Tacho options for a petrol dash?

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Re: Tacho options for a petrol dash?

Post by CovKid »

VW must hate you Chris :D
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Re: Tacho options for a petrol dash?

Post by CJH »

The SAK215 ICs arrived this morning, so I couldn't resist the chance to destroy my expensive tachometer :lol:

I carefully sorted out a complete set of parts. R6 is bigger (0.5W) than the others (0.25W) only because the place I bought it from didn't have any 0.25W versions. The variable resistor should have been 22k, but again the supplier didn't have that value, and I guessed that 20k was close enough. I don't know the first thing about different capacitor types, and the only 220nF version I could find that wasn't stupid money is the one in the photo - it doesn't look like the type used on the original board, but hopefully the most important thing is the capacitance value. And the last thing I wasn't sure about was the diode. All I can tell from the photos is that it's a zener diode from the BZX55 series - I can't tell its value. It seems to be there to protect the IC from over voltage, and since the SAK215 has a maximum voltage of +/- 20V I went with a 5V6 zener.
Image

Then I assembled everything. I went with a fly-lead for the connection to the dashboard, since I don't have an original dash with the bespoke VW connector. I also used a fly-lead for the connection the the gauge, so that I can easily swap the original Diesel PCB back in.
Image

Image

So far so good. I made up a temporary connection to the coil, started the engine and plugged in the PCB. Good news - nothing popped, banged, smoked or otherwise failed. Bad news - even at idle the tacho needle swung all the way round to the right. So I swapped a couple of the components whose values I wasn't sure about - the circuit schematic said one value but the photo seemed to show a different value. The schematic shows R8 to be 602 Ohms (I used 620 Ohms), but the photo seems to show 100 Ohms - so I swapped it for a 100 Ohm 0.25W resistor. And because I was worried that my variable resistor only went to 20k rather than 22k, I swapped R6 from 16k to 20k.

Plugged it back in - no change. So the last thing that I'm able to identify as a possible problem is the value of the zener diode - apart from protecting the IC against too high an input voltage, does it have any other function in scaling the output somehow I wonder. Beyond this I'm a bit out of my depth. I'll see if I can find an exemplar circuit for the SAK215 to see whether there are any obvious blunders in the schematic that I was working from.

Right - time for a bit of a tidy up. My house is starting to resemble a branch of Maplins (before they sold off all their stock, obviously).
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Re: Tacho options for a petrol dash?

Post by CovKid »

AngeloEVS probably your best bet here. I wonder if its possible to run it in a simulator.
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Re: Tacho options for a petrol dash?

Post by CJH »

I didn't get around to tidying up! I've been trying to understand the spec sheet for the SAK215 IC, as well as experimenting with the bare gauge and my bench power supply. I *think* I'm onto something.

The gauge seems to be a 10mA gauge, i.e. full scale deflection occurs when the current through it is 10mA. The SAK215 converts frequency to current and therefore the output current needs to be matched to the full scale deflection of the gauge. As far as I can tell, the schematic that I've been working from is for a 50mA gauge, which explains why the needle slams into the end stop. At the very least, the resistor that scales the output current needs to be changed. I used a 200 Ohm resistor (the schematic had 191 Ohms and I thought 200 was close enough), and by my calculations it might need ~950 Ohms. I might try a 1K variable resistor.
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Re: Tacho options for a petrol dash?

Post by bigbadbob76 »

try a 6.8v. :lol:

Image

It won't stop your tacho hitting fsd though.

Sounds like you've sussed it though, if 5krpm=50ma then 1krpm=10ma=FSD on your gauge.
only just not enough to bounce the needle off the endstop and bend it or burn it out. phew.
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Re: Tacho options for a petrol dash?

Post by CJH »

That's a version of the SAK215 circuit that I haven't seen. R2 in that diagram is the one that controls the full scale current I think. Do you know which gauge that works with? The value of 200 Ohms suggests that's also for a 50mA gauge, unless I'm completely misreading the spec sheet. There are a few extra components in that schematic - presumably to help smooth out the needle response.

I do have another issue, in that without the engine running I have a small current through the gauge, so the needle sits at around 1000rpm. My PCB has a few extra components compared to the basic test circuit shown on the spec sheet, so I'm going to strip my PCB back to that state and see if that cures the residual current.

