| Author |
Message |
Nick
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 13, 2005 - 2:19 am: | |
A friend who once worked as head mechanic for a city bus company told me I should be able to use a 12 volt tachometer on my 24 volt MCI 8 by putting a resister in the line to drop the load down to 12 volts, just as we did for our tow vehicle lights. Any ideas how this would work and where I would mount the tach sending unit or pickup to? |
Pete/RTS Daytona (Pete_rtsdaytona)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 13, 2005 - 9:04 am: | |
Nick
for Tack input:
If you have a gear driven Detroit Diesel 50DN Delco 270 Amp 24 volt alternators - (I'm pretty sure you do)
Use the "R" terminal hookup on the alternator -
If provides a "12 volt" pulse signal @ 17.58 pulses / ENGINE revolution (not alternator revs - ENGINE revs)
Here's why: -> (2.93 gears x 3 pole stator x 2 (for full wave rectified) = 17.58 pulses/rev)
If you don't have 12 volts up by the dash to power the tack - then you need to determine the input resistance of the tach - and add that valve resistor in series to drop the 24 volts to 12 volts
use a volt/ohm meter (VOM) to measure the resistance -or- hook up the tack to your 12 volt car and use an amp meter to determine the current draw and then divide the 12 (actual more like 13.2) by the measured current to get the resistor size
Resistance = voltage divide by current
you can possible get away with just hooking up your 12 volt tach to a small battery charger and read the current gauge and do the math
confused ?? -- sorry
call if you have questions (386**672**0571) |
Kevin Black (Kblackav8or)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 13, 2005 - 9:56 am: | |
There are also magnetic trigger models out there you can get. Uses a small magnet that you epoxy to a pulley or some other crank speed shaft. Some are bolt on. There are other generic alternatives if the above post doesn't apply. |
Gary Stadler (Boogiethecat)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 13, 2005 - 5:09 pm: | |
While you "can" stick a resistor in series with the tach to drop the 24 volts down to 12, it's not a good method to use, design-wise.
This is because in doing this, you're assuming that the tach draws exactly a constant and never changing amount of current, which does not usually happen in real-life. In this case, if the tach drew less current for whatever reason, the voltage across it would rise...conversely if the tach drew more current, the voltage available to it would drop. This can happen as the needle motor is commanded to move, etc. and it coud potentially damage the tach.
The better way to do it would be to make a little 24 to 12 volt solid state dropping regulator, which will insure your tach gets 12 volts no matter what load it is presenting to it's power source.
A simple way to do it is to use a $1 solid state regulator, generically known as a 7812, which will take in up to 35 volts and convert it to a tightly regulated 12 volts. It's a three lead device, input, ground, output, and you'd need to get two capacitors, .1uf each, and hook one from the input to ground, and the other one from output to ground. These capacitors are necessary to keep the regulator stable under all conditions.
Both the regulator and the capacitors are affectionately known as "Garden variety" meaning that they can be bought at Radio-shack, Mouser, Digikey, etc...virtually everywhere.
Nick, if you want to go this way and it totally perplexes you, email me privately and i'd be happy to just send you one all soldered together with a connection diagram. For anyone who knows electronics, it's a 5 minute job, and I'd be happy to help
Cheers |
Pete/RTS Daytona (Pete_rtsdaytona)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 13, 2005 - 7:49 pm: | |
Gary
Your absolutely right - I must have been sleeping at the wheel - that's what I use on my bus
Radio Shack 7812 -- Part# 276-1771
.1uf capaccitors 50 volt metal film --> 272-1069
heat sink (for good measure) --> 276-1363
sorry
Pete RTS/Daytona |
John MC9
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 13, 2005 - 8:04 pm: | |
Gary (or Pete) -
Is it possible to post either the schematics or a picture of the
wired project? A 24 to 12 converter is in fairly high demand
around here! How much power can these handle? Is it possible
to make one designed for around 40 amp or so? |
Gary Stadler (Boogiethecat)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 13, 2005 - 10:35 pm: | |
Here you are. I made it representational rather than schematic.
http://www.heartmagic.com/12voltconverter.jpg
This is good for an amp max, if you give it a heatsink. For loads under 1/4 amp or so it might not need a heatsink. The mounting tab is internally the same electrically as ground, making heatsinking easy because you don't have to isolate it.
For big converters like 40 amps (or anything over an amp for that matter) a DC to DC converter is a better bet. These use switching technology and are much more power efficient, meaning that they don't get hot.
Some can be had at Digikey and Mouser, some can be had at NewEngland solar. The choice depends on application and actual load and there's way more to choose from than I'd write in this post....
BUT just for example, here's one from Digikey, $156, 24 volt in 12 volt out 240 watts, good for about 14 amps output, part# AK80A
click here for a data sheet
You'll be surprised how small these are...
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John MC9
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, November 14, 2005 - 8:16 am: | |
That looks easy enough to wire up, and probably easier to make
than anyone could have imagined.
Thanks! |
Donald Lee Schwanke (Dontx)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, February 12, 2006 - 5:38 pm: | |
Might take a look at your tach. As I recall I bought an Isspro new, and the only difference between 12 and 24 was the light bulb that illuminated it. Might check on their web site. |
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