How to properly check a wire for 12V?

[suchamoneypit]

Having an issue with my hotbed on my 3d printer randomly losing power.

 

I set my multi-meter to voltage (the right mode for what im doing). Grounding the ground on a large bare-metal server case laying under the table, I probed the 3 listed points with the positive. 1 is the hotbed cables coming from the main board, 2 and 3 are positive leads going to the bed, and from the PSU, which are on a MOSFET which was added afterwards as an upgrade. While the bed was properly heating, I seemed to read around 0.35-0.45V when probing all these points in this way. When the bed began to malfunction and started dropping temp, I again probed these points, but only was seeing around 0.3-0.35V. Im not sure if this is saying something or if its within margin of error.

 

Am I probing this wrong? Im trying to figure out if the main board is sending power to the hotbed to try and figure out why my hotbed is randomly not working.

 

[gnomik]

Use the ground in your circuit, not some metal plate.

[suchamoneypit]

5 minutes ago, gnomik said:

Use the ground in your circuit, not some metal plate.

using the ground in my circuit i seem to still get no reading, with the machine on and bed properly heating. 

[suchamoneypit]

set up like this.

[gnomik]

3 minutes ago, suchamoneypit said:

set up like this.

You cant really go wrong there lol. The multimeter doesnt have an indication if its in  dc or ac?

[suchamoneypit]

Just now, gnomik said:

You cant really go wrong there lol. The multimeter doesnt have an indication if its in  dc or ac?

yeah im testing with those settings. No indication thats its in DC or AC. Even when i would be expecting to see 12V because it is heating properly im not.

[wirs]

YOu're measuring AC right now (See that little wave next to the V indicator on the top right of the multimeters screen) Press the Select button to switch over to DC

[artuc]

^ Yup. Spot on.

~AC

-DC

 

Both symbols on top of each other is measuring both, not all will have that.

 

Do that and use a 0v reference (negative) that is the same as the hot bed power supply. Keep your black probe on that and your red probe on the other points and record the voltage.

 

DC supplies can be isolated and floating and have nothing to do with your mains earth or some random chassis under your desk.

[suchamoneypit]

15 hours ago, wirs said:

YOu're measuring AC right now (See that little wave next to the V indicator on the top right of the multimeters screen) Press the Select button to switch over to DC

 

12 hours ago, artuc said:

^ Yup. Spot on.

~AC

-DC

 

Both symbols on top of each other is measuring both, not all will have that.

 

Do that and use a 0v reference (negative) that is the same as the hot bed power supply. Keep your black probe on that and your red probe on the other points and record the voltage.

 

DC supplies can be isolated and floating and have nothing to do with your mains earth or some random chassis under your desk.

This was exactly my issue! Thank you.

 

With this info i was able to properly probe the points in a working and non-working example. I am uncertain to specifics about MOSFET upgrades like what was performed on this printer, other than its highly recommended for safety. 

 

I got 12V readings steady from the mainboard and on the receiving positive and negative on the MOSFET board (the cable labeled 2 is the positive and to its left the negative).

 

The issues appears to be on the outgoing end. When the bed was dropping temp and not properly heating (this might also happen when its working?) If I grounded to the negative left of "2", I got a 12V reading from "3" (even when bed was dropping temp and not working), which is the outgoing power to the hotbed. However, When grounding with the negative to the left of "3", I was getting no readings at all from "3".

 

So this means the ground coming from the hotbed is bad. Im not exactly sure what this means (break in cable on the way to the hotbed or something? What does this point to considering Im having on-off issues where it will work but as soon as it starts to reach temp it loses heat and drops off to never return. How could a bad ground lead to this? I have now made sure its not the mainboard not sending the signal to heat, its something in between.

[Unimportant]

4 hours ago, suchamoneypit said:

 

This was exactly my issue! Thank you.

 

With this info i was able to properly probe the points in a working and non-working example. I am uncertain to specifics about MOSFET upgrades like what was performed on this printer, other than its highly recommended for safety. 

 

I got 12V readings steady from the mainboard and on the receiving positive and negative on the MOSFET board (the cable labeled 2 is the positive and to its left the negative).

 

The issues appears to be on the outgoing end. When the bed was dropping temp and not properly heating (this might also happen when its working?) If I grounded to the negative left of "2", I got a 12V reading from "3" (even when bed was dropping temp and not working), which is the outgoing power to the hotbed. However, When grounding with the negative to the left of "3", I was getting no readings at all from "3".

 

So this means the ground coming from the hotbed is bad. Im not exactly sure what this means (break in cable on the way to the hotbed or something? What does this point to considering Im having on-off issues where it will work but as soon as it starts to reach temp it loses heat and drops off to never return. How could a bad ground lead to this? I have now made sure its not the mainboard not sending the signal to heat, its something in between.

The MOSFET board uses a N-FET, judging from this picture:

( http://www.cecb2b.com/batchupload/inventoryother/zfa_ic_inventory/201495/zfa__332199_f13c9a0a038c37904c57614f101672ad.PDF )

A N-FET will switch the negative side, the positive side is simply connected trough.

So when you're "getting no readings at all from '3'" while "grounding with the negative to the left of '3'" that's most likely just the MOSFET being switched off.

 

Measure the control signal (the small white connector on the left side in the picture I've shown) and see if you have a different voltage there when you have the failure versus when it is working correctly.

 

[suchamoneypit]

1 hour ago, Unimportant said:

The MOSFET board uses a N-FET, judging from this picture:

( http://www.cecb2b.com/batchupload/inventoryother/zfa_ic_inventory/201495/zfa__332199_f13c9a0a038c37904c57614f101672ad.PDF )

 

A N-FET will switch the negative side, the positive side is simply connected trough.

So when you're "getting no readings at all from '3'" while "grounding with the negative to the left of '3'" that's most likely just the MOSFET being switched off.

 

Measure the control signal (the small white connector on the left side in the picture I've shown) and see if you have a different voltage there when you have the failure versus when it is working correctly.

 

 

So the source of the power from the mainboard is the circled "1" , when reading this Im getting 12V here when using the screws at the top of that green connector the cables come off of. Im not exactly sure how to read the wires on the connector on the MOSFET, but sticking the probes into the plug where I believe im making contact with the wire, Im only reading around 0.4V, when the machine is not properly heating. Havent been able to get a reading with the bed working yet.

 

Do you think this is an issue with the mosfet or do I not have enough info yet? Would replacing the mosfet solve issue?

[Unimportant]

46 minutes ago, suchamoneypit said:

Do you think this is an issue with the mosfet or do I not have enough info yet? Would replacing the mosfet solve issue?

I don't think it's a MOSFET issue. It looks like the MOSFET is not properly receiving a signal to turn it on. There should be at least several volts on that white connector on the MOSFET board to turn it on.

[suchamoneypit]

17 hours ago, Unimportant said:

I don't think it's a MOSFET issue. It looks like the MOSFET is not properly receiving a signal to turn it on. There should be at least several volts on that white connector on the MOSFET board to turn it on.

Even when the machine is not set to heat the bed, Im getting a 12V signal properly when I probe the end of the plug (unplugged from MOSFET) thats coming off the mainboard into the MOSFET.

[suchamoneypit]

Update: I was able to catch the signal when the bed switched from working and not working. I saw that the 12V dropped off the outgoing lines from the MOSFET, while it remained present going to the MOSFET. I just replaced the MOSFET, and now the printer is heating up and maintaining very high heat like a champ.