up with some other activities which kept me from prototyping the circuit, but finally I was able to get back to it. I seem to be having some unexpected behavior though, so I could use your help guys!This was built on a breadboard! .

    First, I tested the Current source alone without the HighVoltage, using a stationary source (attachment 1, FIRST TEST). I used an oscilloscope to see the voltage at RLoad, and from there I calculated the current. The waveform oscillated A LOT from when ON, it followed the timings but with a lot of oscillation. I tried by replacing the bjt with a mosfet, but the output was the same. Later I added 2 resistors and a capacitor as shown in attachment 1, TEST 2 (from a post I saw online), and it did stabilized the output. Attachment 2 shows the clean output for a 100Ω resistor at 50mA. -> V = 5V, WITH THE MOSFET.

    So after that, I decided to use the 200V converter circuit as was decided before. Using the R18k, if I input 4 V to the converter, it outputs 220V, and the caps charge up to such voltage. The converter's output current is 7mA at start up (datasheet says Io= 6.25mA max), then drops exponentially to about 120mA as the caps are fully charged. The input current is about 350mA (max input current) initially, then drops as well.

    Here is the problem tough:
    I let the caps charge, turn on the function generator (I applied a square wave of 400mV, 250µs, 25 Hz) , then using a mechanical switch I connect the 200V to the RLoad. Immediately after I do that, the converter's output drops to exactly 149V, R18k has a voltage drop of 100V, so both caps stay at 49V, the converter's output current is 6.42mA (still above max from datasheet) and the input current is 325mA (pretty much max input current), the current in the 18k resistor is 5.73mA. The mosfet voltage is also 49V. The voltage at Rload is shown in the last attachment ("4"). You can see that the voltage reaches the expected values 4V in Rload= 100Ω to get 40mA, but instead of going to 0 when the square wave is at 0, it stays 500mV above ( again, see attachment "4"). I believe this may be affecting the High voltage circuit, and that is why the caps are at 49V. BTW their voltage doesn't seem to fluctuate at all.
    Also, the output current of the converter stays at 6.42mA, which is more than the 5mA I was expecting.
    So basically, what could I be doing wrong? What could I do to make sure that the voltage at the RLOAD goes to 0V when the input square wave is also 0V. And finally how can I make sure that I limit the Output current so that I don't burn the 5-200V converter?

    BTW just as I was about to leave the Lab, I tested the circuit with a RLOAD=330Ω, applying 20mA, and got the expected 6.6V, but the same voltages and current for the 5-200V converter, which are higher than or almost the absolute max ratings.

    Thank you guys this forum has been extremely useful for a beginner such as myself. I hope I'm doing an OK job at explaining the situation, thank you!!!
     
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  8. jpborunda

    Thread Starter Member

    Apr 9, 2014
    55
    0
    OK, it was an embarrassing mistake. After looking around I realized that I was making a mistake when measuring. I basically was shorting with the probe, so that's why the signal was dropping. Also one of the probes for the oscilloscope I was using was damaged, because the x1 x10 switch was not actually changing the scale.

    Just wanted to clarify it so I don't make anyone think in vain...

    Thank you guys.
     
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