Tacho adaptor

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Tacho adaptor

Postby DaveEFI » Tue Oct 24, 2017 4:41 pm

I've been having a play with the relay coil pulse amplifier circuit used to drive a tacho normally fed from a single coil negative. Not for myself, and sadly I can't try any results on the Smiths unit in question, as the car is at the other end of the country.

The owner reported the circuit sort of worked at low revs with the relay chattering. Raise the revs above about 1500 and the chattering stopped - and so did the tacho.

So I'm playing with it here. The relay is a generic 30 amp SPST. DC resistance is 86 ohms and inductance 0.3H

Pulse output from JS3 is solid at any revs.

With the relay chattering, the waveform reads about 50v peak on the scope. When it stops chattering, more like 20v - but a much cleaner pulse.
Tried a 1 amp relay I had lying around. That chatters happily at any revs on the stim and produces much the same high voltage pulse as the larger relay when chattering. But not sure how long it would last in use. And would have to be mounted somewhere you didn't hear it. :D

I'll post some scope pics if anyone is interested. I didn't realise how much effect the armature moving would have on the generated pulse.
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Re: Tacho adaptor

Postby Six_Shooter » Tue Oct 24, 2017 9:09 pm

You need to remove the moving part of the relay.
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Re: Tacho adaptor

Postby DaveEFI » Wed Oct 25, 2017 1:31 am

Six_Shooter wrote:You need to remove the moving part of the relay.


Simply wedging it so it can't move gives the lower peak voltage of about 20, which doesn't seem to be enough to trigger this tacho.
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Re: Tacho adaptor

Postby DaveEFI » Thu Oct 26, 2017 3:28 pm

Can I ask if anyone has got this circuit (2N5551 and relay coil, 1k resistor to base) to work with a Smith's rev counter from the '70s? MS1 V3 fed from JS3.

On test here, I was only seeing about 20v peak from it - and the 2N5551 got very hot. When switched on from cold, it did produce a spike which soon disappeared as it heated up. Volts set to 13.8v

So decided to try a TIP 122. Coil is from a Bosch 30A SPDT type.
Massive great spike.

Image

Will report later if it actually drives the tacho. Could not get the original circuit to work reliably on my car which has a later Smiths tacho, also fed from the coil negative, but didn't experiment as now due to time.
Last edited by DaveEFI on Sat Oct 28, 2017 1:45 am, edited 1 time in total.
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Re: Tacho adaptor

Postby Quagmire » Fri Oct 27, 2017 1:09 pm

Dave, my P6 rev counter works with this circuit, not sure what transistor I used at first, it was a plastic bodied one though. That worked for a few years and then when that died I ended up using a tip122 instead as that was all I had to hand.

Both versions used/use a 40A relay from maplin with the contacts removed to silence them.
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Re: Tacho adaptor

Postby DaveEFI » Sat Oct 28, 2017 2:05 am

Quagmire wrote:Dave, my P6 rev counter works with this circuit, not sure what transistor I used at first, it was a plastic bodied one though. That worked for a few years and then when that died I ended up using a tip122 instead as that was all I had to hand.

Both versions used/use a 40A relay from maplin with the contacts removed to silence them.


Thanks. The original circuit didn't work with the P6 rev counter. Or rather did at very low revs, but stopped above about 1500 rpm. I'll report back if the new one is OK after it's been installed - hopefully next week. The original circuit did much the same with my SD1 rev counter too.

I tried various relays. A 12v 1amp one produced a much nicer waveform using the 2N5551, but made a horrid noise. And being small and encapsulated, not so easy the remove the armature.

The P6 owner had bought a pro made unit called a TachFix which he couldn't get to work either.
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