The Stovebolt Site Forums The Shop Area The Electrical Bay 12 volt to 6 volt adapter direction change

The 7 Adapter won't be made. The lack of interest has confirmed what I suspected, but you never know until you ask. Instead I'll be working on a generic adapter. Actually the mockup is finished and it will now be tested thoroughly. What's different? It isn't plug and play. My hope was to offer a fuel gauge adapter unique to the AD trucks along with a generic adapter for use on other trucks, cars, etc.

But now the project will move along focused only on a generic adapter. In making the mockup I learned of at least 5 or 6 things I'll change. That's common...at least it always happens with me. Some changes will hopefully streamline and simplify assembly. One change will make it a bit more durable. Others may make it easier to install. A few design questions still remain. In case you're curious, 3 images are attached. In this form, it will install by: disconnecting the battery, disconnecting the right hand wire on the fuel gauge (looking from the back side), placing the adapter on the right hand post of the fuel gauge, tightening the screw to hold it in place, attaching the wire you removed in the first step to the post above it and installing a nut and then finding a ground source and attaching the pronged connector.

Again if you're curious or if you have questions or suggestions, please let me know.

Not to hijack the thread but what's the proper way to test a Runtz? After removing the nonworking Runtz from my 53 I tested for continuity between where it fastens to the gauge and the terminal where the vehicle wiring would then connect. That showed no continuity. Is that the correct way to test it?

No problem at all, Tiny. Happy to help if I can. I think I'd do this: find a known good ground connection on your vehicle and connect the ground wire from the runtz device to that. Using an alligator clip test wire or similar, I'd connect 12v positive (from your battery or anywhere you might find 12v) to the threaded post on the runtz and then check voltage from the part of the runtz with no post (the part designed to connect to your fuel gauge) back to ground. As I recall the connection spot is on the bottom side of those. From that connection piece to ground, your voltage tester should read 6v, but since the runtz device used a fixed 8 volt chip, it could read higher than that. Please let us know what you find. If it reads 0 or any voltage around 3v, it is a goner.

One note on the 7808 chips that the Runtz uses: it needs to have a load before it can properly regulate the voltage, so don't be surprised if the un-loaded voltage is higher (like 10-12V).

If you have a small-ish (500-5000ohm) resistor, put that between the output terminal and ground, then test at the output terminal and the voltage should be spot on at 8V.

If you see the 0V or 3V on the output without the resistor (like Jon said), that does mean it is le-dead (fancy french technical term).

Use an analog voltmeter. The small amount of current used, while measuring voltage, is enough to make the chip's output sense circuit operate.

Is there some reason not to use an old school potentiometer, and dial in whatever resistance is needed to handle the load of the gauge? Simply connect the ends of the resistor to power and ground, and dial up the movable contact to get whatever voltage makes the gauge work most accurately. Lots of potentiometers have a lock nut to anchor the shaft down once the proper voltage is achieved on the wiper contact.
Jerry

If I am reading this correctly (which I am very unsure of...), you are suggesting using a potentiometer like an old rheostat to drop the voltage to an "ideal" value for the load that the gauge puts on it. The difficulty with that is the load from the gauge varies by about 50% depending on how much fuel you have. So putting any kind of a static voltage divider in the circuit is problematic.

If I am not reading your suggestion properly, please correct me.

I've been waiting for Tiny's response, but as for the 7808 chip, you might want to check yours with a known good power source/known good chip. Here are 2 images of the 7805 chip (a fixed 5 volt brother to the 7808---same exact chip family, just a different voltage output rating). Running it on 12v with no resistance at all (aside from the wires connecting to the breadboard) it runs happily at 4.96v using the cheapest handheld lcd meter I've ever seen (got it free at Harbor Fright Tools). Hope this helps. Also, please keep in mind the runtz gizmo does have 2 capacitors and 1 diode. I'm sure whoever thought up that circuit had a plan in mind, but I never would have used just one diode with 2 capacitors. Seems fundamentally flawed to me.

Ha, ha...just noticed in my haste I hooked up the polarity backwards, but interestingly the little Harbor Fright meter picked up on that.

There's a 7808 chip, in case anyone is curious...showing 8.01v output. I thought I had seen one of those in my misc box. Just took me a while to find it. Also checked resistance through the HF handheld meter. Didn't show any measurable resistance with the lowest possible setting on the APPA meter.

Jon, I assume you mean testing the Runtz? The fat kid's been pretty busy planning and buying materials for a long delayed master bath remodel. I'm a LOT poorer than I was a week ago and I'm not done casting money to the wind yet. I'll try to get to it in the next day or two.

Ah, well I understand casting money to the wind and bathroom remodels, Tiny. Hope all goes well with that. Easy way to separate men from money at today's lumber prices. No sweat on testing the runtz thing. No rush at all.

I finally got the time to test the Runtz by attaching it's ground and 12v terminal to the battery via jumper wires. I then touched the positive lead of my meter to the output of the Runtz and touched the ground lead to chassis ground. The reading at the output connection was 7.9 volts. According to that it tests good, albeit a bit higher than expected, so I have no idea why it didn't work on the car. The guy who installed it snapped a picture of the install and it was connected correctly according to his photo. The only connection I couldn't see in his photo was the ground so it's possible the ground wasn't good. Anyway the replacement reducer is working fine so I guess the Runtz will go on the shelf instead of in the trash.

My 54-3100 has two deep charge 6 volt batteries and the starter spins fast yet the 6v generator keeps everything nice and charged. I also have a by pass for twelve volt for my a/c and radio system. On six volt systems it is always the ground connection. This vital connection must be checked every 6 months or so here in damp Florida and I use the double ground system (one braded one heavy duty cable), never have problems. Only modification was extending the under floor board battery shelf, batteries slide out through battery floor plate. Doc

Thanks very much for letting us know, Tiny. 8v is what a few of us have expected. That's 33.33% higher than 6 and it does keep alive the question of "why did somebody choose this chip when the correct one was freely available?" Like the design of the circuit, not certain we can unscramble that egg.

