Electrical help

[muddy]

Morning all, this is something petty that has been bugging me for quite some time, I have an aftermarket EGT gauge and every time I crank the engine the volts must drop and the gauge resets to its default colour blue and I like it green so have to change it. Slightly annoying is there anything I can put in its power supply to stop this? Feed and earth is both good and also exactly the same as the boost gauge next to it that doesn't do it. 

 

Thanks, will.

[rtbarton]

You could try a blocking diode and a capacitor, something like this

 

[zardos]

Sizing the capacitor might be a lot of trial and error and getting the right capacitor or capacitors (multiple for 12v) needs some thought and could be expensive to get the right capacitors

It might actually be better to replace the capacitor with a small 12v battery rechargeable and again using a blocking diode to stop cranking from drawing power from the small battery.

It might be worth doing some measurements of the gauges characteristics before deciding what to do.
i.e.
measure at what voltage does the gauge reset at (if the gauge resets at 11.5v for example then you will probably need a very large capacitor)
measure the current draw of the gauge

[Bowie69]

Is it actually the voltage dropping causing it to drop out, or is it just connected to an ignition live that goes dead when cranking?

Most vehicles have both circuits... i.e. one live when cranking and one that goes down, power headlights, rear screen etc, to make sure starter gets the full whack from the battery.

[zardos]

Good thought Bowie, as on more thought the gauge sounds very electronic (having a programmable colour scheme) and therefore probably is internally running at 5V and so to get a on crank voltage drop to below 5V would probably mean your car battery is on the way out and needs replacing.

[muddy]

Thanks, the gauges have both an ign live and permanent live, the gauge does it even with an 1100amp tractor battery that's been on charge from my mxs180 however not if you jumpstart therefore I think it's very sensitive to a small volt drop.

 

will anysize diode do? I have some I used on my led spotlights when wiring into the reverse circuit.

 

will.

[zardos]

No you need the right type of diode and one with a suitable breakdown voltage, current rating and forward voltage. They also create a voltage drop which if enough could mean that it could run off the capacitors or second batteries stored charge when ignition is on but engine not running (engine running would be ok because of the higher voltage from the alternator)

But saying that the ones used with your led spotlights might be OK as they probably have a suitable breakdown voltage and forward voltage BUT might not have a suitable current rating.

You should be able to look up the specs of the diode from it's number.

[rtbarton]

A rectifier type diode will do as they can take some current, although we are talking milli-amps here if you use a capacitor the initial surge will be quite high when you first connect the battery.

The voltage drop across a typical diode is about 200 mV, insignificant in this application.

I can't comment about the LED light diodes without seeing the circuit they're used in.  As Zardos says we need to know the current draw from the permanent live input, from which the value of a suitable capacitor can be calculated.

[FridgeFreezer]

If it does it from any size battery but not on jump-start I'd ask about your battery-to-starter cable (assuming the ignition feed is from the starter post), if that's ropey or under-sized you'll get voltage drop across it no matter what.

The diode & capacitor solution may be as easy as anything, might cost upwards of 50p in parts I suppose.

[rtbarton]

2 hours ago, FridgeFreezer said:

If it does it from any size battery but not on jump-start I'd ask about your battery-to-starter cable (assuming the ignition feed is from the starter post), if that's ropey or under-sized you'll get voltage drop across it no matter what.

The diode & capacitor solution may be as easy as anything, might cost upwards of 50p in parts I suppose.

That's a good point, if the battery cable/connections are iffy it could still fail on a jump start, assuming you connect the 2nd battery direct to vehicle battery.    You could take 12v to the gauges straight from the battery (via a small fuse) to see if it makes a difference.

[HoSS]

On 9/10/2017 at 6:35 PM, zardos said:

Sizing the capacitor might be a lot of trial and error and getting the right capacitor or capacitors (multiple for 12v) needs some thought and could be expensive to get the right capacitors

Zardos sorry but you're talking a lot of rubbish.
Trial and error?   Not at all it's called electronics.
Multiple capacitors? -  No reason at all for that
Expensive?  - Not at all, less that the cost of a can of beer.

