8 Replies Latest reply on Nov 3, 2018 9:20 AM by geralds

Sparkfun Voltage Regulator for Nichrome Heater

I'm wanting to use a 5V voltage regulator from Sparkfun. https://www.sparkfun.com/products/107 . I want to make sure I'm constructing the circuit correctly. I just want to run current through a heater wire. I've already made sure that the load resistor will pull less than the max rated 1.5A. I've just never used a 5V regulator.

• Re: Sparkfun Voltage Regulator for Nichrome Heater

Okay - you have a calculation for the current, presumably the steady state current, although the resistance of nichrome doesn't change too much with temperature.

The power in the heater will be this current times 5 volts.

The power dissipated by the regulator will be its input voltage minus 5V times the current... (Vin - 5) x current

This power will be all converted to heat which must be dissipated or the regulator will get too hot.

Here is a typical site that shows how to calculate what heatsink you will need:

There are other sites if this seems too complex.

The package for your regulator is TO-220 which is covered on the above site.

Note that if the input voltage is over 10 volts, the regulator will generate more heat than the heater wire.

Here is a typical circuit for your regulator:

4 of 4 people found this helpful
• Re: Sparkfun Voltage Regulator for Nichrome Heater

Sorry Doug,

I didn't see that you had a diagram when I first read your reply. It is quiet here tonight so I was just looking for something to play with. I even hooked up a piece of Nichrome wire to see if I could measure a resistance change but you were very correct. A little research showed that Nichrome has only a tenth of the temperature coefficient of copper so the delta 100C that I exposed my nichrome wire to made no measurable difference to my meters.

John

3 of 3 people found this helpful
• Re: Sparkfun Voltage Regulator for Nichrome Heater

Hi Brandon,

Are you considering your nichrome wire as the load resistor or are you putting a second resistance in series with the Nichrome wire? If you can tell us the input voltage it would help. Otherwise dougw  has covered most of the bases and problems that you may encounter. Here is a simple schematic of what you are describing. Data sheets may mention some decoupling capacitors but if all you want to do is heat up some nichrome wire the capacitors may not be necessary.

John

3 of 3 people found this helpful
• Re: Sparkfun Voltage Regulator for Nichrome Heater

Hi Brandon,

The excellent suggestions from everyone will help you, also jw0752 makes a good point, regarding what is the input voltage and purpose of the resistance. What is your use-case if you don't mind me asking? Is it a nichrome heater you're making? Or one you've purchased? What is your actual end goal? Such heaters do not require any stable voltage, but you can control them by changing the voltage, and you can do that by modifying the source supply (in which case no circuit is needed), or adding another resistor in series (a power resistor) instead of the voltage regulator circuit, or even just extending the amount of nichrome wire you're using (the latter may not apply of course if you're already using some ready-made nichrome heater).

3 of 3 people found this helpful
• Re: Sparkfun Voltage Regulator for Nichrome Heater

The purpose is important - but another thing worth mentioning is the fact that linear regulators dissipate unwanted voltage as heat and while the regulator is capable of running up to 1.5A, this is highly dependent on your heatsinking! Don't take it for granted that an LM7805 can run a maximum current of 1.5A and maximum voltage of 35V.

The chip has an operating junction temperature up to 150C, although if you can avoid it, you should try not to operate it at this temperature. The TO-220 package has a thermal resistance of junction-to-case of 5C/W maximum and junction-ambient of 50C/W maximum.

Assuming you have no heatsink, and an ambient temperature of 25C, then your maximum power dissipated before shut-down is approximately:

(150-25)/50 = 2.5W

If your input voltage is 12V and output voltage is 5V, then you are dropping 7V across the regulator. Maximum current is hence 2.5/7 = 0.357A

If you're going to use a super-beefy heatsink, you can probably buy a heavy aluminium heatsink metal with about 2.5C/W thermal resistance. The net thermal resistance is hence:

5+2.5 = 7.5C/W

This increases the allowed dissipated power to:

(150-25)/7.5 = 16.67W

The maximum current is hence 16.67 / 7 = 2.38A, but this is not achievable, so you can now achieve the 1.5A rating due to the regulator's internal limits.

