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milem
Joined: 11 Mar 2008
Posts: 9
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 11 Mar 2008 11:07 Costant current with Atmel 89C52 |
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I need schematic to kontrol current with microkontroler.
It is used to charge kapacitors and then with window komparator (from 1V to 2.5V) generate impuls who is proprtional with time of charge kapacitor.
Any idea is welcome!
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laktronics
Joined: 06 Jan 2007
Posts: 945
Helped: 156
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11 Mar 2008 12:02 Re: Costant current with Atmel 89C52 |
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Hi,
What is the current range, Steps of Change and Accuracy required?
Regards,
Laktronics
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milem
Joined: 11 Mar 2008
Posts: 9
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11 Mar 2008 12:41 Re: Costant current with Atmel 89C52 |
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Range is to maximum about 4mA.
And maximum steps are 7. (One port 7 for selecting current and 1 for empting cpacitor).
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laktronics
Joined: 06 Jan 2007
Posts: 945
Helped: 156
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11 Mar 2008 15:46 Re: Costant current with Atmel 89C52 |
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Hi,
Check if Analog Devices ADN8810 suits you. Google for 'Programmable Current source' for more products.
Regards,
Laktronics
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milem
Joined: 11 Mar 2008
Posts: 9
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11 Mar 2008 16:35 Re: Costant current with Atmel 89C52 |
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Sorry I can't use them!
I must use only transistors to get
current and must have Voltage on capacitor biger than 3V!
I have found one current source with PNP transistor and Zener diode.
Now, problem is how to regulate current!
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manish12
Joined: 21 Nov 2006
Posts: 1020
Helped: 31
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11 Mar 2008 17:00 Re: Costant current with Atmel 89C52 |
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as the diff between the charging voltage and cap voltage get reduced, respectively charging current will also reduced .
but you can use dac / resistor network for for maintain the same diff between it !
or charge it from the ramp like w/f / saw tooth w/f.
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drbizzarow
Joined: 24 May 2006
Posts: 276
Helped: 9
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11 Mar 2008 18:02 Costant current with Atmel 89C52 |
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| this ciruit is not a constent current source but voltage source.
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laktronics
Joined: 06 Jan 2007
Posts: 945
Helped: 156
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11 Mar 2008 19:02 Re: Costant current with Atmel 89C52 |
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Hi,
Want to try this circuit?
Regards,
Laktronics
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11 Mar 2008 19:02 Ads |
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milem
Joined: 11 Mar 2008
Posts: 9
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12 Mar 2008 11:41 Re: Costant current with Atmel 89C52 |
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That circuit I have, only diference is I use zener diode, not two standard diodes.
Only problem with your circuit is that current always go trough condenzator.
Then when circuit going to empty condenzator trough another NPN transistor
connected between + and - pin of condenzator with 100 ohms resistor.
Posible thing will be breakout that transistor.
Do you have any idea how to build circuit who stop current flow when empting
condenzator. Then, when charging condenzator you kontrol how much current
will go trough.
I have idea that I can use one NPN transistor, to control current like in picture!
What you think about that!
[url=http://images.elektroda.net/44_1205318173.jpg][img]http://images.elektroda.net/
Regards!
Added after 4 minutes:
Sorry, i think link is not working!
This is picture!
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laktronics
Joined: 06 Jan 2007
Posts: 945
Helped: 156
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12 Mar 2008 18:50 Re: Costant current with Atmel 89C52 |
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Hi,
The way you have put the diodes, q1 will not get base bias to conduct. If you are planning to use it as a switch to cut of collector current, then one input will do to Q1's base.
In my circuit, you can even short the collector output to ground, without any problem. So, when you use an NPN with a 100 ohm resistor to discharge the cap, the capacitor will get discharged to 0.4V (100 ohm x 4mA). If you want still less voltage use a 47 ohm resistor to discharge, the NPN intially will have to sink a current of 3V÷47 ohms ≈ 60mA, which a 2N2222 type transistor can easily take. With 2N2222, you can even go down to 10 ohm discharge resistance, with a resistor drop of only 40mV. You have to also provide higher base current to the NPN to make it sturate.Anyhow, with a typical Vcesat of 0.3V, there is no point in worrying further about this end discharge voltage.
Regards,
Laktronics
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milem
Joined: 11 Mar 2008
Posts: 9
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13 Mar 2008 12:17 Re: Costant current with Atmel 89C52 |
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I know that diodes are wrong orientation!
I forgot to change in circuit!
I agree with your comment, also I get these results!
I try my design, and change some details and I have found right design
who will work like I think.I can regulate current from couple nA to 2.3mA (in simulation for now).
I write that I need 4mA but real I change to max 2mA and something less!
Then I can use resistor at 100ohms.
And i change levels of voltage of comparator from 1V to 2.5V. Reason is that
I have also some current from comparator that bring voltage, in no working state,
to around 0.3V.
I need in couple days test circuit in real world and i will try my design and your!
Then I will decide what I will use. I think your is better, but I will see!
Thanks anyway!
Added after 2 hours 26 minutes:
What do you think about this schematic!
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