You need the capacitor to provide the current the gauge needs for the cranking time, without dropping below the minimum voltage.
A capacitor has several useful equations
Q=IT, Q=CV (where Q = charge, I = current, C = capacitance, V = voltage)
The discharge will be exponential, and the voltage after a certain time and current will be given by, V= Voriginal . e ^-t/RC

You dont know the R of the gauge, but you can approximate it by measuring the current when operating and use R = V/I
To make it easier here's a couple of calculators:

http://www.learningaboutelectronics.com/Articles/Capacitor-discharge-calculator.php#answer   Use this one in our case, it simple.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capdis.html#c2 A slightly more complete explanation.

Example: If i assume you gauge is drawing 10 mA at 12V , its equivalent resistance would be R=V/I = 12/0.01 = 1200 Ohm
Plug that in to the equation, or the calculator link above with lets say 3 seconds for crank, and play with the capacitance value until the result is over say 10V (where the gauge would drop out - it could be lower)
The result for that example would be a 15,000 uF capacitor.

For a diode use a 1N4148 (one of the most common and easily found diodes) 

But, something doesnt add up from what you said.
Does your gauge not have a permanent and a switched feed? Otherwise how is it retaining its (colour) memory when the vehicle is off.
It sounds perhaps like you have not wired that correctly. i.e. the perm feed has been wired to the switched acc supply.

[Bowie69]

10 minutes ago, HoSS said:

Does your gauge not have a permanent and a switched feed? Otherwise how is it retaining its (colour) memory when the vehicle is off.

Have to say, that confused me as well...

[rtbarton]

12 minutes ago, HoSS said:

Zardos sorry but you're talking a lot of rubbish.
Trial and error?   Not at all it's called electronics.
Multiple capacitors? -  No reason at all for that
Expensive?  - Not at all, less that the cost of a can of beer.

You need the capacitor to provide the current the gauge needs for the cranking time, without dropping below the minimum voltage.
A capacitor has several useful equations
Q=IT, Q=CV (where Q = charge, I = current, C = capacitance, V = voltage)
The discharge will be exponential, and the voltage after a certain time and current will be given by, V= Voriginal . e ^-t/RC

You dont know the R of the gauge, but you can approximate it by measuring the current when operating and use R = V/I
To make it easier here's a couple of calculators:

http://www.learningaboutelectronics.com/Articles/Capacitor-discharge-calculator.php#answer   Use this one in our case, it simple.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capdis.html#c2 A slightly more complete explanation.

Example: If i assume you gauge is drawing 10 mA at 12V , its equivalent resistance would be R=V/I = 12/0.01 = 1200 Ohm
Plug that in to the equation, or the calculator link above with lets say 3 seconds for crank, and play with the capacitance value until the result is over say 10V (where the gauge would drop out - it could be lower)
The result for that example would be a 15,000 uF capacitor.

For a diode use a 1N4148 (one of the most common and easily found diodes) 

But, something doesnt add up from what you said.
Does your gauge not have a permanent and a switched feed? Otherwise how is it retaining its (colour) memory when the vehicle is off.
It sounds perhaps like you have not wired that correctly. i.e. the perm feed has been wired to the switched acc supply.

Spot on.  Couldn't remember the calcs, a long time since I did my O level physics!  If the colour is OK when switching the ignition on then it must have remembered it OK from the last time it was switched off.  IIRC the accessories terminal is dead when the key is taken out.  If subsequent cranking causes the gauge to lose it's colour then it looks like a voltage drop condition, as Fridge-Freezer suggests.

[zardos]

1 hour ago, HoSS said:

Zardos sorry but you're talking a lot of rubbish.
Trial and error?   Not at all it's called electronics.
Multiple capacitors? -  No reason at all for that
Expensive?  - Not at all, less that the cost of a can of beer.

You quote a lot of equations that I'm well aware of, BUT as in your example you are making assumptions. We don't know about any of the electrical properties of the gauge to use with these equations.

I did also suggest getting the data so the equations could be used to remove the trial and error of trying a couple of capacitors or combinations (combining capacitors might get you a better cost vs a single capacitor) or go for overkill and just buying a very large capacitor (which RS suggest was not cheap)

[zardos]

2 hours ago, HoSS said:

Does your gauge not have a permanent and a switched feed? Otherwise how is it retaining its (colour) memory when the vehicle is off.

There are many ways to store that data (like which colour to use) without power.

E.g.

A small internal rechargeable battery
A large internal capacitor
Flash memory
...

