Mystery Diode Identification

Thread Starter

6-volt

Joined Jun 18, 2020
4
I'm copying a circuit from an old aftermarket Japanese automotive tachometer an not having any luck identifying a diode in the circuit. The circuit is a one shot multivibrator using two NPN transistors controled by an RC circuit. The diode (D1) is the on the trigger signal from the primary ignition circuit of the engine. I used a 1N4002 diode on the breadboard mock-up because I had one on hand and it appears to work fine. The markings on the mystery diode are "TT 2.F". What would be a good replacement diode?



 

Thread Starter

6-volt

Joined Jun 18, 2020
4


Here is some background on this project. I have a Sun tachometer from 1955 that I want to put back into service. The meter itself is a D’Arsonval movement that works like an ammeter. It's a well made meter and it looks cool. The circuitry to drive the meter was located in a separate box in the engine compartment. It consisted of a relay triggered by the primary side of the ignition which would charge up a capacitor that would power the meter. Voltage was supplied by two 1.35V mercury batteries in series. The problem with this design is the mercury batteries are no longer available, the batteries get old and corrode, the points fail, and the capacitors go bad. Here is the schematic of the original circuit.




What I am attempting is replacing this 1950's technology with a reliable solid state circuit while maintaining the 1950's look. I have removed the guts from the transmitter box and plan on installing a PCB with my new circuit in the old transmitter box.

I disassembled a 1970's era Japanese tachometer and made my schematic directly from the circuit board in the donor tach. I have checked the circuit several times and my schematic in the original post is accurate. Could the mystery diode be part of an RC integrator circuit?

Here is my first test of the circuit.
 

Thread Starter

6-volt

Joined Jun 18, 2020
4
The frequency of the input signal is a square wave at only 467Hz with the tach at 7,000 RPM (full scale). I didn't design the circuit I only copied it. Could this be a clipping diode? Do you think I should replace the 1N4002 with a faster 1N4148?
 

AlbertHall

Joined Jun 4, 2014
11,610
If it is connected to a points/coil system, then the input is going to go up to a couple of hundred volta when the poits open so the diode has to be able to withstand that and at 467Hz high speed is not what is needed from the diode.

The 1N4002 is rated at 100V so I think you would be pushing your luck with that. I would suggest at least a 1N4004.
 

Thread Starter

6-volt

Joined Jun 18, 2020
4
If it is connected to a points/coil system, then the input is going to go up to a couple of hundred volta when the poits open so the diode has to be able to withstand that and at 467Hz high speed is not what is needed from the diode.

The 1N4002 is rated at 100V so I think you would be pushing your luck with that. I would suggest at least a 1N4004.
I tried it with a 1N4004 and it's working great just like it did with the 1N4002. I will take your advise and use the 1N4004.

I tried it with a 1N4148 and it didn't work at all.
 

cornishlad

Joined Jul 31, 2013
222
When I played around with a DIY car tachometer, rather than connecting directly to the ignition circuit I got one of those small toroidal chokes commonly seen in switch mode PSU's and passed the wire to the ignition coil through it. thus making an isolation transformer. Just a thought...
 
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