Using an NTC ?

[LuxLuthor]

I was having some conversations with Northern Lights, and thought this was important for people using NTC's to be aware of.




These NTC's (specifically the SL12-1R010) have gotten popular with some users because they start out with a higher resistance to block initial startup high voltage spikes which can flash your bulbs, and then when heated by current, their resistance lowers so they don't signfically block optimal voltage/current.

One of the problems with using NTC's is their variation in resistance related to their ambient temp, so take this into account. This came to my attention when seeing this NTC installation right next to a bulb:



The Ametherm SL12-1R010 NTC specifies it has:

• 1 Ohm of resistance at 25°C (77°F) "cold"
• 0.06 Ohms (60 milliohms) at 50% of Max Current rating (5A)
• 0.02 Ohms (20 milliohms) at 100% of Max Current rating (10A), and correlates to an NTC temp of 182°C (360°F)

That sounds great, but look at what happens to the resistance of the NTC depending just on ambient temps. I took these resistance readings by clipping Fluke 179 leads onto legs of NTC.

• 1.8 Ohms while resting on my countertop. Laser thermometer reports counter has temp of 64°F
• < 0.1 Ohms blowing my Heatshrink Gun towards NTC for about 3 seconds.
• 4.3 Ohms putting NTC in freezer (with Fluke leads coming outside). My freezer temp is about - 3°F

Then I checked resistance in my 64°F room by running precisely calibrated 1.0 Amp into the NTC, and time required to reach stable resistance level

• 0.628 Ohms @ 1.0 Amp after 4 minutes

The important thing to remember when using the NTC is the wide variation in resistance based upon ambient temperature, which in part is affected by use outside in cold or hot weather, but also can be affected by placement of NTC near the bulb.

Finally, the NTC can get very hot. I ran 5.5 Amps into the NTC, and touched the Fluke 179 temp probe tip against NTC, and it heated up to 201°F

It's one of those "Catch-22" situations...as you need it to get hot, so the NTC reduces its resistance, but the heat needs to be taken into account with its position/placement, and ambient temps. Once it is heated up it takes a while for it to cool back down into the range where it gives spike voltage protection. There's a number of Pro's and Con's to using an NTC.

 

[Northern Lights]

I just finished a mod and some experiments this weekend too and I am surprised at the results, I got resitance readings much higher than the theory, specs and equaitions said I should, all because of ambient temperature and another feature of the Ametherm NTCs. They have at least a 20% tolerance and some of the NTCs I had on hand were way out of line for use in lights.
On one 5 Amp circuit I expected the NTC to be at 0.060 ohms or resitance and it actually measured 0.17 ohms. You cannot measure that resistance when the NTC is in use but you can measure the Vbat loaded, Vbulb, and circuit curent and then apply Ohms law and calculate this value.

A bulb like the 5761 has only 1.9 volts between its OEM spec and flashpoint. 6.0-7.9 Volts. That is 765-1848 bulb lumens, but the graph is not linear.

In the above example I would expect with a 5 amp bulb for the NTC to drop the voltage .3 Volts when the NTC reached stasis at 0.06 ohms. At .17 ohms the loss was .85 volts. The target was 6.9-7.1 volts and the result ended up being 6.5 volts. The target was 801 torch lumens and the result was 656 torch lumens using the hot rater calculations.

That is darn noticable.

For this build I think I found a solution. I have the NTC in a fairly stable environment. I used actual meter readings and set up parallel resistors to the NTC circuit to adjust the circuit resistance to cut the voltage overhead down to the target start up voltage.

I see no difference in this than using drivers which contain trim pots to achieve the final fine adjustment. Now off to test it.

 

[jimjones3630]

Body temp, see data sheet in link below, of the NTC SL 12 1R010 at 100% current rating is 182C or 360F. There is nothing surprising to find the body temp within its published operating specs. So, at 201F the NTC is almost at 1/2 of 100% operating temp.

At a cost of less than $1 they are easily replaced when found outside of it's specs. $1 vs 25, 60, or more.

http://www.ametherm.com/Data Sheets/SL12 1R010.pdf

 

[yellow]

the happens 99,9 % at the very 1st initial ON of the fully cold bulb (with the fully cold NTC).
When the NTC is/was heated, the bulb wire is also heated and can stand any "new" spike.
Reduction of temp of the wire + the NTC correlates and helps the bulb not to be ed again, when "half-cold"

Unfortunately NL is right on the NTC stealing a bit of current/voltage/brightness, so better not mount it a the body metal to get the heat away. Protection against full cold strike, thats its prime function.
(PS: test resistance simply by shortcutting the NTC in the running setup - one can see the brightness increase. But thats ok, for the very easy, small and rugged protection against bulb failure and increased lifetime)

 

[jimjones3630]

Now flush with ntcs have noticed a spread in spec ohm of 1.4 to 1.7. This is within published 20% tolerance claimed. At the small voltages we use in mods a difference of 0.3ohm will make a noticable difference.