But as Will says:-
 

On 9/10/2017 at 7:46 PM, muddy said:

the gauges have both an ign live and permanent live

and he does not say the gauge is the wrong colour when the ignition is turned on (therefore it has a method to remember it, most likely from the permanent live), only that it changes when cranking

[HoSS]

59 minutes ago, zardos said:

You quote a lot of equations that I'm well aware of

I really don't think you are. I've no intention of being offensive, but you insist on posting a lot of incorrect waffle.
People ask questions beacuse they need help, and it's important to give correct answers. Your waffle does not help anyone.

[zardos]

Now you are being offensive.

People ask questions, to give the best help possible, it's best to ask questions back if more info is need instead of making assumptions which tend to lead to bad answers.

[zardos]

Anyway to move this forward for Will, may be instead measuring the key parameters to determine the capacitor size, may be you can tell us the make/model of the EGT gauge or a web link to the gauge?

Though a quick google for example EGT parameters did not turn up many useful specifications, a few gauge manuals said just to put a 2 or 3 Amp fuses on the permanent live wire

I did find that SPA did actually provide some useful details in their manuals ( http://www.spa-uk.co.uk/SPA/docs/DG60 manual issue 1.1.pdf for example)

This gave an operational voltage range of 10v-16v (so in my opinion using 10v for this type of gauge is a bit borderline, I would calculate using 10.5v)
and
48mA power consumption when on and backlit (it is better to use this higher consumption number as you don't want the capacitor only working some times)

I would also be a bit more generous on the crank time than 3 seconds just to build a bit more of a buffer in the calculation.

[zardos]

While Hoss might know capacitor equations he fails to take in to account that diodes are not perfect.
Looking at the 1N4148 diode spec sheet (instead of using the equations) it looks like the voltage drop across this diode will be about 0.8v for the load and expected temperature when using my example of the SPA EGT gauge.

If you are interested in diodes then https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/introduction-to-diodes-and-rectifiers/ gives some background

plugging the numbers in of
11.2V start voltage (12v - 0.8v)
250hms (48mA @ 12v - this is a bit of an estimate as we don't know the current consumption at 11.2V)
5 second crank time
and a 10.5v target voltage

Gives a capacitor of around 300 mF need (about 20 times bigger than HoSS example)

But then again your particular gauge might have different parameters.

You could also think about the initial charge current of the capacitor being higher than the diode can handle, you could calculate this or you could work around it by initially charging the capacitor without the diode in place. (this would be a one off concern as once connected the charge in the capacitor would not go low enough to cause a high current spike after initial charge) or add a current limiting resistor.

You could also do a parameter sweep DC simulation in QUCS (Quite universal Circuit Simulation http://qucs.sourceforge.net/), this is something I have just started playing with but that is probably overkill.

[muddy]

Yes they have both perm and ign live connections, both boost and egt gauges are glowshift black 7 colour gauges, the boost one always stays green the egt is green when you ign on but drops out during cranking, they are wired into the same feeds, I will try and get a measurement this weekend as any further progress is useless without, I hear you fridge but I think the cabling is fine it fires the tdi up pretty much instantly. 

I did take a video but can't see how to upload it from my iPhone?

 

will

[HoSS]

6 hours ago, zardos said:

While Hoss might know capacitor equations he fails to take in to account that diodes are not perfect.

and the waffle continues....
Its clear you have no idea what you are talking about, please stop confusing people with your incorrect information and guess-work.

[Bowie69]

@muddy, you can upload a video directly from phone by using the "Insert other media" button, and then take the video directly.

Or, if you have already taken the video, use the "Click to choose files" link and then choose "Documents" and find it in the file manager. This assumes you are on Android, I don't know about iOS.... (Deliberately).

[HoSS]

2 hours ago, muddy said:

Yes they have both perm and ign live connections, both boost and egt gauges are glowshift black 7 colour gauges, the boost one always stays green the egt is green when you ign on but drops out during cranking, they are wired into the same feeds, I will try and get a measurement this weekend as any further progress is useless without, I hear you fridge but I think the cabling is fine it fires the tdi up pretty much instantly. 

Measure the perm feed to see if it is dropping during cranking. This may be hard to see on a multimeter due to response time. A scope would be the ideal tool.
In any case try with the cap/diode circuit shown above in the PERM line, the current in that line should be very small (a point our electronics expert seems to miss) it should be just keeping the volatile memory alive which should be less than 1mA.
This should fix it.

[muddy]

Ah nearly worked but it's over 10mb 

[Bowie69]

Stick it on YouTube and then link to it?