Tacho options for a petrol dash?

[CJH]

Hope you don't mind some input ....

On the contrary



That's the photo of the petrol board. The pink resistor to the right of the SAK215 is the one that's being interpreted as 803. Looks like grey [8], black [0], orange [3] (or red [2]), black [x1], brown [1%]. Other way round would be 1_0_3 x1 0.05% (or 10% if it's silver).

The smaller white resistor below it is the one being interpreted as 191. Brown [1], white [9], brown [1], black [x1], brown [1%]. Other way round would be 1_0_1 x1G 1%.

[AngeloEvs]

Here is the problem, the grey could have been originally silver and the tolerance band but aged over time meaning the value is brown, black and orange at 1% = 10k. Apart from the fact it would be impossible to manufacture a resistor of 802 ohms it would also be pointless with a tolerance of 1%. Resistor values also change with time and ambient temperature. You might as well use an 820 ohm but the real issue is regards desired voltage or current parameter. In a series circuit of that magnitude there is no significant difference between a 802 or 820 ohm. Use ohms law and check it yourself. Calculate the current through the chain and then measure the volt drop across 802 and compare to volt drop across an 820. It will be insignificant.

[bigbadbob76]

All noted and corrected.

Could the 802 be a capacitor?
I have seen capacitors in similar packages to resistors. Not common but can be very confusing.
What does your ohm meter say?
Just to be safe, put +ve probe to pin 4 of chip and -ve to pin 1.




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[AngeloEvs]

Just had a quick browse at SAK chip circuits, omit the 802 and use a 220ohm pre set (small variable potentiometer) in place of the 191.

Leave the cap across R5, where it was.

Pity you dont have a signal generator to emulate the coil pulses.

[CJH]

All noted and corrected.

Could the 802 be a capacitor?
I have seen capacitors in similar packages to resistors. Not common but can be very confusing.
What does your ohm meter say?

I don't actually have an original petrol board to measure - that would make it too easy! I'm only working from that photo.

[CJH]

Just had a quick browse at SAK chip circuits, omit the 802 and use a 220ohm pre set (small variable potentiometer) in place of the 191.

Leave the cap across R5, where it was.

Ok, thank you. I had it running ok yesterday with the 802 omitted and R4 set up by trial and error (in conjunction with RV1) - I later measured it at 149 Ohms. So with a 220 in there instead, I'll just have to tweak RV1 a bit to compensate. It doesn't seem to be critical that it's 191, as you suggest, which does make it seem odd that they've used this specific value.

I've got a variable resistor set to 803 at the moment, so will test with and without it this evening.

Pity you dont have a signal generator to emulate the coil pulses.

Yes, it would make it a little easier, but it's easy enough to connect to the engine using my little test harness. With a third hand to hold the engine at a given speed it would be even easier. Actually, I'll take my little vice out with me so that I can hold the two potentiometers in place while I adjust them and hold the throttle open at the same time.

[CJH]

Leave the cap across R5, where it was.

Just to be sure - do you mean C1 in Bob's latest schematic? And do you mean put it on the same side of the Zener as R5 (as per his earlier schematic, or the opposite side (as per his latest version)? My board has it the opposite side - is it important?

[AngeloEvs]

If you use a 220 ohm pre set instead of the fixed value 191 you can adjust it as necessary to whatever value is best.

The capacitor location isn't critical as it is only a low value non electrolytic but other examples of that chip have it across R5.

Some circuits have a electrolytic capacitor across the meter which could help stabilise the movement but you can sort that later.

[CJH]

If you use a 220 ohm pre set instead of the fixed value 191 you can adjust it as necessary to whatever value is best.

The capacitor location isn't critical as it is only a low value non electrolytic but other examples of that chip have it across R5.

Some circuits have a electrolytic capacitor across the meter which could help stabilise the movement but you can sort that later.

Ok, thank you. I have a 1K in there instead of a 220, and I've pre-set it to 191 for my next test. I have a feeling it does a very similar job to RV1 somehow - both seem to scale the output current - so whatever value the 191 ends up as, I can compensate with RV1.

In my brief tests yesterday the needle seemed nice and smooth without an additional capacitor.

[CovKid]

I've got a signal generator Chris Two in fact

[CJH]

I've got a signal generator Chris Two in fact

Two? Are you a collector? Thanks Ralph - I'll keep that in mind if my in-situ testing hits a problem.

[bigbadbob76]

10K does sound more likely.
When driving a meter like that using an op-amp you would usually set the gain and the offset separately.
It looks like VR1 sets the offset by injecting a DC voltage into the feedback loop and R4 effectively sets the gain.
So you should be able to adjust both to give the right readings at each end of the scale.

[CJH]

10K does sound more likely.
When driving a meter like that using an op-amp you would usually set the gain and the offset separately.
It looks like VR1 sets the offset by injecting a DC voltage into the feedback loop and R4 effectively sets the gain.
So you should be able to adjust both to give the right readings at each end of the scale.

Separate controls for gain and offset would seem sensible. However, normally R4 is a fixed value, and the different Diesel versions can apparently be adjusted to suit the different pulley sizes (gain) using only RV1.

And can either of you explain how a 5-band resistor with grey (or silver); black; orange (or red); black; brown can be 10K? I get that in reverse order that could be:
Brown: 1
Black: 0
Orange: if this was to mean 'x1K', wouldn't it have to be a 4-band marking? We'd still be left with black and silver (or grey).

[AngeloEvs]

As Badbob points out, inductors and capacitors can appear to be resistors. Metal film resstors have 4 colour coding bands and carbon resitors of that period would have just three with a tolerance band. Brown, Black and Orange is 1, 0, and three 0's = 10,000 ohms.

I would omit that 802 ohm resistor and replace it with a low value non electrolytic such as a 3.3nF but it should be fine without.

The only other part of the circuit I would change is the zener and I would have it in parallel with the capacitor C1 and R3 to clamp the input to a fixed value of e.g. 5.6v. R5 could then be removed.

[bigbadbob76]

I know what you're saying about the zener Angelo. Looking at the spec sheet the max input voltage on pin 2 is 20v. With the potential divider on the input and the zener as well dropping the input pulse volts it'll be fine as is. [THUMBS UP SIGN]


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