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Author Topic: AC square wave constant current  (Read 1145 times)
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free
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« on: May 29, 2008, 02:50:41 02:50 »

hi friends,

Do you have an idea or circuit for "AC square wave constant current generator" with short circuit protection ?
The spec:
1. The square wave output is 24v 200mA.
2. Max load's resistance is 800 ohm
3. When the load's resistance is changed,the voltage will drop to a value but the current always keep 200mA.
I have no idea how to keep 200mA. 
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Giovepluvio
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« Reply #1 on: June 07, 2008, 02:41:08 14:41 »

Maybe using a feedback controller. But even in this case can't regulate voltage AND current. Should be quite good for current only regulation.
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« Reply #2 on: June 11, 2008, 05:02:36 05:02 »

1. Do you really mean AC where the current reverses every half cycle or do you simply mean pulsed DC operation?

2. What frequency, low, audio, RF?

Constant current implies short circuit protection.


Quote
The square wave output is 24v 200mA.

You mean the maximum voltage compliance should be 24 volts, right? You're not suggesting the circuit should maintain 24 volts and 200 mA while the load resistance varies, are you?
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DTiziano
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« Reply #3 on: June 11, 2008, 12:04:41 12:04 »

1. Do you really mean AC where the current reverses every half cycle or do you simply mean pulsed DC operation?
2. What frequency, low, audio, RF?

Additionally

24 V on 800 ohm get 30 mA so you have to check your specification.
For 800 ohm load you need at least a generator with 160 V.

For a simple on/off a switched constant current generator can do the job (the frequency can be the problem if too high).

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« Reply #4 on: June 11, 2008, 02:40:28 14:40 »

Quote
For a simple on/off a switched constant current generator can do the job (the frequency can be the problem if too high).

Why do you think I was asking him, numb nuts?
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« Reply #5 on: June 12, 2008, 04:10:32 04:10 »

I'm sorry for the delay.

Quote
1. Do you really mean AC where the current reverses every half cycle or do you simply mean pulsed DC operation?
2. What frequency, low, audio, RF?

1. The current reverses every half cycle.
2. The frequency is 70 Hz

Quote
You mean the maximum voltage compliance should be 24 volts, right?
Yes.
 
The goal:
 I must get the every fluctuating resistance value.
 The part of data(ohm),
 ..., 10, 10.7, 15.2, 89, 90.1, 90.4, 125.2, 224, 224.2, 201,...
 
Quote
For a simple on/off a switched constant current generator can do the job (the frequency can be the problem if too high).
Could you explain "the simple on/off switches"?

« Last Edit: June 12, 2008, 04:28:44 04:28 by free » Logged

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« Reply #6 on: June 13, 2008, 08:26:38 08:26 »

FriskyFerret , your questions was OK, I just say there are additional points, still unsolved (800 ohm = 160 V).

The switch idea is quite simple, take a constant current generator and by a mosfet switch on and off the input line. Since you need to reverse the current direction this solution get more complicated and no more attractive.

P.S.
A simper constant current generator can be built by a LM317 (positive) or LM337 (negative).
Line input on input pin, connect the out to the adj with a resistance Res = 1.25 / I , take the current from the adj pin. The max power dissipated by the regulator is  = Vin * Iout – Res * Iout.
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