Constant current source adjustable

Thread Starter

O.K.

Joined Dec 10, 2014
19
Hi!

I want to make a led driver with adjustable current. I want to use an atx computer psu. Is it possible to build 5 separate step up dc-dc converter circuits each with max 40V and 700 ma output? Or it will shortcuts the psu because of high peak current. The psu is rated for 300 W. What design do you recommend? Thanks for reply.
 

ronv

Joined Nov 12, 2008
3,770
Hi!

I want to make a led driver with adjustable current. I want to use an atx computer psu. Is it possible to build 5 separate step up dc-dc converter circuits each with max 40V and 700 ma output? Or it will shortcuts the psu because of high peak current. The psu is rated for 300 W. What design do you recommend? Thanks for reply.
It should be OK, OK.:D
 

ErnieM

Joined Apr 24, 2011
8,045
40 v at 0.7 a is only 28 watts, or 140 watts for all 5 units. Even I the dc-dc converters only get you 50% efficiency you are still below your 300 watt limit.

You are good to go sir.
 

Thread Starter

O.K.

Joined Dec 10, 2014
19
Hi!

The current needs to be adjustable in 200 - 700 ma range. This is the schematic:
1.jpg
I tried to build it but doesnt work. The current grows so high that the 0.56 r resistor starts to burn. If i take it off the psu turns of because of current limiting.
The mc34063 calculator sad ipk is 5 A. If I have 5 of this circuit it means current can reach 25 A wich makes psu turn off.
Thanks for advice.
 

crutschow

Joined Mar 14, 2008
25,114
Why does U2A have such a complex circuit?
Doesn't it just need to be a non-inverting amp amplifying the voltage across R5 with a couple of feedback resistors (one a pot) to control the gain from about 1.7 to 6.25 for a current of 700mA to 200mA?
 

Thread Starter

O.K.

Joined Dec 10, 2014
19
The opamp substracts the voltage of potmeter's output (-0,55 ÷ 1,05 V) from the voltage of sensing resistor (r5). The negative voltage results summing, so the output of opamp will be 1,25 V if the current is in the range of 200 - 700 ma. This helps to reduce waste power on r5. This part is working, I have trouble with the boost converter.
 

ErnieM

Joined Apr 24, 2011
8,045
I tried to build it but doesnt work. The current grows so high that the 0.56 r resistor starts to burn. If i take it off the psu turns of because of current limiting.
The mc34063 calculator sad ipk is 5 A. If I have 5 of this circuit it means current can reach 25 A wich makes psu turn off.
Thanks for advice.
If the current is too high then L1 may be saturating.

What frequency is this running at?

What is the part number for L1?
 

gootee

Joined Apr 24, 2007
447
Hi!

The current needs to be adjustable in 200 - 700 ma range. This is the schematic:
View attachment 99257
I tried to build it but doesnt work. The current grows so high that the 0.56 r resistor starts to burn. If i take it off the psu turns of because of current limiting.
The mc34063 calculator sad ipk is 5 A. If I have 5 of this circuit it means current can reach 25 A wich makes psu turn off.
Thanks for advice.
What is the power rating of R1, the 0.56-Ohm resistor, that you used? For 5 A, for example, R1 would dissipate 14 Watts and you would want to use one that was rated for significantly more, or use a heat sink.
 

Thread Starter

O.K.

Joined Dec 10, 2014
19
The inductor's part number is DPT 047 A5.
I tried to test the 34063 ic without the mosfet and I think it's dead, because the output voltage is same as input. I will try to replace the chip and try bigger inductor.
 

ErnieM

Joined Apr 24, 2011
8,045
That part number "DPT 047 A5" yields no hits. Please provide data sheet link.

Same for any part you try as replacement. The inductor is rather critical in this application. If it cannot support the considerable DC current passing thru it then it looses its inductance and acts as a fairly low short circuit.

And don't forget to state your measures (or calculated if you must) frequency of operation.
 

dl324

Joined Mar 30, 2015
10,938
The mc34063 calculator sad ipk is 5 A. If I have 5 of this circuit it means current can reach 25 A wich makes psu turn off.
I realize you were asking for help troubleshooting your step up regulator, but have you considered wiring your LEDs in series+parallel so you can operate from the 12V supplies?
 

Thread Starter

O.K.

Joined Dec 10, 2014
19
It would be easier. I have 5 different colored leds. 10 of of each color and I want to individual control the brightness of each color. So it's cheaper to make 5 step up converter than e.g. 20 current limiter.
 

dl324

Joined Mar 30, 2015
10,938
It would be easier.
It's been two days and it doesn't seem that you're any closer to understanding why your step up regulator isn't working. If you used some simple 2 transistor current regulators and wired your LEDs as 2 strings of 4, you'd be done.
 
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