Coil secondary resistance data.

Discuss all things Model T related.
Forum rules
If you need help logging in, or have question about how something works, use the Support forum located here Support Forum
Complete set of Forum Rules Forum Rules

Topic author
Luke
Posts: 585
Joined: Fri Dec 13, 2019 1:04 am
First Name: Luke
Last Name: P
* REQUIRED* Type and Year of Model Ts owned: 1926
Location: New Zealand

Coil secondary resistance data.

Post by Luke » Wed May 31, 2023 1:48 am

As a result of Craig's recent thread (https://www.mtfca.com/phpBB3/viewtopic.php?f=2&t=36757) we measured the secondary resistance of 25 'standard' coils today. There were more coils, but they appeared to have open-circuit windings (note that doesn't necessarily mean they won't produce a spark!).

The results are shown below. I have not attempted any sort of analysis, the data is simply presented here as a record in case it's of use to someone.

Also included is a scatter plot which gives a visual idea of the resistance spread across the set. And before anyone complains, the dotted lines have been drawn simply for visual purposes, there is no particular relationship between the points.

Resistance was measured using a Meterman 5XP that had been previously checked against a series of 1% resistors within a few k of the requisite range.

Coil# Resistance (Kohms)
1 3.34
2 3.05
3 3.28
4 3.19
5 2.67
6 3.35
7 3.28
8 3.41
9 3.05
10 3.31
11 5.02
12 3.03
13 3.49
14 3.21
15 2.11
16 2.98
17 3.59
18 2.2
19 2.21
20 3.1
21 5.04
22 3.38
23 2.63
24 3.26
25 2.79

[note the data doesn't present the same as in the editing screen - there is a good space between the coil number and it's resistance - however I believe it should make sense, please let me know if not!]

tcoilresistance.png

User avatar

MKossor
Posts: 458
Joined: Sun Jan 06, 2019 7:30 pm
First Name: Mike
Last Name: Kossor
* REQUIRED* Type and Year of Model Ts owned: 1927 Touring
Location: Kenilworth, NJ 07033
MTFCI Number: 22706

Re: Coil secondary resistance data.

Post by MKossor » Wed May 31, 2023 8:33 am

That is quite a range of resistance values. Possible explanations for the variation in secondary resistance values is use of different diameter wire (resistance increases as wire diameter decreases), different number of turns of wire or both. This presumes all coil secondary windings are wound using the same wire material (copper) with negligible differences of impurities. Measurement of the secondary inductance would provide more detail as to the number of turns of wire used on each coil. The number of turns of wire is important because the turns ratio primary winding to secondary winding determines the voltage step up ratio for a given number of primary turns. The higher the turns ratio, the higher the secondary voltage gets for the same primary voltage.

The random order of coil measurements is good because it shows the possible ranges one can expect from selecting 4 random coils from the box for use in a car. Here is Luke's coil secondary resistance data sorted by value; low to high to better visualize the range of values. Three distinct groups of coil resistance is more evident with the center group that conforms to a nominal range (3.11 K Ohms +/- 15.4%)
Lukes coil measurements sorted.jpg
Lukes coil measurements sorted.jpg (21.6 KiB) Viewed 621 times
I-Timer + ECCT Adjusted Coils = Best Model T Engine Performance Possible!
www.modeltitimer.com www.modeltecct.com


TXGOAT2
Posts: 6411
Joined: Sun Feb 14, 2021 10:08 pm
First Name: Pat
Last Name: McNallen
* REQUIRED* Type and Year of Model Ts owned: 1926-7 roadster
Location: Graham, Texas
MTFCA Number: 51486
Board Member Since: 2021

Re: Coil secondary resistance data.

Post by TXGOAT2 » Wed May 31, 2023 8:40 am

("There were more coils, but they appeared to have open-circuit windings (note that doesn't necessarily mean they won't produce a spark!)."


I'd think an open secondary indication would reveal an open secondary coil, regardless of the fact that such a coil might still throw a spark.

It could be that the high voltage could "jump the gap" in the defective secondary winding and thus still emit a spark at the high voltage output. But I'd think such a coil unit would be on its way to failure, and not reliable.


Scott_Conger
Posts: 6428
Joined: Sun Jan 06, 2019 11:18 am
First Name: Scott
Last Name: Conger
* REQUIRED* Type and Year of Model Ts owned: '13, '15, '19, '23
Location: Clark, WY
Board Member Since: 2005

Re: Coil secondary resistance data.