A 5761, 2 emoli 2670mah cells and a ntc to bring down the over head voltage from 8.4v have measured 6.9vbulb previously. Now if using a high end ohm spec ntc that 0.3ohm difference could cause a noticable reduction in output. Hand picking the choice spec ntc should pay off in increased output.
jim

edit. after going thru 50 more ntc now have a spread of 1.3ohm to 1.9ohm

 

[LuxLuthor]

I measured the 21 NTC's I have after determining the ambient testing temp is 64° F

After 1st set of readings, I repeated to make sure there was no temp variations during testing, and they all gave exact same reading again. I used the Fluke 179 with Aligator leads.

1 @ 1.3 Ω

3 @ 1.7 Ω
11 @ 1.8 Ω
4 @ 1.9 Ω
1 @ 2.0 Ω

1 @ 2.2 Ω
​

I marked the 1.3 & 2.2 Ω which I will likely not use. The next step is for me to test resistance of each under several levels of current after enough time allowed to reach a stable temp.

 

[Bunk3r]

^whilst you are doing that it may be an idea to measure the bulbs resistance at the same time intervals after use so you can see the combined effect of both the NTC and Bulb cooling down on the next turn on current (as per yellows post). I suspect the bulb will cool down quicker.

 

[Northern Lights]

Now flush with ntcs have noticed a spread in spec ohm of 1.4 to 1.7. This is within published 20% tolerance claimed. At the small voltages we use in mods a difference of 0.3ohm will make a noticable difference.

A 5761, 2 emoli 2670mah cells and a ntc to bring down the over head voltage from 8.4v have measured 6.9vbulb previously. Now if using a high end ohm spec ntc that 0.3ohm difference could cause a noticable reduction in output. Hand picking the choice spec ntc should pay off in increased output.
jim

edit. after going thru 50 more ntc now have a spread of 1.3ohm to 1.9ohm

I measured the 21 NTC's I have after determining the ambient testing temp is 64° F

After 1st set of readings, I repeated to make sure there was no temp variations during testing, and they all gave exact same reading again. I used the Fluke 179 with Aligator leads.

1 @ 1.3 Ω

3 @ 1.7 Ω
11 @ 1.8 Ω
4 @ 1.9 Ω
1 @ 2.0 Ω

1 @ 2.2 Ω
​

I marked the 1.3 & 2.2 Ω which I will likely not use. The next step is for me to test resistance of each under several levels of current after enough time allowed to reach a stable temp.

In the above example I would expect with a 5 amp bulb for the NTC to drop the voltage .3 Volts when the NTC reached stasis at 0.06 ohms. At .17 ohms the loss was .85 volts. The target was 6.9-7.1 volts and the result ended up being 6.5 volts. The target was 801 torch lumens and the result was 656 torch lumens using the hot rater calculations.

That is darn noticable.

For this build I think I found a solution. I have the NTC in a fairly stable environment. I used actual meter readings and set up parallel resistors to the NTC circuit to adjust the circuit resistance to cut the voltage overhead down to the target start up voltage.

I see no difference in this than using drivers which contain trim pots to achieve the final fine adjustment. Now off to test it.

We are all experiencing the same phenomenae, but it is well known to the manufacururer, Ametherm. They show in the article discusion that the NTC has wide variances and that using tight tolerance resistors in parallel circuits can help that immensly.

Using resistors in parallel for me was a flash of inspiration that I thought up to fix my problem. Someone else has already been there, done that, I just did not know it.

It is not an original idea as you can see by this link, they suggest a parallel resistor circuit to compensate for resistance changes due to temperature problems. Temperature is in most part a function of the amps and the NTC rating:
NTC Thermistors
Temperature Compensation Circuits

Same principle applied to our hot wires should work to bring that resistance down to where we want it. It worked in the light I built, I got my target Vbulb.

 

[LuxLuthor]

Yeah I agree NL, as we have been discussing in emails, the easiest thing is to add a resistor in parallel to NTC, but I wanted to see how much variation I had in mine. Other than the two at extremes, the rest of them are very close to each other. I started this thread mainly to give the average user more basic information about using NTC's, and what sort of temp ranges they should expect. Here are actual resistance readings I measured at calibrated Amp rates after 3 minutes with one NTC that started at 1.7 Ohm

Ambient Temp 64° F

• 0.0 Amp - 1.7 Ω

• 1.002 Amp - 0.607 Ω

• 2.004 Amp - 0.298 Ω

• 3.002 Amp - 0.185 Ω

• 4.003 Amp - 0.127 Ω

• 5.002 Amp - 0.099 Ω

• 5.503 Amp - 0.085 Ω

The Ametherm specification of 60 mΩ at 5 Amps & 77° F is pretty close to my reading of 99 mΩ, and if I were to raise ambient temp from 65° F to their 77° F standard, it would be even closer.

