Strange voltage from UBEC Buck Converter

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adafruit_support_mike
 
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Re: Strange voltage from UBEC Buck Converter

Post by adafruit_support_mike »

There's no circuit in the last photo.

The word 'circuit' comes from the same root as 'circle', because current can only flow in a loop. For any segment of the loop, current comes in from one end and goes out the other. The wire between the meter's probes isn't part of any loop.. it's just a one-way connection from the black clip to the red one, and 0v is the expected reading for that kind of connection.

DO NOT try to complete the loop by connecting a wire from the black meter clip to the negative terminal block.. that will short your battery and probably set something on fire. You'd have a loop, but one side of it would be the battery and the other side would be three pieces of wire.. a whole lot of current would flow, and it would do bad things along the way.

Your first two photos do show correctly connected circuits. In the first one, the meter (which has extremely high resistance) is closing the loop across the battery. Very little current will flow through that loop because of the meter's high resistance, but there will be enough for the meter to measure the battery voltage.

In the second photo, both the UBEC and the meter close loops that include the battery. The meter's resitance is much higher than the UBEC's, so most of the current that flows through the battery will also go through the UBEC. The meter pulls just enough current to know that the voltage across the UBEC is about 3.5v

That voltage is a lot lower than I'd expect from an UBEC, which makes me wonder how much current is actually flowing through the UBEC.

Your meter can measure that, but you'll have to move things around a bit:

1) Unclip the meter probes and the UBEC from everything.

2) Move the red probe from the hole on the right side of the black probe to the hole on the left.

3) Move the meter selector knob to DC Amps, all the way clockwise on the dial.

4) Connect the UBEC's black input wire directly to the negative terminal block.

5) Clip the black probe to the UBEC's red input wire.

6) Clip the red probe to the wire from the positive terminal block.

For this kind of connection, you want the meter to act like a piece of wire between the UBEC's red input wire and the positive terminal block. That puts the meter and the UBEC in the same loop, and any current that flows through the UBEC will also have to flow through the meter. That allows the meter to measure the current flowing around the loop.

For this kind of measurement, the meter has extremely low resistance (probably 0.01 ohm). That's exactly opposite of how the meter behaves when it measures voltage.

DO NOT connect the meter directly between the positive and negative terminal blocks when the probes are connected this way. It would be the same as shorting the terminal blocks together with wires, and would be equally bad for everything involved.

Billy10mm
 
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Re: Strange voltage from UBEC Buck Converter

Post by Billy10mm »

In this arrangement, I'm getting the same 8-12 volts I was getting when no UBEC was involved:
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Billy10mm
 
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Re: Strange voltage from UBEC Buck Converter

Post by Billy10mm »

But apparently 0 amps (which makes no sense to me, the light that this thing powers aft of the negative terminal block is lit):
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adafruit_support_bill
 
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Re: Strange voltage from UBEC Buck Converter

Post by adafruit_support_bill »

In this arrangement, I'm getting the same 8-12 volts I was getting when no UBEC was involved:
...
But apparently 0 amps (which makes no sense to me, the light that this thing powers aft of the negative terminal block is lit):
It looks like you are still trying to insert the meter into the circuit. As Mike and Zener have said before. This is not the correct way to measure voltage with a multimeter.
In the first one, the meter (which has extremely high resistance) is closing the loop across the battery. Very little current will flow through that loop because of the meter's high resistance, but there will be enough for the meter to measure the battery voltage.
These guides show some basic multimeter measurement techniques
https://learn.adafruit.com/multimeters/current?view=all
https://learn.adafruit.com/collins-lab- ... t?view=all

Billy10mm
 
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Re: Strange voltage from UBEC Buck Converter

Post by Billy10mm »

I hooked it up exactly the way Mike asked me to in the first post on page two. I've read it twice now and can't see what I'm doing incorrectly. And I wasn't trying to measure voltage, Mike asked me to measure amperage. I threw the voltage reading in in case someone wanted that. I'm very thankful for the help.

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zener
 
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Re: Strange voltage from UBEC Buck Converter

Post by zener »

There are two possibilities that would give you the zero amperage reading:

1) There is no current flowing

2) The fuse inside the meter is blown (and as a result, no current is flowing)

The best way to verify the fuse is pull it out of the meter and measure the resistance of it. If it is blown it will read a very high resistance or "OL" If it is good it will read below 1 ohm, like .1 ohms or something. (The fuse is not necessary for the meter to read resistance, it is only used when measuring current)

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Keebie81
 
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Re: Strange voltage from UBEC Buck Converter

Post by Keebie81 »

Looking over the pictures I notice you dont have a fuse on the power line going from battery to the ubec. Thats something you should add for safety

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adafruit_support_mike
 
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Re: Strange voltage from UBEC Buck Converter

Post by adafruit_support_mike »

If you don't have any load connected to the UBEC, the current through it will probably be low.

Try connecting the 5v load you want to power and see if the current reading changes.

Billy10mm
 
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Re: Strange voltage from UBEC Buck Converter

Post by Billy10mm »

Keebie81 wrote:Looking over the pictures I notice you dont have a fuse on the power line going from battery to the ubec. Thats something you should add for safety
The power line is fused. This is inside the car and the entire overhead light setup runs on a 10AMP fuse. We are not directly connected to any power source.

Billy10mm
 
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Re: Strange voltage from UBEC Buck Converter

Post by Billy10mm »

adafruit_support_mike wrote:If you don't have any load connected to the UBEC, the current through it will probably be low.

Try connecting the 5v load you want to power and see if the current reading changes.
I did, multiple times. The load in this case is supposed to be my dash cam. I received a second UBEC yesterday as I'm out of ideas, and just went down a few minutes ago to hook it up. No dice. 8 volts clearly on the input side, 0 on the output side.

I still just do not fundamentally understand why this can be. When i test this thing by hooking up 4 AA batteries on the input side, I get 5v on the output side. But hook it up to an 8v lead in my car, and nothing. So confused.

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zener
 
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Re: Strange voltage from UBEC Buck Converter

Post by zener »

Did you measure the voltage on the input pins on the board while it was connected to the car?

Billy10mm
 
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Re: Strange voltage from UBEC Buck Converter

Post by Billy10mm »

zener wrote:Did you measure the voltage on the input pins on the board while it was connected to the car?
I did in the last post on page 1. For some reason, when measuring voltage across the wires connected to the UBEC input side, while attached to the car, it only reads 3 volts. Anyone care to guess on why that might be?

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zener
 
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Re: Strange voltage from UBEC Buck Converter

Post by zener »

Billy10mm wrote:For some reason, when measuring voltage across the wires connected to the UBEC input side, while attached to the car, it only reads 3 volts. Anyone care to guess on why that might be?
Yes. Because it is not a good source of 12V in the car. That is why your regulator does not put anything out. It cannot output with only 3V in. You need to connect it to a good circuit. You need to find a circuit that maintains battery voltage when the load is connected to it.

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