Post by Scott_Conger » Wed May 31, 2023 10:52 am

Those data are not unlike what I have experienced when I was rebuilding coils. One could make an assumption that the process was very much not in control, or one could make an assumption that there is a natural range of variability with some shorted windings thrown in, or a 3rd option where there are at least 2 different winding procedures/protocols involved. The 5K measurements are so far out of family and so nearly identical, I would be inclined to believe that they are late KW coils, made post-T production period. I have seen coils made in the '50's and '60's which were up around 6.5K Ohm and higher.
Scott Conger

Tyranny under the guise of law is still Tyranny

NH Full Flow Float Valves
Obsolete carburetor parts manufactured


Topic author
Luke
Posts: 585
Joined: Fri Dec 13, 2019 1:04 am
First Name: Luke
Last Name: P
* REQUIRED* Type and Year of Model Ts owned: 1926
Location: New Zealand

Re: Coil secondary resistance data.

Post by Luke » Wed May 31, 2023 5:40 pm

Mike,

Thanks for ordering the data in your plot, it nicely shows the possible three 'classes' of sec coil resistance.

My original intent when measuring these coils was simply to see what the overall range was and nothing more. However, having established there is a possible class difference (interesting to see in Craig's thread https://www.mtfca.com/phpBB3/viewtopic.php?f=2&t=36757 that he has a 5k coil too), I think it worthwhile more closely inspecting the coils to see if there's anything further one might determine.

To that end I agree an inductance measurement would be useful, and a good look to see if there was any physical identifying feature between the classes. For me, as most of these coils belonged to a fellow T enthusiast, it'll be a few days before I can have another look at them, however of the four coils I have to hand here's my initial inductance measurements:

Resistance (KOhms) Inductance (H)

2.78 12.01 *black plastic coil
2.62 12.34
3.24 15.97
3.37 19.24

Obviously this is not enough of a dataset to come up with any useful conclusion, particularly as one of the coils is of a very different age/manufacturer than the others, however it's a start. If you and/or Scott had time to test any coils (or indeed had some pre-existing data) it'd be good to add that - mind you as you're heading into summer I suppose you've probably better things to do, like driving your cars!

There is one important caveat I should mention with regard to my data; I long ago sold my 'better' equipment and so these measurements were taken with an inductance meter of possibly dubious accuracy. While I expect it would be adequate for determining relative differences between coils I'm less certain about absolute measurement. That said it may in fact be ok, and if someone with a 'proper' meter came up with some data it will be useful to see if there's any correlation with what I found.

Scott,

I've never really considered the physical aspects of the coils (other than the obvious difference of the black plastic unit), is there a way to ID the late KW coils from their predecessors?

You also mentioned shorted turns. When I was developing the arduino coil tester I did contemplate adding a test for shorted turns, but ultimately decided against it for several reasons - have you tested for this issue specifically? It'd be good to know if you have and, if so, whether you found many afflicted coils.

Luke.


Art M
Posts: 840
Joined: Wed Jun 19, 2019 12:57 pm
First Name: Art
Last Name: Mirtes
* REQUIRED* Type and Year of Model Ts owned: 1923 Touring
Location: Huron, Ohio
MTFCA Number: 32489
MTFCI Number: 24068
Board Member Since: 2016

Re: Coil secondary resistance data.

Post by Art M » Thu Jun 01, 2023 9:47 pm

Luke, thanks for posting the induction data. I have thought that measuring the reactive impedence of the primary and secondary coils would be a better measurement. I just have got around to ask my wife to order a meter to do this. They are not very expensive, less than a tank of gasoline.

Art Mirtes

User avatar

dykker5502
Posts: 410
Joined: Mon Jan 07, 2019 7:01 pm
First Name: Michael
Last Name: Deichmann
* REQUIRED* Type and Year of Model Ts owned: 1914 Touring, 1921 Roadster P/U, 1922 Fordor (danish custom body)
Location: Rågeleje, Denmark
MTFCA Number: 29116
Board Member Since: 2007

Re: Coil secondary resistance data.

Post by dykker5502 » Fri Jun 02, 2023 5:19 am

These observations underpins Mike Kossers philosphy behind the ECCT.
Back in the day when you tested and adjusted 4 coils out of the same batch, you will most likely have coils with similar inductance, resistance etc since they are built of the same copper wire, the same number of turns etc.
When using the hand cranked tester adjusting to single spark and 1,3 Amps, you may get similar dwell on the 4 coils.
Today your 4 coils may be of different year, different factory (KW or Ford or modern) and adjusted with the HCCT you may get different dwell measurements and hence not perfect engine performance.
The ECCT measures the actual dwell which may give different current draws on the coils but an even engine operation.
You could most likely get similar variance in data for the original capacitors, but we swap them with new these days, right?
Ford Model T 1914 Touring
Ford Model T 1921 Roadster Pick-up
Ford Model T 1922 Fordor (danish build body)
ECCT, Strobospark, HCCT(Sold), Rebuilding coils

Post Reply Previous topicNext topic