 

[Northern Lights]

I just measured a few I have here, no load:


Ambient Temp 74° F

• 0.0 Amp - 1.1 Ω
• 0.0 Amp - 0.9 Ω
• 0.0 Amp - 1.2 Ω
• 0.0 Amp - 1.2 Ω
• 0.0 Amp - 1.1 Ω
• 0.0 Amp - 1.3 Ω

I have a new home for the 0.9 Ω

​

 

[LuxLuthor]

Ambient Temp 64° F

Ambient Temp 74° F

LOL....You live in an oven.:sweat:

 

[Northern Lights]

LOL....You live in an oven.:sweat:

Land of extremes. You should have seen it last night... pitch dark and then 12 hours later, magic! Bright as sunshine outside! No use for flashaholism except for half a day.:nana:

Seriously, snow on the mountains yesterday, I scraped ice off my windshield to go to work this morning and 74 degrees was outside temperature by 4 pm, we had the doors open by afternoon. We close them in the summer when it is 110 out.

50-55 degree temperature ranges daily are common in the Sonora Desert, today it was 42 degrees difference. Spring is here, flowers are blooming, birds nesting and in two to three weeks the best yet...the spawn and fishing season peaks! And then I will post the results of my new mod and invention, a green LED fishing light that is on the line ahead of the lure!

​

For letting the next million dollar invention secret out of the boat!​

 

[Northern Lights]

I have now tested several dozen of the new (specifically the SL12-1R010) which we have been procuring through Newark.com.
I found in my test fixture and also by tapping into an almost competed 2x emoli 5761 that the NTCs are runing steady state resistance of round 0.2xxx ohms at 4-5 amps. Now the spec sheet states at 50% 10 amps 0.060 ohms is expected, =-20%. The curve although not linear can be figured over the small differnces between 4-5 amps and that usually gives teh 0.3 volt drop we are seeking in the emoli and is acceptable in other hot wire applications,e.g. single click start ups of AW C liion cells.

It is not happening. I wonder if NTCs are made by mixing and baking up a batch of the chips and testing and binning them like most other solid state components.

If the NTC was close at stasis to 0.010-0.18 you can get them by using parallel resistors into range to run the 5761 at a decent voltage and still have soft start. With the emoli you only need 0.28 ohms to start that 5761 at 6.0 volts and then at about 0.06 ohms to run it at 7.3 volts.

I am waiting for some help on the problem from Ametherm, makers of these NTCs and by the way they are the best priced ones at $0.70 when other similiar brands are big bucks and $15 handling per order.

It is apparent the uses and most applications of NTCs do not consider very much the importance of the final reisitance and for sure the resistance that lies between 0-100% of the Imax (maximum amps rated for the NTC) is not linear. For the Amertherm series we want the lowest ohms at 5 amps and the one above is it. Others rated for 5 amps are higher in resistance when max amps, 5, is reached ! It looks to me it is more predictable and likely obtainable to find an NTC where max amps is nearer to 5 amps of the 5761 and similear bulbs and where the final resistance is below that 0.10 and closer to the 0.060 range. Even if it is lower a series resistor or series/parallel array will correct the problem.

Here is the candidate list so far:
http://www.farnell.com/datasheets/76062.pdf look at slg130!

2.
GE CL-150 could be useable with parallel resistors, 0.11 ohms at 4.7 amps.
GE CL-101 would be @ 0.09 at 4 amps a little lower at 5 amps. good stuff. All within range.
http://www.gesensing.com/downloads/datasheets/920_325a.pdf

Keeping this in mind I am researching for alternatives and so far this is what I found in Newark.com as possible candidates. If the last batch of NTC some of us got were within the advertised specs there would be no need for this. Maybe they were mislabled.

Another idea is to manually obtain the desired high resistance for soft start and then a lower value for running. This could be achieved mechanically by using a switch that as you press it closes the high resitance circuit and when you reach the click and switch engagement it adds a lower parallel circuit or switches over to an altogether lower circuit. After all it only takes a milisecond or two to warm the filament.
An aternative is to use a solid state relay, like fet, to open on circuit and then another.

Please, someone with electronic skills look at that idea and tell me if there is such a thing as a solid state relay that could be tripped with a delay cutout. Turn on the light the relay is on and a higher resistant circuit is in then cut out when the relay tips in a few miliseconds.

