Electrical help

[zardos]

It is true that I'm no electronics designer, but again HoSS is making guesses about the gauges design that the permanent live only powers the memory part of the gauge, it could be that it powers all of the gauge and the accessory power just acts as a switch.

I go back to my initial question - what is the power consumption by measuring it, a simple multi-meter can do this, instead of guessing.

[FridgeFreezer]

FWIW I think both Zardos and Hoss are taking this way too seriously, it's a shonky old Land Rover and  tacky eBay gauge, the error margin on the entire thing is about 200% so pick any old diode and whatever size cap you can find from the nearest dead PC power supply or old telly that you can squeeze behind the dash, join 'em together and give it a squirt. It'll either work or it won't.

[HoSS]

1 hour ago, zardos said:

It is true that I'm no electronics designer, but again HoSS is making guesses about the gauges design that the permanent live only powers the memory part of the gauge, it could be that it powers all of the gauge and the accessory power just acts as a switch.

I go back to my initial question - what is the power consumption by measuring it, a simple multi-meter can do this, instead of guessing.

Lol you never suggested measuring it until after my post. In your first post you suggested trial and error and then waffled about irrelevant diode parameters, adding batteries and charge circuits...
But i see you have since much edited your original posts to reflect some of what i said.
(For the record i'm a degree qualified electronics engineer with 25 years of experience.)

[HoSS]

1 minute ago, FridgeFreezer said:

FWIW I think both Zardos and Hoss are taking this way too seriously, it's a shonky old Land Rover and  tacky eBay gauge, the error margin on the entire thing is about 200% so pick any old diode and whatever size cap you can find from the nearest dead PC power supply or old telly that you can squeeze behind the dash, join 'em together and give it a squirt. It'll either work or it won't.

I take it seriously to see bad advice posted, and i try to give a complete answer, it may help people pick something up.
But yes in practice a largish cap and any old diode may work. But 1N4148 is the easiest to find at Maplin for example. And measuring the actual current avoids fitting a cap to large or too small.

[zardos]

On 9/10/2017 at 5:35 PM, zardos said:

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

Clearly you might know something about electronics, but it is also clear you have a problem reading as this is a second time you have missed reading previous posts.

[HoSS]

24 minutes ago, zardos said:

Clearly you might know something about electronics, but it is also clear you have a problem reading as this is a second time you have missed reading previous posts.

My mistake for reading your original posts and not your later heavily edited versions. 

 

[zardos]

Do you have admin / moderator powers? As us normal users can only edit a post for a short period after posting and the original post was way past that time limit when you first posted.
And it's not just my posts you have failed to read properly.

And yes I do edit posts to correct typo's and re-word things but usually that is straight after I've posted them as I know there is a time limit on editing.

[muddy]

Ok chaps, with multimeter connected in series of the permanent colour wire the it's reads nothing with the ign off and .05 with ign on. It must be drawing something as unplugging the wire loses the memory. It's hard to judge when the gauge drops out but ~10s cranking shows a low of 9.61v I'd estimate it drops out at around 10v.

 

the diodes I have read SB5100  pec5200 and are a similar size to the coloured band on a male spade terminal

 

will.

Ps, fridge I'm insulted! This is the finest tat the Stars and Stripes have to offer carefully nailed into a British motoring icon!

[rtbarton]

The meter may not be sensitive enough to measure the quiescent current - it could be a few micro-amps.  You say it shows 0.05, but 0.05 what?

Where are you measuring the cranking voltage?  Where is the voltage for the gauge taken from?