Everything has been ordered for the ones I'll be making, but it will be a week or 10 days before it all arrives. More to follow. Thanks again.

Normal "engine running" 6 volt charging system voltage is 7.5 or a bit more. The only time an actual 6 volts will be seen is when the engine is stopped. Whomever designed the Runtz system knew what he/she was doing.

Normal "12 volt" system operating voltage is 14.7. People get so wrapped up in terminilogy that they conveniently ignore what's actually happening.
Jerry

Sorry Jerry, I'm 180 degrees out with you. In my mind I can't get past, "when Runtz units fail, why"? Surely the guys did their homework before coming up with the circuit to reproduce in quantities. I have one failed Runtz unit and the chip itself is bad (output very low voltage). All the other components tested good.

The other thing is there are general designs available and we ask everyone's opinion but shouldn't there be more investigation first?

What voltage do the downstream components need? How accurate is the fuel gauge with a 6 volt, 7 volt or 8 volt source...maybe the voltage has less impact than we mentally perceived? Maybe the actual need is stable current rather than stable voltage? Would a resistor, choke, zener diode or combinations give a better bang for the buck, or be just as accurate and possibly have a longer lifespan than a chip circuit? Is there a need to protect against inductive kickback from the fuel gauge coils? Are there conditions where the voltage system (6 volt or 12 volt) of the truck causes lethal voltage spike to the design?

My brother does independent computer modeling projects for most of the major automotive manufacturers, designing electronic components on a computer, and then running them to failure with a sophisticated simulation program without ever actually building the part. 10 years ago, he was charging $1K a day for his services. The manufacturers' primary concern was cost cutting- - - -they didn't care if a part worked well, as long as it could meet the warranty period without an unacceptable failure rate. Chances are, there's a bean counter somewhere in the woodwork who is dictating what kind of components that product can use.
Jerry

Bean counters?? In corporate America?? Whoda thought.........

I didn't say an 8 volt converter wouldn't work. What I said is it was the wrong chip to choose, and I stick to that. Why? Easy. If you're going to sell an adapter described as a 12 to 6 volt device, at least be honest and use the 6v chip. Otherwise call it a 12v to 8v device and don't fib about what it is because you think you'll be able to sell more of them to people looking for a 6v something or other. And also, take a look at how the runtz gizmos are being marketed today. Whoever is selling them has recently decided to claim they can handle 4 amps. Is that the truth? If they are still using the 7808 chip, it can't take 4 amps for even a nanosecond before shutting down and/or internally frying. Even if they've changed the chip, ask anyone...4 amps is not a piddling slug of current for a chip like that. It will require a heat sink which the runtz thing doesn't have.

Why are they failing? Not sure, but I have an idea. Take a look at the circuit. It has the 7808 chip, one capacitor on the input side and one on the output side with a single diode forward biased back from the output side to the input side. To me it looks like you sort of have protection for short circuits, but what about protection from capacitor discharge? The makers of most voltage regulator chips suggest various "typical circuits" in their data. I've yet to see a diode protected circuit with only 1 diode. To me that's like a doctor saying "Mr Hodpecker, we set your broken leg with a nice plaster cast and normally we would give you 2 crutches to get around with, but today we only had a termite-eaten piece of 1 x 2 and nurse Scooterpie coated one end of it generously with axle grease for fun. Good luck...you're on your own."

What I also said was it was likely the voltage mattered less than may be expected for the fuel gauge (and I explained how that gauge would work and why I believe current will be more important than voltage). But will it work on 6v? Without question it will. Please remember the 6v battery is made of 3 cells. Each of those when brand spanking new outputs 2.1~2.2 volts and whatever amperage the battery was designed to provide. As it ages, you can say adios to the 2.1v and the "new battery" amperage. That's what happens when any battery gets old, and it matters not whether you have an 8v battery (4 cells) or a 12v battery (6 cells)---and if you're curious why a 12v battery when you get it home registers apx 13.2 volts, multiply 6 times 2.2. They all go south when they start showing age. Just like the alkaline cell in your digital clock, remote control, Maglite, etc.

Yes, more investigation is always good. So is more knowledge. If you recall I had about 3 things I wanted to achieve in a "new design" voltage regulator and all of those have been accomplished. It has unavoidably taken longer and cost more, but it is going be able to handle 1.8 amps continuously and it will even be adjustable (something else new) so that you can make it run at any voltage you want (below 12v, naturally). Hopefully in a couple of weeks I'll have something to show you.

Jon, all I'm suggesting is that people who experiment with this stuff should factor in some real-world numbers to their calculations. "12 volt" electrical systems run at 14.7 volts in the real world of driving down the road. "6 volt" systems operate at 7.5 anytime the charging system is working against a fully charged battery. Shouldn't a researcher take the actual figures into account instead of chasing some pie in the sky ideal condidions? "Constants aren't- - - -variables won't!"
Jerry

As an update to this, the work is still ongoing and there have been five different versions so far. I believe there will be one and possibly two more. I'm still awaiting some parts and the final version depends on those. I've been testing the best version of the five for a while and I like it. It is working fine powering the gas gauge. At first I thought it was making the gauge needle jitter a bit, but it turned out that was me. I had caused the truck to shake pretty well when getting my head under the dash and I guess the gas was still sloshing around when I checked it.

In case anyone is curious, the rough prototypes look like the images below (front/back) and there is also an image showing the thumb screw plus the small screw which is used to adjust the output voltage.

More to follow in time...