Tach resistor

[Jman]

Hi
I converted my 66 AH sprite to negative earth. I was wondering if anyone knew the value of the resistor in the tachometer that you have to reverse. When I was unsoldering it to do the swap it broke in half. I have figured out that it is a 150 ohm resistor with a 10% tollerance, but I don't know what wattage it is.

Thanks
Jman

 

[ecurie_ecosse]

Ohms Law might help you here. I=V/R. This gives the amps,for watts,you multiply your answer by the voltage.
Not sure if this wll be helpful,but it's worth trying.

Stuart.

 

[Trevor Jessie]

1 watt should work... but that is based on my faint memories of high school physics.

 

[sparkydave]

Um, what exactly are you "reversing"? Resistors are not polarity specific, only the connections to the tach would be. You won't need more than 1 watt, FYI. (12 * 12)/150 = 0.96.

 

[Jman]

I'm not reversing the resistor. I am following this guide for changing the tach polarity.

https://www.mgcars.org.uk/midgetspriteclub/MascNew/TechTips/PosNeg.html

I appreciate the info though. just out of curiosity, where did you get the 12*12 from in your formula?

Thanks

 

[dklawson]

He's multiplying (12V * 12V)/Ohms = power.
V=I*R so I = V/R,
P=V*I, therefore P = V * V/R = Watts
(However, 14V would be better here since the tach is only running when the charging system is working... 14*14/150 = 1.3 Watts )

It's been a while but I believe it's the diode that is reversed. See:
https://www.lotus-cortina.com/electric/convert.htm
see also:
https://www.hazelden.ca/austinhealey/RebuildingBJ8Tach/
and
https://www.classictiger.com/techtips/motach.html
for a general schematics

 

[Jman]

Thanks guys, I really appreciate the info.

 

[Sarastro]

I read the section from the MG and Sprite Club website, and I think I see what you're doing. You're switching that resistor from one spade terminal to the other, not simply reversing it.
That, and reversing the loop, are all you need to do. Don't reverse any diodes or anything like that.

Astley's book, "MGB Electrical Systems," shows this as 120 ohms, 1/2 watt. But that's for an MGB, and it's possible that Spridgets could be different. In any case, the tach will probably work with either value, but the value does affect the reading.

The circuit shows a 5.6V zener diode in series with the resistor, so the voltage across the resistor is more like 9V, less if you have a generator instead of an alternator. I think the half watt is marginal, and not enough if you have an alternator. No harm in going higher in wattage, anyway, so I'd use a 1 watt resistor. The resistance value probably isn't critical, either. In any case you will certainly have to readjust the tachometer to make sure it reads correctly. There's an internal adjustment for that.

 

[Jman]

I read the section from the MG and Sprite Club website, and I think I see what you're doing. You're switching that resistor from one spade terminal to the other, not simply reversing it.
That, and reversing the loop, are all you need to do. Don't reverse any diodes or anything like that.

Right, just changing the terminal it's attached to. I ordered a 150 ohm 2 watt resistor. I didn't figure it would hurt to have to much wattage. I've already reversed the loop, and now I'm just waiting on the resistor. Pretty simple fix actually if it works. The only reason I even had to get a new resistor is because the other one disintegrated when I went to change it. I hope that's the only one that is that fragile.