# Limit high current using LDO regulator [closed]

I need to heat up a nicrome wire using 4Amps to break an O-ring. I don't know exacly what is the size of the nicrome wire, so I cannot put a resistor in series, also I cannot dissipate all that heat. That's the reason why I need a way to limit the current. Is there any way to limit it? I'cm thinking in using LDO regulators, but I'm not sure it would work.

• Any reason you can't just use a lab supply and bring the voltage up until you get the effect you want? Why exactly 4A? If the wire is too thick then 4A may not be enough to get the required heating. If the wire is too thin then it may burn out prematurely. Apr 2 at 21:34
• Do you need to break 1 O-ring, or do you need to break many O-rings? If you need to break many O-rings, will you always use the same diameter and length of nichrome? Or will you possibly use different diameters and/or lengths? Apr 2 at 21:52
• This is a remote order, so I would like to limit like a lab supply in a PCB. And I will use different diameters and lengths, I want to make it 'stupid-`proof' because I will not the only one who will use it. Apr 2 at 21:59
• do you want to limit the current to protect the power supply, or to control the temperature of the wire? Apr 3 at 6:28
• Limit the current to the power supply Apr 3 at 7:46

I would use a pulse a pulse width modulated supply to do this. This could consist of a small microcontroller or 555 IC driving an appropriately sized MOSFET.

• How can I do it? I have absolutely no idea. Apr 2 at 22:33
• learn about PWM and then implement it. without knowiung the details of your requirements we can but guess at what approach might be suitable, Apr 2 at 23:42

A linear regulator, be it an LDO or regular linear regulator used as a current source would dissipate exactly the same heat as the resistor you'd have to put in series to achieve the same current limit.

So, if you can't dissipate the heat that resistor would produce, you can't dissipate the heat of your linear regulator, either.

So, no. An LDO would not work.

• Sorry, I didn't explain it well enough. Just imagine I put a 1 ohm resistor and 11V Battery. If the nicrome wire is too thick, it has neglible resistance, so it would circulate 11 amps and break my pcb. If I put 2 ohm but the wire is narrow (2 ohm), it would pass less current and the resistor would have to dissipate a lot. Using a LDO of 1.4V and 5Amp, the max it would have to dissipate is 7W, and it won't stop the flow of current if the nicrome wire is narrow. Apr 2 at 21:57
• yes, but the LDO would dissipate exactly the same amount of power as a resistor bringing down your supply voltage to 1.4 V. Again, this is not a solution: an LDO is just a "self-adjusting resistor", if you will. Apr 2 at 21:58
• Wait, so if I'm using a 11V battery with a LDO of 1.4V dropout, that means that in normal conditions, I would be able to draw 9V6maximum. My idea is to maintain those 9V6 and limit the current. But if the current increases my output voltage decreases? Apr 2 at 22:02
• Your wire sets the relation between current and voltage. It's fixed, you can only control one, the other automatically is defined by the resistance of the wire. This is really basics: Ohm's law! Apr 2 at 22:10
• Sorry. What I meant, can I mantain an 9V6 5Amp? Apr 2 at 22:12

It's a time vs. money thing.

If you have little time but enough money, use a lab supply that has a constant-current control option.

If you have time but not (as much) money, a buck DC-DC could be hacked to monitor a sense resistor in series with the nichrome wire.

• yes for example a buck designed to be used an LED driver or a lithium cell charger would probably be a good fit here. Apr 2 at 23:45
• Can u give any example? Apr 3 at 7:46
• Here’s a TI appnote: ti.com/lit/an/snva829/snva829.pdf Apr 3 at 14:55

Use a LED power supply

They are designed to provide constant output current at some defined voltage range. There are many models available with either DC or AC input.

To select a suitable model, consider:

• Input voltage range
• Output current: this may be adjustable or constant
• Output voltage range: this has a minimum and maximum value. The voltage drop of your NiCr wire should fall in this range. Pay attention to that the NiCr resistance increases a lot when it gets hotter, so you will need a supply with wide output voltage range.

Internally the LED power supplies operate with switching mode converters, so they do not dissipate as much heat as a linear regulator would.

Note that often it may be desirable to use constant voltage supply instead, as the increasing resistance of NiCr will then regulate the temperature. With constant current supply it is easier to overheat the wire, as the dissipated power increases when the wire heats up.

For this, select a power supply where the maximum output voltage matches what you get when testing for suitable result temperature. The supply will then limit current until the wire heats up, and then operate at maximum voltage.