Working on the 241 from hell again. The truck is converted to 12v. Wanting to still use the original gauges, I bought a voltage regulator/resistor. Is it normal to show 12v on both sides when just checking it with the battery? Thanks in advance. Dan
Do you mean a ballast resistor? Edit: or is what you have made to drop voltage to run just the gauges? When you said regulator I was originally thinking charging or ignition.
If it is a resistor in series with the load (dropping resistor or a "Runtz"), it will show the full 12 v when it is not connected to the load. Once connected, it should show around 6 volts across it.
Budget36. I have a ballast resistor in the coil. I just need to step voltage down from 12 to6v. The resistor I bought shows 12v on each pole just using the bettery to test it
36roadster. I don’t have the runtz but one I grabbed from the local parts store. They said it would run all 4 gauges. So it is normality 12v on both sides until it gets a load on it?
Yes.... using a resistor to drop voltage, requires the load to be about the same resistance as the resistor is. The load of your voltmeter is very low (high resistance), compared to the resistor itself. Kirchhoff's Law says you'll see battery voltage on both sides if the voltage drop resistor, with that little load on it. If you get an electronic voltage regulator (sometimes sold as Runtz), then it will show the drop voltage on your meter, with no load on it. also, after you get the gauges connected, check the voltage and see if it's near 6v, or not.
A resistor does not actively regulate the voltage, it just makes a voltage divider so the total input voltage is shared between the resistor and the load. Unfortunately it isn't automatically evenly shared, the voltage over the resistor goes up as current goes up, so if the current demand changes so will the voltage. Resistors can be an excellent choice to reduce voltage in some cases, as when the current demand is stable and well known. On the other hand it can be a horrible choice if the current demand is unknown or changes, or the voltage has to be exact. Please don't call simple resistors voltage regulators. They don't regulate anything actively, and selling them as such is a horrible way to fool people. Sometimes they're good enough, sometimes not, but calling them regulators makes people think they do something they're unable to do.
On my 55 Ford I used one of these, it is what Ford used on all their cars up until the early 70's to run 6 volt gauges on 12 volt in the trucks, Mustangs, Fairlanes, etc.. https://www.classicindustries.com/p...jNNzxj1Xp56aeYGRnZIaAiRdEALw_wcB&gclsrc=aw.ds
You can make a proper 6 volt regulator, using a fixed voltage regulator IC (LM7806), which will put out a fixed 6V output , regardless of the input voltage (up to about 40 volts in). As G-son said, a resistor only splits the voltage proportionately, and it will change as the supply voltage changes. (This won't be much because that's what your charging regulator does. )
Dodge used them too, I pulled one off a mid 70’s 1 ton truck and used it when I changed over to 12v on my ‘42 Ford PU
Don't dash gauges operate on 5 volts? I never thought of the connection to a 6 volt system, are they really meant to run on 6 volts?
That depends...the old Hudsons used a 5v regulator for the gauges, and they had a 6v (nominal) battery. Ford didn't use a regulator on the gauges until they switched to 12v systems in 56, then they added the 6v gauge regulator. Chevy used magnetic gauges that really don't care too much what the supply voltage is.
The 7806 is an excellent linear regulator, along with the rest of the 78- series (positive voltage) and 79-series (negative voltage). I think they've been around since the 70s. The upsides are that they're cheap, easy to use, and have built in overheat and overcurrent protection. Downsides are that they only can handle usually 1.5A (depending on version), and that they produce heat - the voltage that they "stop" becomes heat in the regulator. Antother downside is that the input voltage needs to be a couple of volts higher than the output voltage, but in this case it isn't a problem. If you use the maximum 1.5A at 6V, and have a 14V input from a running car you'll have 14-6= 8 volts across the regulator, and 8V at 1.5 A means it will develop 8*1.5= 12 watts of heat. In a standard TO220 capsule I think the thermal resistance to air 65°C/W, i.e. for every watt it'll heat up 65°C over ambient temperature. At one or two watts we are hitting the upper temperature range it can handle, at 12W it would heat up 680°C and probably glow red (assuming it hadn't died or shut down from the thermal protection long before that). So, we need to keep the current going through it low, and/or add a cooling fin to the regulator to dissipate the heat. I don't know how much current the gauges need, but you could for example spread it over multiple regulators to keep the heat output from each of them down. As I said, they're great regulators, you just need to know a bit about the working conditions to do the math so you know if one can do the job, and if you need to cool it.
Another way to deal with higher current loads is to use a buck voltage reducer. This is a switching power supply, that is available in different voltage/current ratings. It doesn't just dissipate the extra voltage as heat, instead it puts most of it to work. here is a generic example.... https://www.amazon.com/Converter-Voltage-Reducer-Regulator-Waterproof/dp/B08MZVXHP5
I've used those and they're excellent for the price. If you have an alternator, don't unhook your battery with the engine running. I did that and let all the magic smoke out of the little box as the charging system went unregulated briefly. Phil
Funny story about disconnecting the battery. A year and a half ago I drove an "old" pickup with alternator to a remote property down a fifteen mile rough dirt road. When I got there I thought I'd move the truck, but it wouldn't start, and no dash lights came on. I opened the hood and discovered both terminals had shaken off the battery. I reattached them and the truck was fine, no harm done.
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