Tach 10 pole to 20 pole converter.

[NMShooter]

Folks... I may be able to help us out with this tachometer issue we have with the 20 pole tachometer for our Force engines.

I think I understand the problem.. tell me if I am wrong.

Our engines have 20 pole magnetos on them. Every revolution, we output 20 pulses to the rectifier.

Modern tachometers are set up for things such as 6, 10, and 12 pole.

I think I can design a simple circuit that would allow us to use a standard 10 pole tachometer. All we need to do is to take the 20 pole signal, and divide it by two, then output the signal to the tachometer. However, I need your help....I don't know enough about where the tachometer interfaces to the engine / rectifier. I'll need to hook up a scope to measure the pulses and see what sort of conditioning I'll need to do to divide and buffer out to the tach.

Can somebody tell me where on the rectifier (or engine terminal block or whatever) the tach attaches to? Does it attach to the input of the rectifier?

Thanks...

Frank

 

[roscoe]

Re: Tach 10 pole to 20 pole converter.

Stator attaches to the input of the rectifier.
One output goes to the battery.
The other output goes to the tach.

Seems like a lot of work for this non problem.
A whole lot easier to just buy the correct tach if you need to replace one. They are readily available.

 

[NMShooter]

Re: Tach 10 pole to 20 pole converter.

Is it a bridge rectifier, or something else?

 

[NMShooter]

Re: Tach 10 pole to 20 pole converter.

I keep seeing folks posting comments about 20 pole tachs no longer being made. Is this true? I have had a difficult time finding one myself. Seems like 12 poles is the max.... is there a tach still available for these?

 

[pnwboat]

Re: Tach 10 pole to 20 pole converter.

20 pole tachs are still available. Not as common as the 4 6 8 10 or 12 pole, but they're out there.

 

[KenOverton]

Re: Tach 10 pole to 20 pole converter.

20 pole tachs are still available. Not as common as the 4 6 8 10 or 12 pole, but they're out there.

Please don't take this as me trying to be rude, and if it sounds that way I apologize beforehand.

I have been looking for about 4 days now and have not found anyone that has one. Lot's of sights have them listed. but when I call they do not actually have one. I have a few more sources to try tomorrow and another idea while I am out of town later in the week though.

So, NMShooter, let me know if you need anything for assistance, I would be willing to help out.

Ken

 

[Shife]

Re: Tach 10 pole to 20 pole converter.

Shoot. I have an oscilloscope and a Force with a 20 pole tach output, but I've got the boat and motor torn apart right now.

I'd be glad to give you some readings once I get the boat back together, but I can't say for sure when that will be. Could be next week. Could be August. Who knows these days.

 

[KenOverton]

Re: Tach 10 pole to 20 pole converter.

I do not own an oscilliscope, so I cannot help with that.

 

[chuckz]

Re: Tach 10 pole to 20 pole converter.

You don't need an oscilloscope. You know that the output of the full wave bridge rectifier will be a maximum of 17 or 18 volts. The pulses are going to be a bit larger than that. Feed the output though a three pin 5 volt regulator, without any external filtering, and you will get five volt peak pulse that look very much like square waves. I used a 3 pin regulator in a test set-up and it passes pulses down to about 0.5 microseconds, much shorter than the pulses you will get at maximum RPM. You now have a TTL compatible signal that you can feed into a flip flop which will provide the divide by 2 function. Use the output of the flip flop to drive the base of a transistor. Voila, a ten pole output referenced to the battery (Use the battery voltage to feed the transistor collector and ground the emitter. Wire a 10 pole tach in series with the transistor).

 

[Josh P]

Re: Tach 10 pole to 20 pole converter.

Ok chuck any way you can help me im with ken and trying to find a 20 pole tach and its not happening. How much would you charge to build me something like that? Help me out. If not im gonna call teleflex and see how much it would be to build the tach i was looking at, might buy a coulpe and throw them up on ebay.

 

[KenOverton]

Re: Tach 10 pole to 20 pole converter.

Ok chuck any way you can help me im with ken and trying to find a 20 pole tach and its not happening. How much would you charge to build me something like that? Help me out. If not im gonna call teleflex and see how much it would be to build the tach i was looking at, might buy a coulpe and throw them up on ebay.

