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Author Topic: Variable current-fixed voltage source needed with pwm control  (Read 1744 times)
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snowman
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« on: August 22, 2017, 11:54:02 23:54 »

Hi friends,
I need to control a hydraulic valve. Its manual says works with current not voltage. Voltage is fixed and its 24 volts, truck battery. But current varies from 0 to 40 mA so valve changes liquid flow (Zero value is important). I'm familiar with voltage regulators but never designed any current source. First choice would be LM317 as current regulator but you know it never drops voltage to zero if used as voltage regulator so I have doubts. At the end I'll need to control it with arduino pr raspberry pi so I need pwm control (I can handle that).

I could not find ant useful starting point. Not here, not in google. Of course I'm sure there is but.... Also I could not find any ready to use module for arduino or raspberry pi at ebay or aliexpress (buying would be much easy).  I looked into battery chargers, lab power supplies but I could not find an easy solution. I don't want to place an entire power supply circuit in my project (there are plenty of pwm controlled lab supply schematics but circuits become huge).

Can anyone supply an sample variable current source or point me to a right direction please ?

best.
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sadman
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« Reply #1 on: August 23, 2017, 09:50:12 09:50 »

here is design how used lm317 as current regulator you can used pwm signal to instead of variable resistor to control current 

sadman
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mars01
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« Reply #2 on: August 23, 2017, 12:26:42 12:26 »

You can have a look here:
http://www.analog.com/en/design-center/reference-designs/hardware-reference-design/circuits-from-the-lab/cn0151.html#rd-commonvariations

It will require that you generate negative voltages from your single positive supply but that should not be an issue. No longer PWM control but control over SPI.
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dennis78
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« Reply #3 on: August 23, 2017, 01:08:26 13:08 »

What is exactly model/type of hydraulic valve?
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Signal
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« Reply #4 on: August 23, 2017, 01:22:06 13:22 »

I'd use PWM switched voltage output filtered by coil inductance or by additional filter stage with current feedback from sense resistor in ground to ADC input. But better I'd search for recommended circuit for that (unknown for us) valve and not invent the bicycle.
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jzaghal
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« Reply #5 on: August 23, 2017, 04:35:10 16:35 »

Hi,

You may find this block diagram useful.
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snowman
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« Reply #6 on: August 23, 2017, 04:38:08 16:38 »

Hi,
I don't know the valve part number YET. I'll get it probably in a week.

I found LT3086 chip and looks promising. But unfortunately there are no proteus library. I'm looking for it now.

http://www.linear.com/solutions/4475
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Old_but_Alive
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« Reply #7 on: August 23, 2017, 07:54:17 19:54 »

I use Linear Tech products wherever I can.

Their products are superb.

don't ever use proteous its rubbish.

Use LTspice, and you will be happy.

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LithiumOverdosE
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« Reply #8 on: August 23, 2017, 08:44:44 20:44 »

don't ever use proteous its rubbish.

That statement is quite misleading.
Do you have any argument to support your claim aside from lack of specific LT models?
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Old_but_Alive
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« Reply #9 on: August 23, 2017, 09:20:14 21:20 »

its not real.

the CCS site wont accept anymore proteus virtual simulations.

I may have been overzealous, but real hardware, a scope etc is the only way to work
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LithiumOverdosE
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« Reply #10 on: August 24, 2017, 12:11:25 00:11 »

I fail to see what CCS has to do with analogue simulations of current regulators in Proteus?

While sims will never entirely replace hardware testing in many cases they significantly cut down on development time and costs.
Proteus is quite capable SPICE simulator and IMO in some aspects superior to LTSpice so I don't quite understand why you recommended to avoid using it "ever".
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PICker
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« Reply #11 on: August 24, 2017, 12:26:06 00:26 »

I dont know if this component can help you in the design:
http://www.ti.com/lit/ds/snvsa90/snvsa90.pdf
just try to read the datasheet.
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snowman
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« Reply #12 on: August 24, 2017, 10:31:15 10:31 »

So folks,

for days I'm searching for answers and your suggestions. I keep hitting to articles and schematics about LED driver circuits. As I see they obtain stable current to LEDs and some of them can change brightness. like LM5021 mentioned my PICker. I know LEDs work with current no voltage so...

LED drivers can give me adjustable output current right? And they can solve my problem. Am I right ? I'm asking because I don't know how LED drivers work (yet), I made a simple reading and it looks they can solve my problem.

Again, I need an adjustable current source to control current driven valve, 0 to 40 mA is enough, and operating voltage is 24v DC, a truck battery.


And there are plenty of pwm controlled schematics there.

