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Tach resistors

bernd_st

bernd_st

Bronze Level Sponsor
Tom,

great finds ! Appreciate you took the time to make real measurements.

However usually we don´t experience higher than 80F temps over here in Europe. Therefore a slightly low reading may not be so important plus turning the engine more than 5000rpm can be regarded a rare case.

Anyhow interesting so next step would be to find out the actual Smiths/Jaeger thermistor compensation curve. Quite sure that we could figure out at least a close replacement...
 
H

howard

Donation Time
After reading this post, I believe it explains why my tachometer no longer works accurately: I replaced the engine and it needs recalibrating!
 
T

Tom H

Platinum Level Sponsor
Just so everyone knows the NTE158 pieces are replacement TRANSISTORS, not thermistors.

Paul, I recently spent several hours looking for and finding a near replacement for the thermistor. Since no one had come forward with a part number or any info on it I had to spend time, first to empiricaly determine its characteristics, and then search for a device with those characteristics. A lot of work! Then I saw your post and thought "arghhh, someone already knows the part number!" . But then I looked it up and see it's not the thermistor. Good to know my effort was not wasted.

I've repaired 5 tachs and have not seen any bad transistors, (nor bad thermistors) but it would be good to have a few just in case. I plan to buy a bunch of thermistors (a fifteen cent part) and if my tests show they do the job I'll offer them around also.

How much do you want for 2 NTE158s? Send me an e-mail.

Tom
 
R

RootesRacer

Donation Time
Tom,

I should have chimed in on this earlier, I doubt you'll be able to find any modern day thermisters with curves like the ones lucas used.

There have been such changes in electronic materials that it seems very unlikely to find a match.

Probably the best you could do will be to find a curve that matches at two points (hopefully useful points).
 
T

Tom H

Platinum Level Sponsor
Jarrid, I already thought that it would not be easy. I know a bit about thermistors, but mostly about those used as temp sensors for tempp measurement- typically 2k-30k values.

I measured the thermistors at 6 temps from 10C to 45C. That tells me enough. It is plainly a 150 Ohm thermistor (@25C) and has a rather steep curve ( B constant about 4700). I found a 150 Ohm thermistor in a catalog from GE Thermometrics (Material 9.5), that looks like a match. But I couldn't find any stock and I don't need to order 5000 pcs!! But I did find a near equiv , 150 Ohm thermistor in stock at Digi-Key. It's a SMD part , 1.6mm X .8 mm. I think I can solder pair of leads to it. It has a less steep curve, but my tests have already shown that the original thermistor overcompensates by quite a bit, so a less steep curve will probably work even better than original.

Futher, I have found that by adding a 33 ohm resistor in series with the original thermistor to "detune" the compensation results in near perfect temperature stability of the tach. With the replacement devices I think I will still need to add some series R to "detune" the compensation, but I calculate it's only 12 ohms or so.

Fun stuff.

Tom
 
R

RootesRacer

Donation Time
Tom,

Sounds good.

Give me the DK part number if you would, my tach suffers from vibration sensitivity, I suspect the thermistor leads are making intermittent contact to the substrate.

I'll tell you a little secret on how to use an SMD part in a thu-hole application, and to expect it to survive the lead vibration.

Take the largest gauge solid wire the board will accept, wrap several turns around the SMD package before bending the wire downward, solder the wire to the SMD pad. Do the same to the other side of the component, then encapsulate the mess in 5 minute epoxy.

That way it will be the wires at the PCB that break off eventually, not at the component.
 
T

Tom H

Platinum Level Sponsor
Jarrid,

Here's a link to the DK part.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=490-2435-2-ND

The last digits after the = sign is the part no.

If you buy any, send me a couple. Thanks for the tip on leads. I have never worked with SMD parts

IN one case of bad leads I was able to re-solder leads onto the thermistor. The solder blob on the thermistor is pretty solid.

By the way, just today I pulled the tach from my Alpine (nice 55 Deg day today!) and temperature tested it. With no mods to the compensation it tracks dead on, +/- less than 100 RPM, at 6000 RPM over 65 to 110 F span. So what the heck? I'm tempted to open it to see if they used a different Thermistor or maybe a different zener??