Another thing I can't tell is whether there's any adjustment available to calibrate the needle. The only adjustment in the test circuit is PR2 in the schematic you showed above. It goes to pin4, which is the feedback input, so I'm not convinced that is for trimming the circuit. R2 is where, if I've understood correctly, the output current is scaled. So I'd have thought R2 ought to be a variable resistor, but I haven't seen that in any schematics.
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Re: Tacho options for a petrol dash?

Post by 937carrera »

Wlring diagrams in Polish now - I'm struggling to keep up already :rofl
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Re: Tacho options for a petrol dash?

Post by CJH »

Actually, you know what, I AM misreading the spec sheet. It says I can calculate the value of that resistor from V6/I5p. I know I5p, the peak current, but how do I know the voltage on pin 6? It's connected to ground through the resistor I'm trying to calculate, and according to the spec sheet it can also be connected to battery voltage through a 25uF capacitor.

The spec sheet says "Since the current flowing into pin 5 is equal to the sink current of pin 6, the adjustment resistor can be calculated as V6/I5p....." I took that to mean that the chip matches the current on pin 5 (the gauge negative) to the output current from pin 6, so by limiting the pin 6 current I can control the gauge current. But the calculation requires me to know the voltage on pin 6, and I'm stuck.

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Re: Tacho options for a petrol dash?

Post by CJH »

Tsk - and I've just noticed that the sample circuit (Fig 2) in the spec sheet is for a 10mA gauge, and uses only a 160 Ohm resistor.

Confused now - the only things I'm sure of are that my gauge needs 10mA for FSD, the gauge has a resistance of 150 Ohms, and that my PCB is delivering much too much current for my 10mA gauge.
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Re: Tacho options for a petrol dash?

Post by bigbadbob76 »

937carrera wrote:Wlring diagrams in Polish now - I'm struggling to keep up already :rofl

yeah, I found it here https://www.elektroda.pl/rtvforum/topic2141048.html it seems to be a Motometer tacho for god knows what, I havn't worked through the translation.

I'd have to study the SAk datasheet to be of any help and I'm on my 2nd JD allraedy. :lol:
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Re: Tacho options for a petrol dash?

Post by bigbadbob76 »

I was thinking about this while walking the dog just now, you might need to scale the input circuit to suit your 5.6v zener.
If there's too much current into the input of the SAK it won't detect the pulses, it will always be on, or partially on, just like our fuel led circuit does with the 5v6 zener.
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Re: Tacho options for a petrol dash?

Post by CJH »

That's a possibility, although I did find a post that said the circuit worked with a 5v6 and not with a 6V8.

I was thinking about the V6 problem, and it makes some sense if pins 5 and 6 are respectively the collector and emitter of a transistor. The spec sheet refers to V5/6, as though they are the same, which would be the case apart from the 0.2V drop. And since the collector and emitter are essentially in the same circuit, controlling the current in the emitter leg would automatically control the collector side too. So the SAK215's job is to control the base-emitter current, and the job of the adjustment resistor is to limit the current when the transistor is saturated.

So in my case, I know R(gauge) is 150 Ohms. For a running engine, the ignition voltage is around 14V, and I want a maximum current of 0.01A. I want to know R(Adj)
14 = 0.01 * (150+R(Adj))
R(Adj) = 1250 Ohms
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Re: Tacho options for a petrol dash?

Post by CJH »

Taking into account the 'pulse duty factor' of 0.7, R(Adj) comes out as 850 Ohms.
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Re: Tacho options for a petrol dash?

Post by Cobra88 »

Hi Chris

You probably already have this one (I have no idea how to shorten the link :oops:

https://redirect.viglink.com/?format=go ... nziner.gif

Rick
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Re: Tacho options for a petrol dash?

Post by CJH »

Thanks Rick

That one seems to have a 19 Ohm resistor, so, unless I'm seriously off target (which is still a distinct possibility), that might suggest it's designed for a gauge with a much higher full scale current and/or internal resistance. I think the target gauge characteristics are quite critical, so where I previously thought you could just take a petrol board from any old tacho, I now realise that the gauge resistance and full scale current would have to match.

I'm about to test out my modified board with a 1K variable resistor, to see if I can at least get the gauge into a zone where the rpm makes a difference to the needle position.
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