I need help with this half wave rectifier I am designing!

[simo4295]

Hello all,

I am currently designing a half-wave rectifier to convert AC pulses from an alternator on a moped to 5V DC pulses for an arduino shield to detect, and trigger a 555 timer.

The circuit I made consists of a diode and a voltage regulator to get it down to 5v which should work, but I want to protect the circuit somehow, so I dont fry components on the shield or the arduino.

So my questions are:

How should I protect my circuit, and how can I do it the best way possible?

And how should I connect it to the shield? The shield is the large schematic attached, and the output from the rectifier goes to the "TACH OUT" terminal on the engine RPM. Do I just ground the circuit to the ground on the shield?
 
Thank you very much!

[Audioguru]

The input of the circuit called "Tach Out" is the base of the BC547 transistor that will blow up because it has nothing to limit its base current that needs to be only 0.05mA (0.00005A).
Why are you using a 5V regulator and a 1A fuse to produce 5V pulses? The circuit cannot be powered by the pulses (it needs smooth DC) and the transistor does not care about the voltage since it needs only a small current from the rectifier which can be limited by a series resistor, not by a voltage regulator.   

[simo4295]

It was just an idea that it put onto paper.
So what you are saying is that:
the alternator and diode is good, but I should remove the fuse (silly remnant from a scrapped sketch) and regulator and put in a resistor in series.
Or should I scrap everything and put a resistor from the lead directly from the alternator?
How would I prevent the transistor from blowing to bits?

[Zero999]

It was just an idea that it put onto paper.
So what you are saying is that:
the alternator and diode is good, but I should remove the fuse (silly remnant from a scrapped sketch) and regulator and put in a resistor in series.
Or should I scrap everything and put a resistor from the lead directly from the alternator?
How would I prevent the transistor from blowing to bits?

What about a TL431 circuit?

[simo4295]

I'm getting a bit confused now. How can I easily reach my goal?

Regarding the TL431 circuit, I am not sure how I would implement that.
I started looking into this yesterday, with very basic knowledge.

[mobbarley]

I think the TL431 idea was to replace the 7805 with a smaller regulator, but if you have 7805's why not use them - at least for hobby purposes.

You can get a lot of ideas from automotive design guides by searching for terms such as 'automotive' and 'load dump' for example: http://www.ti.com/lit/an/snva681a/snva681a.pdf

In saying that it might be worth protecting against a load dump type even as I guess a moped's electrical system is very basic. You can do this by using a high enough voltage & current zener or TVS (about the max Vin rating of your regulator) in a crowbar arrangement, then choose the lowest current fuse possible. If a load dump occurs the input fuse should blow before your regulator.

[simo4295]

This draft would work then right? I tried my best at a crowbar circuit, and bear in mind that the names of the diodes and components are not the correct, I will have to look that up.

I am still not sure how to implement the TL431, so if anyone could do a sketch that would be great!

[Zero999]

Hello all,

I am currently designing a half-wave rectifier to convert AC pulses from an alternator on a moped to 5V DC pulses for an arduino shield to detect, and trigger a 555 timer.

The arduino has a 5V supply anyway right?

So why not just use a couple of clamping diodes and a series resistor to clip the voltage to <0.6V within the supply rails?

And why do you need a 555 timer? The aurduino can perform the timing function with the correct code.

[simo4295]

The shield schematic is not my design, but I think it is because the signal from the moped can be noisy, especially if you use a capacitive clamp on the ignition lead for the spark plug, instead of "reading" the lighting coil.

I think I have settled on the last schematic I posted, with the crowbar circuit, because I can understand what is going on with it. I want to eliminate any mistakes with it now, though as it is the first circuit Ive ever made.

[exe]

If you don't need sharp edges I'd just clamped the output with a zener (and a current limiting resistor, 1k-4k would do the job, but check zener's datasheet). Just for, say 5V clamping I would take a zener with like 3.3-4.0V breakdown voltage because zeners are not vary accurate regulators (if not say more) and digital inputs usually do not tolerate voltages above their supply (because they are clamped internally).  Also check voltages before attaching to arduino

[simo4295]

Thank you very much!
This seems to be the most reasonable solution.

[simo4295]

One last question:
How should I connect the circuit with the shield?

The positive lead on the "Tach out", and the negative on the shield ground?
I am not sure because the capacitive clamp just connects to one "port"

[danadak]

You need to handle load dump issues in car designs, google load dump circuit
design or similar for application notes, help.

http://www.littelfuse.com/~/media/electronics/application_notes/tvs_diodes/littelfuse_tvs_diode_automotive_circuit_protection_using_automotive_tvs_diodes_application_note.pdf.pdf

Regards, Dana.

[exe]

The positive lead on the "Tach out", and the negative on the shield ground?

Yes, but how much the "tach out" can supply? The scheme I proposed draws like up to 20mA or so (less with higher-value resistors). This may be too much for car electronics, I don't know...