Sanguinololu LM7805 hot... normal?

Hello, I thought I'd try my hand at putting together this board and I've got Sprinter loaded onto it and, as a quick test before installing it, it's sending instructions to the motors and I'm quite pleased.

For the test, I've hooked it up to a bench power supply at 12V and capped it at 250mA. Without the USB cable attached, it pulls a mere 40mA. When I plug the USB cable in, it hits 160mA power draw, just sittin' pretty.

What concerns me, though, is that the 7805 gets quite hot to the touch. I can't hold my finger to it longer than 5 seconds, and I've measured 67C on its backside.

According to the datasheet, operating temperature is 0C to 125C, and it has built-in thermal protection, but I wonder if it's really normal. I don't have anything hooked up to it (detached the motor when I noticed it got hot) other than Stepstick drivers plugged in, so I'm just looking for a sanity check. Thanks!

Let me guess: plugging in the USB cable makes a connection between USB's 5V and LM7805's 5V. Most likely, they're a bit different, e.g. 5.05V vs. 4.90V, so a current flows.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Yep, there's definitely a voltage difference. I guess the question really is whether I should be concerned about it or just slap a heat-sink on it and pretend I didn't notice.

I actually hadn't measured the +5V from this particular USB hub before and it's pretty disappointing (4.6V). Dollar store special strikes again, it would seem.

You should be concerned about this and cut off one of both 5V supplies.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Cool, thanks for the help!

This sounds a lot like my problem. The 7805 also gets very hot (I actually burned my finger when I touched it) and in addition the printer resets itself during prints > 30 min and freezes then. I cannot measure the current right now, so I am not 100 percent sure if I have the same problem. I will check that tomorrow.

How do I "cut off one of both 5V supplies" then?

sloth Wrote:
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> This sounds a lot like my problem. The 7805 also
> gets very hot (I actually burned my finger when I
> touched it) and in addition the printer resets
> itself during prints > 30 min and freezes then. I
> cannot measure the current right now, so I am not
> 100 percent sure if I have the same problem. I
> will check that tomorrow.
>
> How do I "cut off one of both 5V supplies" then?

Honestly, the best solution is to get a USB hub that hits +5V better than your current setup. Cutting 5V supplies reduces the flexibility of it, IMO, in ways I'll describe below. One word of caution is that I'm just glancing at the 1.3a board, I haven't really rigorously studied it, or attempted any of these myself, so consider this a starting point and not a definitive source.

As far as doing it, there are two ways. The least dramatic change would be to just desolder the LM7805. The side effect of this is that the board will NOT work unless it's attached to a computer with USB, since there will be no way to power the logic from 12V. You can also cut the trace in the attached picture named sanguino_no5_01.png (found on the bottom of the board), but if it's not doing anything, you might as well remove it instead of wasting the power on it.

Another location to cut, which would allow you to use it while the cable is disconnected, is shown in sanguino_no5_02.png. The downside is now you can't program/communicate with the board unless it's getting +12V, since the logic is no longer powered from the host machine. Also, another downside is that +5V on the data lines might not match +5V powering the ATMega. The electrical maximum for input high voltage is VCC+0.5, so if your computer's provided logic high is above +5.5V you're going to potentially kill it. Half a volt is a pretty big deviation, though, and I would find it odd if the computer's +5V was that much higher than your Sanguinololu's +5V.

Thanks for your reply, I will begin with desoldering the 7805, since this won't damage the board. I will give feedback here.

Very good, I removed the 7805 and the printer is back and printing

Thanks for your help!

Thank you Mazaw for posting this. I recently upgraded to a heated build plate and new PC and ran into this problem. I will try desoldering the 7805 when I get home from work. It seems like this solution would make sense and is very likely considering I'm using a different PC than I was before (USB 3.0 vs 2.0). Maybe it's drawing the full 12v from USB 3.0 and giving me this issue. Would it still be ok to remove the 7805? Thank you!

Better to cut the track from the USB connector to the 5V rail.

The problem with Sanguinololu is that both the USB VBUS and the regulator connect to the 5V rail. That is very bad for the regulator and the PC. The regulator will give a much more accurate 5V signal than the USB. The only downside of this mod is you can't talk to the board without the 12V PSU being on, but since it doesn't control the PSU there would be no point.

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[www.hydraraptor.blogspot.com]

Thank you Nophead! I cut the trace and the regulator doesn't overheat anymore, but my power supply still craps out on the first layer of my print. I'm thinking my old pc power supply wasn't enough (250W :/) with the heated bed. So I ordered a 30A switching power supply off of ebay. Hopefully I'll be back printing by Tuesday. And hopefully my underpowering didn't hurt anything too seriously.

Have you got a dummy load on the 5V rail? A PC PSU will not work without one.

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[www.hydraraptor.blogspot.com]

250W PC-PSUs typically have some 8 amps on the 12V rail. Sufficient without a heated bed, no dice with one. Not even with a load on the 5V rail.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

i don't have a dummy load, I only have pins 13 and 14 shorted. And I figured 250 wouldn't be enough after I got my heated bed, but I had it in my head that I had a 350W for some reason. Thanks for the help guys.