DC from one supply works, but not the other?

[MillenniumFalsehood]

So I've been using an AC-DC converter to test the circuitry in my USS Enterprise-D built for a customer. I have used this unit before to test other circuits, and so far it's been just fine as a substitute for a battery power supply. But for some reason, I can't get the batteries in the base of the model to power the ship... The model works fine on the wall supply, but whenever I try to use the installed batteries, they won't power the ship. I have two six-pack C-size battery holders in the base, which produce 18 VDC. I used a voltage divider to get it down to 12.3 VDC (verified with a DMM), which should be fine because that's roughly the same amount that the wall wart provides. I have a resistor and a voltage regulator inside the ship which provides a steady 12VDC supply for the entire model, but aside from the 555 timer strobe circuitry and current limiting resistors for the single LED units, there isn't anything else inside the ship. This problem is really stumping me... you'd think that one DC source is as good as another, wouldn't you?

[EVApodman]

Check your voltage at the regulator and chip in the model with a DMM to confirm you have the right amount using both the battery and transformer. If it isn't then backtrack your wiring to the batteries to make sure it is correct. If that is correct then try another set of batteries as these may be weak and won't work under a load.

[MillenniumFalsehood]

The problem with that is that the circuitry is inaccessible right now. But these batteries should be powering it. They're brand new and producing a strong 18+ volts which is stepped down to 12.3VDC by the voltage divider. I tested them with my DMM like I said, and they produce the correct voltage. They are C batteries, which means there should also be enough amps to go around, though I should probably check how many exactly they're putting out. The wiring also isn't the problem. I learned my lesson about not applying a backwards connection to the voltage regulator, so everything is properly polarized and marked to prevent a mistake in how it's wired. A couple of the LEDs DO light up when power is applied by the batteries, which adds to the mystery, because if they're on the rest of it should also be on. They light up when the transformer is powering it, too.

[EVApodman]

Since the interior circuitry is inaccessible there's not much else you can do to troubleshoot this problem. One other thing you can do with things as they are now is check the current from the batteries and transformer to the circuits by putting the DMM in current mode and seeing how much it is.

Also is the voltage regulator putting out the same voltage for both supplies?

Another thing I would check is the AC ripple,on the input to the regulator and it's output, to see if there are any significant differences between the two supplies.

One thing I just thought of, which LEDs are effected? You said there was a 555 timer chip that I assume is driving some flashing LEDs? Do these come on only with one power supply while the straight resistor LEDs are ok? This may help isolate the problem.

[naoto]

Stupid question -- were you measuring the voltages with the load in place?
https://electronics.stackexchange.com/q ... resistance
(reason for my question: Output from voltage divider will be different between "no load" and loaded condition)

[MillenniumFalsehood]

Actually... no. I hadn't thought of that.

Since the interior circuitry is inaccessible there's not much else you can do to troubleshoot this problem. One other thing you can do with things as they are now is check the current from the batteries and transformer to the circuits by putting the DMM in current mode and seeing how much it is.

I'll see if I can find some time to do that this evening.

Also is the voltage regulator putting out the same voltage for both supplies?

The voltage regulator is on an internal circuit board, so I don't know. I really should have tested it with batteries, since that is what's in the final model, instead of a wall power supply...

Another thing I would check is the AC ripple,on the input to the regulator and it's output, to see if there are any significant differences between the two supplies.

I hadn't heard of AC ripple having this particular effect... Sounds kind of weird. Wouldn't the ripple be a problem for a DC circuit with solid state components?

One thing I just thought of, which LEDs are effected?

All of them, save for the position marker lights and impulse engines. Those tap directly off the +Vcc leg of the voltage regulator.

You said there was a 555 timer chip that I assume is driving some flashing LEDs? Do these come on only with one power supply while the straight resistor LEDs are ok? This may help isolate the problem.

Yeah, that definitely happens. They do not come on when I have the model hooked up to batteries, but when it's on the AC power supply they work fine.

[EVApodman]

Based on what you said about the lights driven by the 555 chip it appears that something has affected it's oscillation, either causing it not to oscillate or to do it at the wrong rate. This could be caused by the source voltage.

All this comes back to the same thing, you will have to access the internal circuitry to find out the problem. Maybe you have a short or miswiring that is causing an excessive amount of current drain that the batteries can't handle.

[naoto]

to go along with the stupid question I'd asked...
http://hyperphysics.phy-astr.gsu.edu/hb ... oldiv.html
https://learn.sparkfun.com/tutorials/voltage-dividers

[naoto]

https://youtu.be/wVpL3csYZ5E

https://youtu.be/-kEh0TYjYYE