Tom
 
puff4

puff4

Platinum Level Sponsor
Tom H said:
By the way, just today I pulled the tach from my Alpine (nice 55 Deg day today!) and temperature tested it. With no mods to the compensation it tracks dead on, +/- less than 100 RPM, at 6000 RPM over 65 to 110 F span. So what the heck? I'm tempted to open it to see if they used a different Thermistor or maybe a different zener??

Tom
Click to expand...

Does that mean that your tach is still better than mine? Grrr... ;):D
 
T

Tom H

Platinum Level Sponsor
Yeah, Kevin, and it puzzles me greatly. Now I don't know whether to simply put it back in the car, be sorry I even bothered to pull it, be happy I pulled it so I know I can trust it, or take it apart to try to see WHY this one is so good. I think I'm too darn curious to leave it alone! Kinda wished it was worse than any so I could be glad I pulled it and glad I had figured out how to fix it! Now that's a Grrrrrr.

I had previously done the mods- replaced the main capacitor, replaced the timing and adjusting Resistors, and calibrated it with a precision Pulse gen, but I had never messed with the thermistor. I'm guessing that mine has a later thermistor with a more suitable temperature curve. Gotta take it apart!

Tom
 
R

RootesRacer

Donation Time
Tom,

My guess would be the tempco requirement relates to an average transistor beta value.

Perhaps your "perfect" tach is one where the transistors have more nominal beta values than others you have seen which seem over compensated.
 
T

Tom H

Platinum Level Sponsor
Jarrid,

I doubt it's the tempco of the beta. The transistors are used as switches- ON/OFF. I know the primary tempco is due to the copper coil in the meter. The circuit applies a Voltage Pulse train. But the meter movement respnds to current. So as the copper coil gets warm, the resistance goes up, so the current goes down and the meter reads low. The thermistor (and a parallel fixed resistor) are in series with the coil. The thermistor has a neg tempco, so as the temp goes up it has an opposite efect from the coil. But on the first 3 tachs I have looked at the themistor changes more than is needed. There are also some secondary tempco effects but I don't think it's beta related

Now as I pulled my good one apart, First I find that resistance of the thermistor is much higher than the other 3 I have checked. Unfortunately as I unsoldered it the lead came off the chip- like you described. Now it may be hard to characterize this "perfect" thermistor.

Tom
 
P. Scofield

P. Scofield

Bronze Level Sponsor
Tom,
Send me your address in a PM and I'll throw a couple in the mail. That way, at least you have the package, part number and manufacturer. Don't remember what I paid so don't worry about it. I may need a tach repaired someday.

Paul

Tom H said:
Just so everyone knows the NTE158 pieces are replacement TRANSISTORS, not thermistors.

Paul, I recently spent several hours looking for and finding a near replacement for the thermistor. Since no one had come forward with a part number or any info on it I had to spend time, first to empiricaly determine its characteristics, and then search for a device with those characteristics. A lot of work! Then I saw your post and thought "arghhh, someone already knows the part number!" . But then I looked it up and see it's not the thermistor. Good to know my effort was not wasted.

I've repaired 5 tachs and have not seen any bad transistors, (nor bad thermistors) but it would be good to have a few just in case. I plan to buy a bunch of thermistors (a fifteen cent part) and if my tests show they do the job I'll offer them around also.

How much do you want for 2 NTE158s? Send me an e-mail.

Tom
Click to expand...
 
T

Tom H

Platinum Level Sponsor
Over the weekend I think I figured out what was going on with my Tach and the thermistor in it. Based on several measurements done before I disassembled my tach, and some done after, I have concluded this amazing results. Apparently the thermistor in my tach had degraded over the years, so much so that what originally had been a 150 Ohm thermistor had become about 700 Ohms. Or possibly the silver contacts on each side had corroded such that there was a lot of "contact" resistance in series with the thermistor. At any rate it was clearly about 700 ohms at room temperature. Furthermore, whatever characteristics were there did a near perfect job of temperature compensation over a temp range from 50 F to 120F. Even more amazing is that I last calibrated this tach in 2005, so this fragile degradation had stabilized over the last 4 years.