You can see how an NTC is a simple solution to this problem if you can get one to operate at 5 amps or more, 0.28 ohms or more at 25 degress C and then drop to 0.02 to 0.17 ohms at steady state 5 ohms.

That is all that is necessary to soft start and run the 5761 on emolis or the AW C protected cells, to fool the protection.

I was lucky when I first stumbled on that original NTC that those did behave within specs.

 

[Mr Happy]

Have you tried insulating one of them so it gets hotter? I'm not quite sure what you could use though -- plastic or rubber wouldn't survive the heat, so you need something heat-proof but non-conductive. I'm scratching my head trying to think what could be used...?

 

[Northern Lights]

Have you tried insulating one of them so it gets hotter? I'm not quite sure what you could use though -- plastic or rubber wouldn't survive the heat, so you need something heat-proof but non-conductive. I'm scratching my head trying to think what could be used...?

Heat is not an issue with these. I have placed them directly on the bulb to generate the maximum heat, heated them with butane and insulated them. They are just not reaching the reduction in Ohms that they should at 50% Imax, or 5 amps.
On the earlier ones, inside shrink wrap in tail caps was enough insulation to keep them at the expected 50% value. I have of lately put them into the bell along the pedestal. You can use ceramic blanket material too, that is also a trade mark of my mods, I use the blankets behind the reflectors. Here is are shots of insulated reflectors, works for bipin insulation on MagChargers too:
http://img72.imageshack.us/img72/2782/3cemoliea123bq3.jpg
http://img208.imageshack.us/img208/7403/3c57613uq6.jpg
http://img67.imageshack.us/img67/8320/m11leex7dz5.jpg
Here is an NTC parallel with resistors rigged onto the pedestal, picture taken before I cleaned up the excess epoxy on the charging jack housing and wired in the bi-pin socket:

The NTC measured .17 ohms hot, and the two .18 ohm resistors all parallel brought the final drop to give me 7.3V at the bulb. 5761 on two emoli. BTW- I hardwire + and - terminals onto the Bi-pin to pr socket. I will use FM, Kai or my own bi-pin to pr sockets.

 

[EvilLithiumMan]

Years ago I saw a clever circuit using a NTC in parallel with a high current (low on resistance) FET. An RC circuit on the FET's gate gave it something like a 5 or 10 second delay before turning on the FET. When power was first applied, the NTC was there to limit current. After 10 seconds or so, the FET switched on, giving the load nearly 99% of the available voltage. Because current was now going through the FET, the NTC would cool off and be ready in short order when next needed.

 

[Northern Lights]

Years ago I saw a clever circuit using a NTC in parallel with a high current (low on resistance) FET. An RC circuit on the FET's gate gave it something like a 5 or 10 second delay before turning on the FET. When power was first applied, the NTC was there to limit current. After 10 seconds or so, the FET switched on, giving the load nearly 99% of the available voltage. Because current was now going through the FET, the NTC would cool off and be ready in short order when next needed.

I read about something like that at ametherm.com site; I may be able to build it and follow the circuit but I cannot design it.

 

[Northern Lights]

I found a search application that is finding all kinds of potential NTCs for this circuit
Type in: Inrush Current Limiters
into the search box to get the application.
http://www.digikey.com/scripts/DkSearch/dksus.dll?KeywordSearch

Here are a few circuits and potential NTCs with this company:
http://www.rtie.com/ntc/ntcappln.htm

Ametherm engineer will talk with me on 4/2 and maybe I can find out what has happened and maybe a better solution.

For the 5761 and similiar bulbs, we want the lowest Resistance at that amperage, 5 amps for the 5761 regardless of the R @ Imax.
That is a paramater that is not easily searched for, you have to read the spect sheets to find it. I have read so many now...

 

[Flipside]

Ametherm engineer will talk with me on 4/2 and maybe I can find out what has happened and maybe a better solution.

NL:

Did you ever hear back from that Engineer? My new FM85 instaflashes on fully charged batteries, and I'm hoping that an NTC could be the solution.

Certainly, I could charge the batteries to 4.0V instead of the normal 4.2V. But then, I'd be giving up run time and absolute brightness...

 

[Northern Lights]

NL:

Did you ever hear back from that Engineer? My new FM85 instaflashes on fully charged batteries, and I'm hoping that an NTC could be the solution.

Certainly, I could charge the batteries to 4.0V instead of the normal 4.2V. But then, I'd be giving up run time and absolute brightness...

Yes, but it is not worked out totally, and I am recuperating from some life altertering medical problems and will not be able to tackle it for maybe a few months. His suggested models and samples did not perform as expected and the circuit needs some work and testing.