24V Light on Power Distribution Board

[joachimbean]

We're having trouble with our Power Distribution Board, the 24V light is not coming on. Everything else is working fine (5V, 12V, etc.).

Does anyone know what the issue could be?

[Team#2057-Vegas]

if you have another from past years or just an extra i'd just switch em out. thats the only thing I can tell you.

[joachimbean]

We don't have any extra ones currently (we're a rookie team), but we might be able to find another one we can use.

[Joe Ross]

Was the light ever on in the past? Does it turn back on if you remove all connections to the 24v supply? If it's the latter, it indicates a short in your wiring.

When you measure the voltage with the DMM, what do you read?

[joachimbean]

Yup, it was all working and powering the cRIO as of a few days ago.

[Matt Krass]

Sounds like there may be a short, I'd go ahead and disconnect everything from the 24V power and see if it comes on. Check it with the meter too, just in case the LED is just flaking out.

I don't know for sure, but since there is some kind of reasonably sophisticated power supply in there to boost the 12V to 24V, I'd hope it has some kind of short circuit protection, and is just shutting down.

[joachimbean]

Nope, disconnected everything from the 24V port, and there's still no light. I also checked with the meter, and there's no power. Should the circuit breakers be configured in any way, or should we just leave them the way they're currently connected?

[WizenedEE]

I'd suggest pulling everything off (except the in power, obviously), including circuit breakers and wires going off to other things and check again. Make sure you're using your meter from all the way on one side of the connector to all the way on the other side (there are two 24V and two GNDs on each connector)

Also, there's a connector that fits on top of the 24V output that you screw the wires into; try taking that off.

[Trent B]

Have you tried a new battery too. Despite being a regulated 24v things can get a little funny with lower battery voltages.

[Al Skierkiewicz]

joachimbean,
The 24 volt power supply will operate down to about 4.5 volts on the battery. If you haven't charged the battery, it is possible for the battery to fall to less than 4.5 volts when you try and power the robot. However, there is always the possibility that one of your batteries has been wired backwards. It happens all the time at the beginning of the season. Check all batteries with your voltmeter to be sure. The battery connector is marked with a + and - as well as the PD. Measure the voltage right at the input to the PD to be sure you have the correct polarity and a battery that is well above 4.5 volts.

[Matt Krass]

Originally Posted by Al Skierkiewicz

joachimbean,
The 24 volt power supply will operate down to about 4.5 volts on the battery. If you haven't charged the battery, it is possible for the battery to fall to less than 4.5 volts when you try and power the robot. However, there is always the possibility that one of your batteries has been wired backwards. It happens all the time at the beginning of the season. Check all batteries with your voltmeter to be sure. The battery connector is marked with a + and - as well as the PD. Measure the voltage right at the input to the PD to be sure you have the correct polarity and a battery that is well above 4.5 volts.

Is the PD protected against reverse polarity?
Also, he said the 5V and 12V outputs were working, which would imply that he currently has the correct polarity, but if the board isn't protected a reverse wired battery may have damaged something.

Do we know exactly what boost converter they use for the 24V rail? I'm curious what its datasheet says about reverse polarity.

EDIT:
I realized after I asked all the schematics are on the FIRST website here: http://usfirst.org/roboticsprograms/....aspx?id=16337

The boost converter is based on the LM3478 chip, and I pulled its datasheet up from NI. I admit I only skimmed it, but it doesn't seem to have any reverse polarity protection, nor does the PD board seem to have. It does have built in short circuit protection, so a short shouldn't have hurt it, but there is a 15A fuse in line with it inside the PD board. If you shorted it, I'm not sure if that fuse would go before the SCP in the LM3478 would kick in, so it's possible that blew I suppose. I guess this is a viable possibility if you guys can't find any batteries wired in reverse.

EDIT2:
I actually think that it's not a fuse but a self resetting internal 15A circuit breaker. I should probably not try to read schematics when half asleep....

[joachimbean]

Yup, all batteries seem to be wired correctly (and getting a reading of 13.5V on the meter). I don't really know what to do at this point.

[EricVanWyk]

You will most likely need to buy a new PD.

Please disconnect the battery and anything from the 24V. Measure the resistance between the positive lug (where the battery is usually connected) and the positive 24V output. If there is a high resistance, the fuse has been blown.

One way the fuse can be blown is if the 24V positive output is shorted to the negative return for a motor or the battery (shorting to its own negative return does not cause this). This can happen if your chassis is not isolated. Please check to make sure that the chassis of your robot is completely isolated from the electronics - and then make sure it is still true when stuff is getting jostled around.

Originally Posted by Matt Krass

The boost converter is based on the LM3478 chip, and I pulled its datasheet up from NI. I admit I only skimmed it, but it doesn't seem to have any reverse polarity protection, nor does the PD board seem to have.

Matt -

Q12 on page 6 provides reverse polarity protection to the 3 regulated power supplies on the PD.

[Garten Haeska]

We had a similar problem, and it turns out it was a short, every other port worked, and the only one that didnt was the 24v for the crio.

[Matt Krass]

Originally Posted by EricVanWyk

You will most likely need to buy a new PD.

Please disconnect the battery and anything from the 24V. Measure the resistance between the positive lug (where the battery is usually connected) and the positive 24V output. If there is a high resistance, the fuse has been blown.

One way the fuse can be blown is if the 24V positive output is shorted to the negative return for a motor or the battery (shorting to its own negative return does not cause this). This can happen if your chassis is not isolated. Please check to make sure that the chassis of your robot is completely isolated from the electronics - and then make sure it is still true when stuff is getting jostled around.



Matt -

Q12 on page 6 provides reverse polarity protection to the 3 regulated power supplies on the PD.

Ah, I didn't see that. Clever design.

Unfortunately, it seems that the symptoms point towards a short permanently taking out the fuse. I suppose opening the PD up to replace the fuse would constitute a non-competition legal modification?