 

[zardos]

Yes confirmation of the unit of the 0.05 number you gave would be good but as Will said the multimeter was connected in series the it is likely to be Amp's or milliamps.
If it's 0.05 Amps then this gauge would be similar to my example gauge I linked previously (that was 10v minimum and 0.046Amps)

The diode specs looks fine (if a little over spec'd) https://www.diodes.com/assets/Datasheets/ds30135.pdf

It is a 5A Schottky Diode rated to 100v, at 0.05A the voltage drop is going to be about 0.25v according to the datasheet (much better than 40% loss the useful voltage range of a 1N4148 and around the figure rtbarton suggested)

So plugging 11.75v to above 10v with a 240 Ohm load (for 0.05A) for a 3 second crank to the formula HoSS quoted or entering the numbers in to a QUCS simulation suggests you would need about a 0.08 F capacitor

The nearest suitable from a quick search was http://uk.rs-online.com/web/p/aluminium-capacitors/5437003/

which is not small or cheap

Lets hope the unit was milliamps not Amps

as 0.05 milliamps would need a 100uF capacitor e.g http://uk.rs-online.com/web/p/aluminium-capacitors/5194059/ at 21p

Alternatives if 0.05Amps?
As I first suggested http://uk.rs-online.com/web/p/lead-acid-rechargeable-batteries/6867253/
and diode might be better, it's around the same size as the capacitor but half the price at it has a much shallower voltage drop profile and a much higher charge storage capacity.
But this still sucks

You say that the boost gauge next to it shares the same feed and earth (is that gauge a similar model/manufacturer?) and it does not show the same problems?
Have you swapped the feed and earth wires, may be there is a bad joint from where they must split at some point?

Another solution might be to put an extra switch on the ignition feed, so the gauges are not powered when cranking and then can be manually switched on after cranking has stopped.
This might solve the problem or make is so that the capacitor need to retain the memory would be cheap and small, as you have seen that it is below your measurement range.
Again this sucks a bit

Buy a new gauge that only has one colour of green?

[muddy]

The multimeter just has one setting that shows 10A max this is where the measurement was taken so I'd guess its 0.05A? 

 

Yes both gauges are the same model and make and all three wires on both come together and are crimped into the appropriate connector.

 

id rather not buy any more they are fairly dear!

 

will.

Edited September 25, 2017 by muddy

[Bowie69]

That'll be 50mA then.

[zardos]

Thanks for clarification of Unit, so yes Amps.

I had another idea, that has lower cost and is much smaller in size but has a limited life (that life could be around 1200 engine starts, which at twice a day is over 1.5 years)

This would use an A23 non rechargeable 12V battery to power the gauge during crank (they are about 50mAh in capacity so could in theory run the gauge for about 1 hour of cranking before it would need to be replaced)
It would need a second diode to stop it being charged by the car battery/alternator (yes the diode is not perfect so will be a very very small charging affect on this non rechargeable battery of around 0.001mA, but I don't expect this level to damage the non rechargeable A23 battery)

Since you can get 2 x A23 batteries for £1.62 https://www.amazon.co.uk/Energizer-Alkaline-Batteries-LRV08-GP23A/dp/B002YOWPHG/

and a battery holder https://www.amazon.co.uk/SODIAL-TM-Battery-Holder-Black/dp/B00DFQQWC6/ for £1.26

This keeps the cost and size down at the cost of needing the battery to be replaced at some point (I was not able to find a rechargeable version of the A23 battery)

 

Other alternatives:-

If you ignition switch has an accessory output (as somebody mentioned before) that is not powered during crank and the gauges ignition switched input is from this then during crank the gauge would actually be switched off and thus a much smaller capacitor could be used .e.g the 100uF I referenced should be enough.

Open up the gauge and add a diode and capacitor in the right part of the circuit for just the memory (very very difficult to do and could damage the gauge)

 

[FridgeFreezer]

For the grand total of £2.59

http://www.ebay.co.uk/itm/171926499750

 

[zardos]

Very useful find and if you want it quicker than delivery from Hong Kong, next day from Amazon https://www.amazon.co.uk/Converter-1-25-30V-Charging-Voltage-Regulator/dp/B00IR883G0/

at a slightly higher price

Edited: Again as original was only a boost

[zardos]

And I learn more about electronics by watching https://www.youtube.com/watch?v=vwJYIorz_Aw

 

[zardos]

I like this better https://www.amazon.co.uk/Adjustable-Converter-Regulator-Step-down-Regulating/dp/B00SKP2IJ8/
as it has a nice case/heatsink