Nigel Goodwin said:I don't see any relevence to a "voltage to current converter"?, but a constant current source is what you use to charge a capacitor in a linear fashion.
The reasoning behind it is VERY simple to understand, if you charge a capacitor then the voltage across it increases as it charges up (which is the whole point in doing it). So if you charge it via a simple resistor then the current through the resistor will decrease as the capacitor charges up, because the voltage across the resistor will decrease - simple ohms law!. It's this decreasing current which gives the capacitor charging it's characteristic curve - using a constant current obviously prevents this happening, as the current doesn't vary.
e-l-e-c-t-r-o said:Nigel Goodwin said:I don't see any relevence to a "voltage to current converter"?, but a constant current source is what you use to charge a capacitor in a linear fashion.
The reasoning behind it is VERY simple to understand, if you charge a capacitor then the voltage across it increases as it charges up (which is the whole point in doing it). So if you charge it via a simple resistor then the current through the resistor will decrease as the capacitor charges up, because the voltage across the resistor will decrease - simple ohms law!. It's this decreasing current which gives the capacitor charging it's characteristic curve - using a constant current obviously prevents this happening, as the current doesn't vary.
Now I am getting it.
So, in my in/out graph, what CCS is needed?
The input voltage range is 0-5 Volts, time in the graph is 10-20 ms.
All I have to do is place a constant current source before the cap to get that "red" line in the output stage?
Are there any CCS circuits out there for that?
thanks for the help, by the way.
Ron H said:The graph you posted is not the way a capacitor will respond to being charged by the current waveform you showed. Below is more representative of the voltage across the capacitor as a result of the applied current. I do think this is what your instructor is after. See the schematic I posted in your other thread.
Ron H said:This is getting complicated. It can be done, but why do you want to do it? We frequently get questions here that are too far into the details. When the real goal is revealed, another approach turns out to be better.
Well, no information is lost unless you are trying to make a slew rate limiter. If you really want to clamp at the control voltage, you are correct in thinking that a diff amp is involved (an op amp is the simplest diff amp you could use). Here's a way to do it.e-l-e-c-t-r-o said:Ron H said:This is getting complicated. It can be done, but why do you want to do it? We frequently get questions here that are too far into the details. When the real goal is revealed, another approach turns out to be better.
oh, the goal was finding reference of a linear charging capacitor schematic.
Goal achieved.Much appreciated.
But the schematic with the output you draw got me really curious on something:
The capacitor charges linearly, but since the initial input remains constant for ever after some time (it gets at the maximum voltage, but it is a constant dc level),
the linear charging capacitor charges up to the supply voltage.
So, a very important information seems to be lost there:
The maximum level of dc voltage of the initial input, no matter what value it will have, the output voltage will always reach the supply voltage (in different times, of course).
I assumed there has to be a way to link input with output voltage levels,
a way to force the output not to increase beyond the level of the initial input.
I just assumed this is trivial piece of info to be lost.I am wrong?
:?:
May be it is not that difficult, a differential amplifier could provide the base of comparison,and what if we could find a way to force the capacitor to stop charging?
hooo, it starts to get interesting!!
Thank you for your responses.
It's not bad - or good. A current source charging a capacitor is not a useful circuit unless it gets discharged sometime, for some reason.e-l-e-c-t-r-o said:All I was qurious was whether with the circuit you suggested, we have an output level that does not relate to the level of the input maximum point.
Cause if the capacitor is charging as long as the input remains constant (i.e for eternity) at its higher value, then the capacitor will continue to charge (not forever, until it reaches the converter's limits).
This is bad, right?
What is your tutor asking? Please answer this.I don't think the slew rate limiter is what my tutor is asking, although I think that clamp generator perform a such a role, right?
You can simply discharge the cap with a switch to GND (or some other voltage, and with or without a series resistor). The switch can be an NPN, a MOSFET, a JFET, etc. You could get more elaborate and discharge it through a current sink, constant or programmable. There obviously has to be some event to trigger this, be it an external event or an output from a comparator.I got it about the inability to discharge.I am curious to whether this ability could be added.
Can u suggest a theory frame for what I am curious to as if it could be built?
clpam? I'm assuming this is means "clamp". The "clamp" generator is just the last block diagram I posted. The controlled current source is not part of a clamp generator.All I suggested is a way to "force" the capacitor to charge up to the point of the initial input's maximum voltage.
I just think that a clpam generator should have this feature.
:roll:
Yeah, I've been thinking about mentioning an integrator, but I got the impression (perhaps erroneous) that he needed to be able to drive a grounded load. An integrator certainly simplifies the V-I conversion process.spuffock said:Try "integrator". An op amp is connected with + input grounded, capacitor between - input and output, voltage signal applied via resistor to - input.
Op amp will keep - input at virtual ground so input current will be constant for a constant input voltage. Cap charging will be linear until op amp output gets near rail. Hope this is what's needed
Ron H said:It's not bad - or good. A current source charging a capacitor is not a useful circuit unless it gets discharged sometime, for some reason.e-l-e-c-t-r-o said:All I was qurious was whether with the circuit you suggested, we have an output level that does not relate to the level of the input maximum point.
Cause if the capacitor is charging as long as the input remains constant (i.e for eternity) at its higher value, then the capacitor will continue to charge (not forever, until it reaches the converter's limits).
This is bad, right?
What is your tutor asking? Please answer this.I don't think the slew rate limiter is what my tutor is asking, although I think that clamp generator perform a such a role, right?
The clamp will cause the cap charging to stop at the control voltage level, but the slope of the output will be a function of the control voltage. That's not the way a slew rate limiter works.
You can simply discharge the cap with a switch to GND (or some other voltage, and with or without a series resistor). The switch can be an NPN, a MOSFET, a JFET, etc. You could get more elaborate and discharge it through a current sink, constant or programmable. There obviously has to be some event to trigger this, be it an external event or an output from a comparator.I got it about the inability to discharge.I am curious to whether this ability could be added.
Can u suggest a theory frame for what I am curious to as if it could be built?
clpam? I'm assuming this is means "clamp". The "clamp" generator is just the last block diagram I posted. The controlled current source is not part of a clamp generator.All I suggested is a way to "force" the capacitor to charge up to the point of the initial input's maximum voltage.
I just think that a clpam generator should have this feature.
:roll:
I see no point in stopping the ramp when it reaches the control voltage. Why does this seem so important to you? Suppose the control voltage was 0 to 100 volts (which we could easily do). What do you do now?
If you are looking to make this do something useful, you could make a voltage-controlled oscillator by connecting this circuit (possibly with some modifications) to a 555 timer chip or some other sort of Schmitt trigger.
Actually, in the text with the schematic, I said essentially that:spuffock said:The voltage controlled current source in Ron H's vco will provide the calculated current less the base current of the transistor. Using a FET will eliminate this. Of course, you will need enough voltage to turn the FET on..
Sorry, it's nit-picking day..
But nit-picking is still good. It must be - I do it all the time. :roll:If you needed it to be precise, you would need to use precision resistors, replace the BJTs with small geometry MOSFETs, and use precision op amps. "
e-l-e-c-t-r-o said:Give into a Voltage to current converter (the one u suggested) the input signal (blue), charge a capacitor linear BUT
when the capacitor's voltage reaches (actually tries to surpass) the voltage level of the input signal,
SOMETHING (maybe a differential amplifier with a few extra components :idea: )
forces the INPUT signal to revert to zero and the capacitor to discharge :shock:
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