Voltage Divider Help: Solving 12V Power Supply Issues for Telescope

[russ_watters]

I blew another 12V (13.5V actually) power supply running my telescope the other night. Primary power comes from a 17 a-h lead-acid battery in a power tank. I run the telescope and a dew heater from it and over the course of a night (10-12 hours), will drain the power tank completely. I keep the charger plugged-in to try to make up some of the difference, but a couple of nights ago, I guess I drained it so far that the charger overloaded and burned-out. It is a 750ma charger. I actually have a couple of issues:

First one way or another, I need a new charger for my power tank (assuming I didn't destroy the battery). From what I understand, "12v" adapters are rarely ever 12V and probably can't be if they are meant for equipment compatible with car batteries, which are actually more like 13.5V. The assumption I'm making here is that virtually any "12v" adapter will be able to charge my power tank. Is this true?

Second, since I don't feel like spending $75 for an adapter that will power everything, I'm thinking of either using a spare computer power supply I have laying around. I think they would work for giving true 12V power to my stuff, but probably wouldn't work as a charger. Correct?

I have a couple of laptop a/c power supplies lying around that I could use. They are 19 or 20V. I understand the concept of a voltage divider, but don't know how to design one or even if it is really a feasible option: does it sound like a good idea and how do I pick the resistors? The voltage divider concept tells you the relative size of the resistors, but not their actual size. Do they get sized based on expected amp draw? Do I need to calculate the resistance of my loads (and what happens when that changes?)? Should I be doing an external voltage divider or is it possible to swap-out a resistor inside the power supply to change the voltage?

Thanks, guys.

 

[NoTime]

There are two types of walwarts nowadays.
The old nominal 12v type is a transformer with a diode bridge.
Thus peak voltage is around 16 volts.
Enough to charge a lead acid battery.
You can probably get away without paying attention to current limiting as long as you just have the battery on the unit.

The other type is a mini switch power supply. They produce the voltage stated.

Using the computer power supply should work for the scope.
The battery will put out about 12v with a load and no charger on it.
So the voltage is the same. You may need to bridge the 5v output of the computer power supply with a resistor so that it will start. These units require some load to run and I'm thinking that the scope draws current intermittently.

For charging the battery you don't want a divider. You need a current limiter (ok) or voltage regulator (better). A resistor will work as a current limiter, but a real charging circuit is better and is only a few components. If you take the +12 and -5 line from your computer supply you can make a charger out of it.

 

[ravenprp]

Hello,

I'm sorry to hear about your power supply issues with your telescope. It sounds like it has been quite frustrating for you.

In regards to your first issue, you are correct that "12V" adapters are rarely ever exactly 12V and can vary in voltage depending on the equipment they are meant for. It's important to check the specifications of your power tank to ensure that it can handle a slightly higher voltage without damaging the battery. Additionally, it's important to make sure the charger you choose is compatible with your specific power tank model. It's always best to err on the side of caution and choose a charger that is specifically made for your power tank.

For your second issue, using a spare computer power supply or a laptop AC power supply may work as a temporary solution, but I would not recommend using them as a long-term solution. These power supplies are not designed to charge batteries and may not provide a stable and consistent voltage, which can damage your equipment.

As for using a voltage divider, it is possible to use one to lower the voltage from a 19 or 20V power supply to 12V. However, it's important to note that a voltage divider will also decrease the available current, which may not be enough to power your telescope and dew heater. Additionally, as you mentioned, the actual size of the resistors will depend on the expected amp draw of your equipment. It's important to calculate the resistance of your loads and choose resistors that can handle the expected current without overheating.

In summary, it's important to choose a charger that is compatible with your power tank and to avoid using power supplies that are not specifically designed for charging batteries. If you do decide to use a voltage divider, make sure to carefully calculate the resistance and choose appropriate resistors to avoid damaging your equipment. I hope this helps and good luck with finding a solution for your power supply issues.