But this is something to be aware of - most TO-220-specific heatsinks are terrible pieces of stamped aluminium with something like 19-28C/W thermal resistance (from memory). This makes it hard to achieve the claimed rated current, but also doesn't keep the regulator cool enough to increase its lifetime. Operating at near the temperature limits is not recommended - aside from the possibility of regulator shut-down due to overtemperature.

The safe-operating area (SOA) is limited automatically by these LM-series linear regulators - Figure 9 in the datasheet  gives you a clear idea of how even the peak current can be limited depending on temperature - the sustained current is governed by the temperature/thermal dissipation characteristics as mentioned above.

Figure 8 gives a good idea of the dropout voltage versus temperature - the dropout voltage being the minimum voltage the input needs to be above the output to ensure proper regulation. At a load of 1A, on a  cold regulator, almost 2V is necessary between the input and output. It is basically "forced" loss that you have to accept, otherwise if you provided 5V to an LM7805, you will see something like 3-4V with no regulation on the output.

So, if you're only going to be operating a nichrome heater wire on a fixed voltage, it makes better sense to just design the heater coil for that voltage as the excess voltage is basically being converted to heat in some rather "expensive" semiconductor material and dissipated through a heatsink. Otherwise, you can even use a fixed large wattage resistance as the nichrome resistance is rather constant - this won't allow you to vary the input voltage but is a simpler solution. Or you could use a switching converter which would reduce losses, but could be a bit complex depending on the level of current required.

- Gough

6 of 6 people found this helpful
• Re: Sparkfun Voltage Regulator for Nichrome Heater

Hi Gough,

Sorry, I would say that this calculation is not just these formulas.
First, you need to see the specific resistance of the Ni-Chrome wire.
e.g. - If you want to operate this wire with 5 V and 1.5 A, you have a power loss of 7.5 W at 20 ° C. That would be 3.3 ohms from this wire.
In addition, there would be the loss of the ambient temperature, as well as the resistance change, temperature coefficients during operation and the contacts, which also act on the wire.
-> at 20°C...… but....
Only then you can start with the calculation of the regulator, because these are the starting conditions at the regulator output.
Then count back to the input-pin, with all the parameters that are necessary.
Here I would also see a temperature-stabilized current limit as necessary, strongly recommend.

In this datasheet you can find the formulas for the beginning, e.g. for selecting the right regulator.

This wire may be a usable wire:

0.5mm thickness and 5.65 Ohm / Meter (20°C).

But, if the OP wants to heating up the wire e.g. 300°C, well, then he must calculate with the specific resistance at this temperature.

So for result this will not work with this small regulator.

Best Regards

Gerald

----

3 of 3 people found this helpful
• Re: Sparkfun Voltage Regulator for Nichrome Heater

Of course it is not just these formulas - I was concentrating on the regulator's ability since the author of the original post said:

I just want to run current through a heater wire. I've already made sure that the load resistor will pull less than the max rated 1.5A.

To think that being less than the maximum rated 1.5A is "sufficient" to specify the regulator is incorrect - that is why I provided examples to calculate (with some more realism) the actual current the regulator may be able to deliver given certain conditions. I wasn't sure what heater the original poster wants to run - whether it's "just slightly less" or quite a lot less, or even what input voltage the poster was using.

Of course, if your heatsink is worse/better than expected, if the thermal paste is poorly applied, if contact pressure is not sufficient, if ambient temperature is >25C (e.g. due to lack of airflow), your regulator will perform differently. But knowing what the realistic current you can expect is an important exercise, as would be understanding the dissipated energy (loss of efficiency, heat).

I didn't consider Nichrome wire at all - because that was not what I was illustrating - the other posters above did mention the issues with temperature coefficient of resistance of Nichrome ... of course this will change depending on which temperature you intend to run at, just as incandescent light globes don't exhibit a constant resistance ...

- Gough

4 of 4 people found this helpful
• Re: Sparkfun Voltage Regulator for Nichrome Heater

Yes, I am with you.

"Heating wire" - it's just a wire that's just unknown to us what type it wants to use. Ni-Chrome, or copper, or simply steel.

Also I can can't see his uploaded image. May be this is his circuit or mountings.

Gerald

---

1 of 1 people found this helpful