I have MPS looking into the cost for Teleflex to build 2. I will let you know.

 

[KenOverton]

Re: Tach 10 pole to 20 pole converter.

You don't need an oscilloscope. You know that the output of the full wave bridge rectifier will be a maximum of 17 or 18 volts. The pulses are going to be a bit larger than that. Feed the output though a three pin 5 volt regulator, without any external filtering, and you will get five volt peak pulse that look very much like square waves. I used a 3 pin regulator in a test set-up and it passes pulses down to about 0.5 microseconds, much shorter than the pulses you will get at maximum RPM. You now have a TTL compatible signal that you can feed into a flip flop which will provide the divide by 2 function. Use the output of the flip flop to drive the base of a transistor. Voila, a ten pole output referenced to the battery (Use the battery voltage to feed the transistor collector and ground the emitter. Wire a 10 pole tach in series with the transistor).

Sounds simple enough. What could I use to verify the actual RPM on my 10-pole tach? Don't want to spend an arm and a leg for test equipment, my timing light with built in tach function reads over 5000 with my motor at idle. That is taking the pulse right off #1 spark plug, with timing light set for 2-stroke.

 

[Josh P]

Re: Tach 10 pole to 20 pole converter.

I have MPS looking into the cost for Teleflex to build 2. I will let you know.

they dont by the parts for them anymore teleflex sold out to VeeThree but its still the teleflex staff, already talked to chris and nothing at all from them.

 

[Frank Acampora]

Re: Tach 10 pole to 20 pole converter.

Not quite so simple! The so called 20 pole alternator stator only has 10 poles for the alternator. (count 'em, two completely different wimdings) The other ten poles charge the CD box capacitors. Compounded by the rim magnets ( I never counted) which produce more than ten pulses per revolution. Besides which, it is an AC signal that you must account for, not a simple pulse.

But, If you want it badly enough, it can be done. ANYTHING in this world can be done if you throw enough cubic money at it.

 

[chuckz]

Re: Tach 10 pole to 20 pole converter.

I have a 20 pole tach on my boat and I know it's not the mini-tach. I will check the manufacturer and let you know. I bought it about three (3) years ago.

If no tack is available I will design a circuit for everyone here. If there is enough interest, I may also build it for you. The shipping is much more than the parts if I only order one or two of each component.

I believe by definition a 20 pole tach means there are 20 pulses per revolution. I will check.

Yes it's AC coming off the stator. The voltage regulator converts it to positive 5 volt pulses.
The pulses are the positive going cycle of the sine wave clipped at 5 volts. The flip flop will convert the clipped pulses into nice clean square pulses. If the pulses need to be bi-polar for the tach to work, we can do that also, but I don't think it's necessary.

 

[chuckz]

Re: Tach 10 pole to 20 pole converter.

To measure the RPM with other test equipment you can use either a scope or a counter. You need to know the number of pulses per rotation to do the math. The math is, as follows:
Output Frequency/Number of Poles*60

For instance if you measure 600 Hz on a 10 pulse per revolution system the rpm is
600/10= 60 rotations per second

60 revolutions per second times 60 seconds in a minute = 3600 RPM

If you are measuring off any spark plug, a two stoke engine fires every other stroke so the formula is:
Frequency/2*60

If you measure the same 600 Hz off the spark plug the rpm is:
600/2= 300 revolutions per second (you're already in trouble)
300*60= 18000 RPM

 

[Nugglet]

Re: Tach 10 pole to 20 pole converter.

i would be interested

 

[chuckz]

Re: Tach 10 pole to 20 pole converter.

Checked my tach, it is a Teleflex.

 

[shytownborn]

Re: Tach 10 pole to 20 pole converter.

id be interested also

 

[Josh P]

Re: Tach 10 pole to 20 pole converter.

Ken and I purchased probably the last 2 teleflex 20 pole tachs. (probably others out there just have to do ALOT of digging) If you build the circiuts im interested i wanted a diffrent gauge sytle anyway, i would sell the tach i just bought. Teleflex no longer manufactures gauges they sold out to VeeThree which is a company that make auto gauges etc. They are staffed by the teleflex crew so they know whats going on, i talked to chris production manager, they no longer buy the parts to make the 20pole gauges is what he told me. In short im in to buy a circuit set up. Thanks Josh