Any thoughts ?
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PICker
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« Reply #13 on: August 24, 2017, 03:05:17 15:05 »

You can try to study the principle and the implementation of the Howland current source:
http://www.ti.com/lit/an/snoa474a/snoa474a.pdf
http://www.linear.com/solutions/Current_Source_Solutions
http://www.edn.com/design/analog/4430167/Op-Amp-Current-Sources--The-Howland-Current-Pump
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Signal
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« Reply #14 on: August 24, 2017, 08:40:25 20:40 »

Any thoughts ?
I have some. Lets start from words "MOST LIKELY".
Most likely there is just a solenoid with spring.
Position of a solenoid's core defines a flow.
Position corresponds to length of a spring that change is proportional to current through the coil.
Current through the coil depends on impedance that should be stable. So closed control loop PWM->voltage->current sense->ADC looks as fast enough.
PWM here is not for power efficiency, but for convenience.
I do not think that you need a real source of current such as bipolar transistor in common emitter connection or analog closed loop with opamp as error amplifier.

Posted on: August 24, 2017, 07:40:12 19:40 - Automerged

Integrated amplifier - and controller electronics for proportional hydraulic valves:
http://www.wandfluh.com/fileadmin/user_upload/Wandfluh/Products/Components/DataSheets/Englisch/1.13%20Electronics%20and%20electro-accessoories%20for%20proportional%20valves/1_13_75_e.pdf

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dennis78
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« Reply #15 on: August 25, 2017, 02:19:50 02:19 »

I think for his system pwm without control loop is enough precise, but without type of valve (simple solenoid, integrated servo or...) we don't know is it pwm adequate type of driving signal.

flow~K*(duty_cycle*(V(24V)/Rsolenoid)).

Reason for variant without current control loop is because without at lest one other measurement (flow, pressure, position of valve moving part, or...) precise  current not solving anything because parameters of mechanical parts of valve is changing over time (temperature, other conditions,..) and precise current don't guarante valve state. Only reason for closed loop in this case is compensating of heating coil winding.

Also, PWM method is good for zero current (duty=0%) - for other(regulated) systems (mostly variants ) is much harder to realize.

If his valve has integrated servo then he must drive it with precise analog current signal to get best performance.
« Last Edit: August 25, 2017, 02:23:00 02:23 by dennis78 » Logged
snowman
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« Reply #16 on: August 25, 2017, 10:25:52 10:25 »


hehe yes I need its schematics Smiley But thats the idea. thanks.
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dennis78
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« Reply #17 on: August 25, 2017, 11:18:54 23:18 »

If you read spiecification from last document Imax=1.5-2A so linear outut stage is eliminated. Only option is buck topology (clasic or  synchrony) driven directly by MCU and regulated from MCU. Only difference than direcltly PWM driving is that they added simple LC filter to output stage because of eliminate switching effects. If you look for some "magic" controller/schematic I think you waste time. Specific switching controller -> more components without any significant advantages. Another problem is how to precise set current in that case.

Something like this (R1 is place for coil of valve Smiley :




In source you can add 10-100mOhm shunt with RC to ADC of MCU to meas (maybe with simple amp x10 or similar). Some solutions use mosfet as shunt but it is little specific for measure and not very precise.
« Last Edit: August 25, 2017, 11:31:28 23:31 by dennis78 » Logged
sadman
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« Reply #18 on: August 28, 2017, 11:22:02 11:22 »

low drop voltage to current source
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Signal
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« Reply #19 on: August 28, 2017, 08:14:41 20:14 »

low drop voltage to current source
While LT1636 is nice the posted circuit looks like a zoo comparing with clean typical application from LT1636 datasheet http://cds.linear.com/docs/en/datasheet/1636fc.pdf.
I do not see any benefits of this circuit (apart from the fact that it is not suitable for task of this thread):
- Too many elements.
- Redundant double supply.
- If R5 adjustment is for current level then current will depend on voltage of +12 rail because of compromised CMRR of differential amplifier.
- Current level is controlled by IRL520 as a "reference" element.
- There are no integrator in closed loop - just proportional that gain is mainly set by current/voltage gain of Q2
- energy inefficient.
- by adding three resistors: one as current limiter and two as divider for LT1004 reference the circuit from LT1636 datasheet becomes "low dropout" too with possibility of direct adjustment of output current without tradeoff.

All this just to use n-channel mosfet? unconvincingly.
What do I miss?
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optikon
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« Reply #20 on: August 29, 2017, 02:41:48 02:41 »

low drop voltage to current source

Another issue with that circuit, have a look at the transient response / bandwidth with that VOM1271 photovoltaic device in there.
Maybe its ok for the OP application but as a current regulator, response to changes in load may be important.

LT is pricey and not really needed here..
« Last Edit: August 29, 2017, 02:54:07 02:54 by optikon » Logged

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Signal
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« Reply #21 on: August 29, 2017, 05:20:26 17:20 »

Another issue with that circuit, have a look at the transient response / bandwidth with that VOM1271 photovoltaic device in there.
Maybe its ok for the OP application but as a current regulator, response to changes in load may be important.
As long as a closed loop is stable a response is not a problem. MOSFET with stable Ugs is a quite stable current source itself that is naturally ready for load and supply change.
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