I need to run a 6v doohickey (only about 1 amp) off my 12v heap. Found one of these in my pile. Has two terminals, one marked Bat. So I hooked a battery up to that, my VOM to the other terminal and the ground test lead back to the battery. Still reads full battery voltage. Opened it up, just a spring connecting the terms, like an ignition ballast resistor. So I grabbed an old ballast resistor, hooked it up the same way, still reads bat voltage. Am I doing something wrong? https://www.ebay.com/itm/New-Voltag...GcAAOxyqUpQ9CsP:sc:USPSFirstClass!94947!US!-1
When you use a resistor as a voltage drop the voltage drop is dependent on the current (ohms law) so you only have the correct drop when the current is flowing. Phil
...and your volt meter draws very little current. So you get very little voltage drop (so little, that you can't measure it) What is the resistance of the resistor you want to use? you can use maths to figure out what it needs to be, to drop 12v down to 6v at 1 amp current.
...when a specific current is flowing, not just any current. Resistors are useful as voltage reducers, but only if the load is constant, or if it is rather unimportant how much the voltage is reduced - as said, at no load there's no drop, at high load there may be almost no voltage coming through, depending on resistance and load current. Basically, resistors are simple, but often not good.
Thanks, I figured it might be something like that. So there are better options? You know those USB cigarette lighter adapters that give you 5v? What is the makeup of those things? Even if I were to match a resistor, it's still a waste of energy isn't it, unless I want more heat in the car?
Squirrel, That's not exactly correct, very little current flow because a voltmeter is a very high resistance so it takes almost all the voltage to push through the small amount of current through the volt meter.
yes, but it's not a lot of waste of energy. A 6 watt heater won't keep you very warm. There are modern electronic gizmos that will reduce the voltage more efficiently, but they have other costs. Knowing more about what it is you're trying to do, could help us help you figure out a better way to do it.
Ok but it's not traditional. It's a wifi thingy. The A/C adapter it comes with is 5v 3.5 amp. I have been running on it off the car using a car USB charger for power but it keeps cutting out, I don't think it has a current capacity. I tried running it off a 6v battery yesterday and it does fine. Could I open up the AC wall adapter, bypass the AC-DC part and hook up 12v to it somehow?
Do yourself a favor and just go buy the proper 12V adaptor. To use a resistor, you'd have to know the impedance of the device which would need lab equipment to find out. Yes, a USB port doesn't put out enough current, it's probably tripping the internal circuit breaker.
Afaik no specific 12v adapter is made for this device. It is normally used in a house, I am using it at a remote off grid location. I suppose I could use an 110 AC inverter with the AC adapter but it seems a waster converting to AC and back.
Good price, good efficiency (unlike resistors and more basic regulators), and you don't want to turn electricity to waste heat unless you have to - at least not when the supply is limited, as it may be in an off grid application.
Take a look here: https://www.jalopyjournal.com/forum/threads/how-to-build-a-voltage-regulator-for-3.448038/ I have used the guts from car phone charger. Just make sure it reduces the voltage to 6V, it is printed on the side as "output voltage 6 volts". It worked fine for a gas gauge on a 6 volt to 12 volt conversion.
The 78**-family of regulators can usually only handle 1-1,5A depending on version (some even less), so not quite suitable for something that may need 3.5A. Also, as it just acts as a voltage divider efficiency will be low, less than 50% for getting 5V out off 12V (the regulators for that are called 7805), unlike the 90-something percent of the switching buck regulator Squirrel linked. 78**-regulators are simple and great for items using low current, but for higher current there are better options today.
Try it with a second load connected in series. The first load (the ballast resistor) has full voltage, but the second load (try a light bulb) will only have half.
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