adding a diode (or two) to allow trim gauge to work without ignition

[chopsbutcher]

Thanks BruceB58, that would be nice to have! Will put it on my wish list.
I'll let ya'll know if I get around to hooking in these diodes and if they perform as expected.
Peace!

 

[sam am I]

If we can ignore the negativity and get back on topic I am curious, Sam I Am, as to why the suggestion for the relay? I think I could add in another diode to the circuit to prevent back-powering other devices. We were having so much fun with diodes, why introduce a mechanical point-of-failure?

I considered that approach, and it's a workable approach but, the non-linearity of Vf/i along with how temp effects those curves of the diodes mainly, it could have an negative effect but, how much? Not sure, you can try it and if things stay normalized over temp and current, It was definitely a choice. The relay in this config would not be effected by the same issues by design........The relay in that arrangement will most likely outlive us both, but sure, it could break someday.

 

[chopsbutcher]

Well that's a little over my head. Are you referring to the forward-current handling of the diodes or something?

 

[bruceb58]

The diodes are fine. The gauge works over a wide range of voltage since it regulates the voltage down in the sense circuit. If it didn't, it wouldn't work over the voltage ranges of the alternators charge voltage which is way more than a didode drop.

Look at your trim gauge with the engine off and then running. It doesn't move even though the voltage went from 12.6V all the way up to 14V.

The forward voltage of a diode varies slightly depending on how much current you are sending through it. Its on the order of a couple tenths of a volt.

 

[sam am I]

Well that's a little over my head. Are you referring to the forward-current handling of the diodes or something?

Sorta, the amount of voltage they drop can change with current and temp and not necessarily in a liner fashion. Give it a shot, you may also need to design around the PIV (google it) of that extra diode as that could be an issues also, the flyback voltage (another google) from those relay coils might also have negative effects (destroy) on the gauge/s internal components/sensitive electronics but, if it flakes out too much, you can always use just the relay, as stated, the relay is immune and prevents this...............

Best practice IMO? With adding a main switch (you really should have one anyway) thou which is probably best course here anyway in your situation as wired, you could avoid any potential issues/skip all the design criteria. That's how I'm wired and I don't have or ever had any issues.

 

[bruceb58]

You don't need to worry about peak inverse(reverse) voltage with what you want to do. Depending on the diodes you choose, the minimum it would be is 50V. If you were getting that big of a spike in a boat's electrical system, you would have much bigger worries.

 

[sam am I]

Just be advised the flyback voltage from the relay coils is a know component destroyer, they easily and solely can create 100's of volts and must be accounted/design for in any design that has perhaps unknown circuit components not able to withstand those potentials .........I personalty won't/didn't risk that. It is a very common occurrence and you don't want to ignore!

 

[bruceb58]

That is only a problem when you are switching a relay with an integrated circuit. If the diode couldn't handle it, they wouldn't be used as back EMF diodes. Been doing this stuff for over 35 years. I am an EE. Kinda mute here since its totally unnecessary to use a relay.

 

[sam am I]

Also anything attached to the coils including the diode/s themselves will also see potentially 100's of volt's, the diodes will conduct, if not short due to this as well as,.........as stated above, unknowns such as internal sensitive less voltage components in the gauges could fry etc. The relay definitely prevents this......

 

[bruceb58]

OP...if you really want to see how much the trim gauge draws...pull off its wire and connect a meter measuring current between its power input and 12V. If it's 100mA or over, don't leave it powered all the time.

 

[bruceb58]

Also anything attached to the coils including the diode/s themselves will also see potentially 100's of volt's, the diodes will conduct, if not short due to this as well as,.........as stated above, unknowns such as internal sensitive less voltage components in the gauges could fry etc. The relay definitely prevents this......

That is why you use a back EMF diode on a relay. In our chassis that we design at my company, all relays that are controlled by our controllers have back EMF diodes installed or built into the relay itself. Only need though if an integrated circuit is driving the relay.

Of course, kinda pointless to use a relay here in this application.

 

[sam am I]

Most boat's analog meter's (including mine and yours most likely) using a standard meter movement that draw 1ma for full scale deflection. Just an FYI if you choose to power it all the time.........Which you said clearly stated above you didn't want.

 

[bruceb58]

Now I want to measure mine!

 

[sam am I]

Another FYI..............The voltage spike (Meg scale) due to flyback voltages present at the trim gauge's power terminal, the relay's (up or down) being driven by an IC or open drain/collector device or switch.



The voltage spike (NOT) present using a relay and at the trim gauge's power terminal, the relay's (up or down) being driven by an IC or open drain/collector device or switch.

 

[chopsbutcher]

I'm pretty eager to try the two (actually three) diode trick. Have no interest in adding a relay with potential failure/voltage spikes, etc.

Maybe I'll just going to grab a bowl of popcorn and let you two figure this out

 

[sam am I]

I'm pretty eager to try the two (actually three) diode trick. Have no interest in adding a relay with potential failure/voltage spikes, etc.

Maybe I'll just going to grab a bowl of popcorn and let you two figure this out

Hehehe.......... Correction tho,.......The third diode app is where you'll get the spike and it can do serious damage, even to your ECU, if ya have one(see plot 1 above), I certainly would advise not too!! That is unless the design gets much more complicated in components, cost, etc, etc not to mention more design intensive than intended. The simple relay in post 14 (probably 100,000+ cycles MTBF) app w two standard RS diodes (see above plot 2) is clean as a whistle and won't potentially fry anything or cause the spikes as measured/seen with the third diode idea........GL

 

[bruceb58]

I'm pretty eager to try the two (actually three) diode trick. Have no interest in adding a relay with potential failure/voltage spikes,

Don't worry...you won't be getting any spike to harm anything! I wouldn't install a relay either..just silly. If the back EMF from something really inductive like your horn does nothing, what you are doing won't either.

 

[sam am I]

And probably your horn is on a circuit by itself and most likely has internal protection (if it's even inductive) so the design is intrinsically safe by design, plus, it's not tied directly onto/into gauge..silly non-sense! As stated, a main switch and not risk or guess anything...GL

 

[bruceb58]

And probably your horn is on a circuit by itself and most likely has internal protection (if it's even inductive) so the design is intrinsically safe, plus not tied directly onto/into gauges, BTW.........

LOL...a horn with internal protection!!! LOL Yes it's inductive..anything with a magnetic coil is going to be an inductive load. That along with windshield wipers, bilge pumps, blower motors, trolling motors...Horn is typically hooked up to the same fuse panel bus that the ignition is on that drives the gauges.

 

[sam am I]

LOL...a horn with internal protection!!! LOL Yes it's inductive..anything with a magnetic coil is going to be an inductive load. That along with windshield wipers, bilge pumps, blower motors, trolling motors...

Certainly they can and sure they are ........you'll need to have the last word here, i know it's important and meaningful to you, ball in your court........