# Variable current limiting circuit.

#### Bmachining

Joined Sep 10, 2018
34
Hi, I am needing to build a new variable current limiting power supply for electroplating.

Previous one went up in smoke (its in a harsh environment) and i cannot locate my circuit drawings. was drawn up by someone years ago.
It was based around a 741 op amp, and 4 of 2N3055 Transistors. 5V output up to 20 A.

Needing 12v and 40 amp capacity for this one.
Power supplied by a switchmode unit.

Can anyone point me towards a circuit? I think 3055s would be old fashioned now!

#### MrAl

Joined Jun 17, 2014
7,594
Hi, I am needing to build a new variable current limiting power supply for electroplating.

Previous one went up in smoke (its in a harsh environment) and i cannot locate my circuit drawings. was drawn up by someone years ago.
It was based around a 741 op amp, and 4 of 2N3055 Transistors. 5V output up to 20 A.

Needing 12v and 40 amp capacity for this one.
Power supplied by a switchmode unit.

Can anyone point me towards a circuit? I think 3055s would be old fashioned now!
That is a hefty order. 40 amps is not usually done in the linear mode but using switch mode topology. If the output has to be especially clean for some reason then we almost always see a switcher front end followed by a linear back end. The switcher handles most of the power while the linear just keeps things clean and well regulated as a typical pure linear would.
A pure linear will use a ton of power with some loads and require big heatsinks. A switcher will relax this requirement.

#### Marley

Joined Apr 4, 2016
348
So this is to control the curent at a constant value? Operating voltage for the plating?
Have you got a power supply?
Depending on the voltage, 40A is quite possible for a switch mode power supply. I would not recommend making one yourself.
If you could get a power supply (possibly second hand or cheap from ebay for example) you could modify the feedback circuit inside the power supply to lower the output voltage when the current gets to the maximum value. You would have to add some method of measuring the current, then add some extra electronics. Best way to measure the current is with a "hall-effect" device. This has a hole to pass one of the wires through.
This would be better than trying to limit the current with linear transistors - with all the power wastage and heat.

#### MrAl

Joined Jun 17, 2014
7,594
Yeah 40 amp power supplies are not for the faint of heart.
High current devices, low esr capacitors, low esr inductor, etc.
Parts can sometimes literally explode like M80's.

#### ci139

Joined Jul 11, 2016
1,464
Previous one went up in smoke
a harsh environment
cannot locate my circuit drawings.
you should resolve each issue to prevent the history repeating itself
as
• overload / short circuit / thermal protection
• appropriate casing that suits your environment
• cloud / CD / USB-Flash / ?Floppy / camera SD-card (unused cell phone or mp3 player) / multiple paper copies at multiple locations . . .
______
??
https://www.powerelectronics.com/technologies/dc-dc-converters/article/21863490/innovative-packaging-yields-40a-synchronous-buck-converter-ic
https://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/dc147a-c.html

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#### Danko

Joined Nov 22, 2017
928
Hi, I am needing to build a new variable current limiting power supply for electroplating.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Needing 12v and 40 amp capacity for this one.
Power supplied by a switchmode unit.
12Vdc Dimmable LED Driver
3 in 1 Dimming, IP65, CC or CV
HLG-600H-12AB $183 HLG-600H-12AB € 163,70 Input voltage 90~305VAC Output voltage 12VDC Output current 40A max Datasheet Last edited: Thread Starter #### Bmachining Joined Sep 10, 2018 34 Danko, that looks perfect, but current Au$ makes it rather expensive.

Marley, Yes i need to hold the Current at a constant value. Voltage is what ever it takes, depending on the Area of the plated item.
I have a new 12v40A Switchmode power supply, chinese made.
I see only one trimpot Vr1 i would think is for fine voltage control.

Is there a circuit i can adapt/make to add current control? Main IC in PS is a 17407YH, TL494CN

#### Marley

Joined Apr 4, 2016
348
Yes, would have to modify the circuit around the trimpot. And add some extra electronics. Will take a little effort but better than limiting the current with a power transistor dropper.