Unfortunately as I removed this thermistor to better characterize it, it fell apart, crumbling into several pieces. I hope you're happy now , Kevin!!!

Fortunately I was able to replace it with an old thermistor from another broken tach. After a few more tests I find that by simply adding a 68 ohm resistor in series with the original I can achieve near perfect compensation with a temperature stability of about +/- <100 RPM. I will try that on at least one other tach for some insight into how consistant these tachs are.

It seems clear that the original thermistor, as observed by me in 4 seemingly good samples, overcompensates the circuit by about 4-6 %. It is also possible that the original thermistor did compensate nearly perfectly, but that the devices have all aged and degraded to where they no longer do so. It is a known fact that thermistors, in general, are rather unstable devices (over time) and very likely ones made 40 years ago were even more unstable. With no part number to compare I cannot tell what the original specs were, I can only measure its characteristics now.

After some study I think any of several replacement thermistors stocked at Digi-Key in the range of 200 to 500 ohms can be used successfully.

Depending on the thermistor value and its Beta curve, some thermistors will have close or near perfect compensation. Of course that also depends on possible variations in other components in the tach. My calculations suggest that a 300 ohm thermistor with a beta of 3900 slightly overcompensates my tach (by about 1.8%) but with a 33 ohm resistor in series with it will provide near perfect compensation and still allow some tweeking, if needed, by changing the value of the series resistor. A different value might accomplish the compensation without need for an extra resistor, but without knowing more about the variations of individual tachs I think I'll stick with the 2 component solution

I plan to buy several of these 300 Ohm thermistors and a few others from Digi and will experiment on two or three tachs. I hope to be able to provide a solid replacement recomendation then.

Sorry for boring all the non-electronic guys with such minutia, but this is one small area where I can contibute.

Tom
 
puff4

puff4

Platinum Level Sponsor
That's some superb research, Tom, and I can't thank you enough for your efforts. I truly am sorry about your tach, though. I have a spare here (quality unknown) - do you need it to replace yours, or as an experimental unit?
 
T

Tom H

Platinum Level Sponsor
Kevin, I was only kidding about my feelings about my tach!:D I knew when I decided to open it up that I was risking something. I have often heard that the biggest enemy of "good enough: is "just a little better"! But I was really curious to find out how & why it performed so well. My consolation is twofold: 1) It may well have failed later in my car anyhow, and with no warning and no way to know it shifted badly. 2) I think I have found a way to make it just as good, but more solid and stable.

Yes, I could use another tach, even non-working, to help research the right thermistor and as a supply of spare parts.

Tom
 
K

kenny_ii

Donation Time
I'm curious as to the results of the testing. Is there a recommended thermistor? I looked at Mouser and didn't see ratings with Beta, they had curve charts. My tach was flaky when I parked the car many tears ago, so I figured to rebuild it while the motor was off being machined.

Tim
 
T

Tom H

Platinum Level Sponsor
Hello Tim,

After much testing and calculations I have determined that the best compensation readily available is a 300 Ohm (3500K) thermistor with a 33 ohm resistor in series with it. :

I found them at Digi-Key :
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PNT109-ND

That's P/N PNT109-ND

You need to add the 33 ohms in series.

I suggest you also replace the main timing cap (0.25 uF) , use a 0.22uF plastic film cap. And also replace several resistors in the circuit. They are old carbon comps and have aged poorly and shifted. I suggest you replace them with more stable metal film resistors. These are the ones I would replace:
1) the 1.3 K that is in series with the 0.25 uF cap
2) the 3 K that is in series with the adjustable pot - replace it with about 8K to reduce the amount of timing resistance from the less stable adjustable pot.

You might also check the big 150 Ohm resistor. I have seen several that are so far shifted above 150 that they provide too little current to the zener diode and the meter then reads low at higher revs because there is not enough current available to drive the meter and maintain the regulated zener voltage

I assume you have read this article that describes a lot about how the tach works:

http://www.classictiger.com/techtips/motach.html

Good luck.

Tom
 
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