MCU control Contant Current Source - Electronic Circuits Projects Diagrams Free

VictorPS · 23rd October 2007, 09:04 AM

I need to design a constant current source/ or current limitter, from a fix 24V, control current from 0 to 0.5A max.

I need it control by PIC MCU, any idea on how to do it?

Currently, what I think off is using I2C + Digital Pot + Mosfet.

Any other suggestion?

**Nigel Goodwin** · 23rd October 2007, 09:21 AM

You don't need a digital pot, although that is one solution, all you need is a D2A, and a simple R2R ladder would be fine (and cheap).

Even cheaper, you could use PWM, there are application notes at MicroChip about using PWM as an analogue output.

eblc1388 · 23rd October 2007, 09:41 AM

You also need to decide how to maintain the current constant.

Either via active feedback loop where PIC reads the actual load current value and compensate, OR via analogue control where the PIC generates a fixed voltage reference proportional to the required current and the hardware compares this reference with the actual load current and control the current accordingly.

VictorPS · 23rd October 2007, 11:48 AM

Sorry, is my fault not state clearly, let me add on more info.

That constant current is use to control LED, a light source for camera, so I cannot use PWM which flickering, so a linear constant current must use. (as I know, may be there is other method)

Also, I may need to control at least 4ch, and max up to 32 ch, each channel is different brightness/ current source.

My idea is use V to I , so choose to use MOSFET or BJT as current control.
The input V can be DAC, but DAC is not cheap, so I choose D-Pot.

The resolution I need is about 8bit, or 256 steps.

I will consider Nigel Suggestion, using R2R, generate using 8bit shift register like CD4094, with this, I can have many ch too.

Thank you,

Sceadwian · 24th October 2007, 01:13 AM

Why would you need 32 channels for a camera light source? PWM is fine for cameras, you just use a frequency much higher than the shutter speed. PWM is going to be simpler to implement than a mosfet controlled current source. For PWM all you need is a single resistor for each LED to limit current, and a then you change the PWM to lower the light level from it's max limit.

VictorPS · 24th October 2007, 01:54 AM

it is use for industrial machine, for vision inspection test, each test need at least 4 channel light source at different angle, for quad test, it will need 16channel.

For knowledge, how to generate high freq PWM for multiple channel? I am using PIC MCU, or any external PWM generator IC I can use to generate very high Freq PWM?

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23rd October 2007, 09:04 AM	(permalink)
VictorPS	MCU control Contant Current Source I need to design a constant current source/ or current limitter, from a fix 24V, control current from 0 to 0.5A max. I need it control by PIC MCU, any idea on how to do it? Currently, what I think off is using I2C + Digital Pot + Mosfet. Any other suggestion?

23rd October 2007, 09:21 AM	(permalink)
Nigel Goodwin	You don't need a digital pot, although that is one solution, all you need is a D2A, and a simple R2R ladder would be fine (and cheap). Even cheaper, you could use PWM, there are application notes at MicroChip about using PWM as an analogue output. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

23rd October 2007, 09:41 AM	(permalink)
eblc1388	You also need to decide how to maintain the current constant. Either via active feedback loop where PIC reads the actual load current value and compensate, OR via analogue control where the PIC generates a fixed voltage reference proportional to the required current and the hardware compares this reference with the actual load current and control the current accordingly. __________________ L.Chung

23rd October 2007, 11:48 AM	(permalink)
VictorPS	Sorry, is my fault not state clearly, let me add on more info. That constant current is use to control LED, a light source for camera, so I cannot use PWM which flickering, so a linear constant current must use. (as I know, may be there is other method) Also, I may need to control at least 4ch, and max up to 32 ch, each channel is different brightness/ current source. My idea is use V to I , so choose to use MOSFET or BJT as current control. The input V can be DAC, but DAC is not cheap, so I choose D-Pot. The resolution I need is about 8bit, or 256 steps. I will consider Nigel Suggestion, using R2R, generate using 8bit shift register like CD4094, with this, I can have many ch too. Thank you, Last edited by VictorPS; 23rd October 2007 at 11:52 AM.

24th October 2007, 01:13 AM	(permalink)
Sceadwian	Why would you need 32 channels for a camera light source? PWM is fine for cameras, you just use a frequency much higher than the shutter speed. PWM is going to be simpler to implement than a mosfet controlled current source. For PWM all you need is a single resistor for each LED to limit current, and a then you change the PWM to lower the light level from it's max limit. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

24th October 2007, 01:54 AM	(permalink)
VictorPS	it is use for industrial machine, for vision inspection test, each test need at least 4 channel light source at different angle, for quad test, it will need 16channel. For knowledge, how to generate high freq PWM for multiple channel? I am using PIC MCU, or any external PWM generator IC I can use to generate very high Freq PWM?