Better to reduce the PSU voltage to get the current you need instead of producing all that extra voltage and then dropping it after!

#### Bmachining

Joined Sep 10, 2018
34
Sounds like a plan! Any ideas where i can find a circuit diagram? Ive searched the internet with no luck on anything high current so i will go the more modern idea you have suggested.

#### BobTPH

Joined Jun 5, 2013
2,392

#### Bmachining

Joined Sep 10, 2018
34
Ive drawn up what i think the circuit should be.
Previous one was powered by a PC powersupply and had the Opamp Neg connected to -5v Will this circuit still work with no -voltage? anything else it needs?

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#### Danko

Joined Nov 22, 2017
928
50A DC step-down power supply module constant voltage, constant current.
US $63.00 Shipping:$1.99
Product parameters:
Input voltage: 8V-40V
Output current: adjustable, up to 50A (recommended for use within 45A, current regulation, only works in constant current state.)
Output power: up to 800W (recommended for long-term work within 600W, please reduce power for large differential pressure)
Working temperature: -20~+65 degrees (When the working environment temperature is high, please pay attention to strengthen ventilation and heat dissipation when using)
Working mode: synchronous rectification
Conversion efficiency: up to 95%
Short circuit protection: Yes, constant current value currently set by constant current
Input reverse connection protection: Yes
Overheat protection: Yes, the fan is activated at 40 degrees, the power output is turned off at 65 degrees, and the temperature is restored to 38 degrees to automatically restore the output!
Output anti-backflow: none, for charging or motor class, please add a diode to prevent backflow! This shop is available for sale!
Wiring method: terminal block or soldering terminal, VIN is input, VOUT is output
Module size: length 140mm width 90mm height 53mm
Weight: 541g

#### Bmachining

Joined Sep 10, 2018
34
Tempting Danko!
But!
My previous experiences with this type of chinesium electrics hasnt been blessed with reliability.

My circuit above did work, might not be efficient but its simple! Does anyone have any sugggestions on it? can it be improved?

#### crutschow

Joined Mar 14, 2008
24,978
Is there a circuit i can adapt/make to add current control? Main IC in PS is a 17407YH, TL494CN
Below is a typical circuit from the TL494 data sheet which is likely similar to the supply you have.

The current limit is determined by the value of R13 and the voltage at pin 15 as determined by R1 and R2.
The values shown give a current limit of 10A.
So if you can find R1 and R2 (resistance values may be different) in your power supply you should be able to adjust the output current limit by changing R1 (replace with a pot of value equal to R1 in your circuit, with wiper connected to one end of pot).

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#### Bmachining

Joined Sep 10, 2018
34
crutschow

Thankyou! i will let you know how it goes.

#### Bmachining

Joined Sep 10, 2018
34
Not too good. found and replaced the resistor, 660ohm. i set the pot to the same resistance for initial power up.
PS led came on, Fan started then 1 second later nothing, it shut down quietly.

Checked internal fuse etc, all looks fine. Possibly another dodgy PS, or could be the input voltage, its switchable between 110 and 220V. My power measures 249V at the power point, standard in my area.

#### ci139

Joined Jul 11, 2016
1,464
i wonder why they always mount the filter caps immediately next to the heat source / heat sink . . . and on the same side of the PCB
my Taiwanese Intel Celeron fried it's caps just like so . . . (next time i should stack a piece of calcium silicate in between ((g.d. smart-S computer designers ...)) )

#### crutschow

Joined Mar 14, 2008
24,978
found and replaced the resistor, 660ohm. i set the pot to the same resistance for initial power up.
PS led came on, Fan started then 1 second later nothing, it shut down quietly.
So does it work if you put the 660 ohm resistor back in?

#### Bmachining

Joined Sep 10, 2018
34
No go with resistor replaced.
Have ordered another PS the same.
Where do i start to troubleshoot the switchmode PS? i am going to need 2 of these power PS.

#### crutschow

Joined Mar 14, 2008
24,978
Where do i start to troubleshoot the switchmode PS?
Those are not easy to troubleshoot without a schematic.
Check to see what points have voltage.