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Heat Bet Voltage Question: smiling smileyRamps1.4

Posted by Janski 
Heat Bet Voltage Question: smiling smileyRamps1.4
June 26, 2014 01:00AM
My Power source reads 12.48 V from the power source and the D8 slot in Ramps 1.4.

However, when I check the Heat Bed terminals they only read 11.82 V. This is resulting in slow heat-up-times and power. Any quick fix? do i need to rewire?


Re: Heat Bet Voltage Question: smiling smileyRamps1.4
June 26, 2014 01:07AM
PS: using 16 gauge wire for the bed.Do i go for 14? the heater extruder runs on 18 gauge so I would not think that would make a difference in bed.
Re: Heat Bet Voltage Question: smiling smileyRamps1.4
June 26, 2014 04:05AM
When you're measuring D8, are you measuring from one pin to the other, or are you measuring from the positive pin to the ground on your RAMPS board/PSU?

RAMPS provides 12V ALWAYS on one pin, and switches the connection on the ground (using a MOSFET).

Not all MOSFETs are created equal, and the ones shipped by default on the RAMPS boards (STP55NF06L) really aren't good for high current loads.

If you get a similar voltage across D8, then I'd look at replacing the MOSFET for the bed. That said, 0.6V loss in the MOSFET is not huge, and I suspect that even with a good MOSFET like the IRLB8743's or an FDP8870 you're not going to see much lower drop than that.

If you suspect the wiring to the bed, while the bed is heating measure the voltage from one end of a wire to the other. eg: One probe on the bed end, one probe on the RAMPS end. If there's any appreciable drop on both wires, you might consider replacing it. I doubt this is an issue in your setup if you're only losing ~0.6V.

Regarding wire to the bed: You want reasonably flexible wire that is capable of carrying a good current. While thickness (gauge) relates to this, you can get types of 18 gauge wire that will carry more current than other types of 14 gauge wire. eg: 18 gauge RC hobby wire will do heaps better than 14 gauge 240V mains wire.

I usually recommend you look at RC Hobby places, as RC vehicles (of all sorts) tend to use very high currents, and the wire they sell tends to be very good at high current and is usually very flexible. They also tend to have decent supply chains so getting your hands on it is not an issue.

Note: The bed draws quite a lot of current. A 200x200mm square PCB bed draws 9A->12A at ~12V (usually more likely to be on the 12A side). Compare this to the hot end (extruder heater) which draws between 1.75A (6.8 ohm resistor) -> 3.4A (40W cartridge heater).

Edited 2 time(s). Last edit at 06/26/2014 04:10AM by Cefiar.
Re: Heat Bet Voltage Question: smiling smileyRamps1.4
June 26, 2014 12:07PM
The voltage drop on a fully open STP55NF06L mosfet passing 12A should be 0.192 volts. To measure that directly you measure between gnd terminal of D8 and gnd of psu itself.

If you do get indeed a 12.4v on main connector and at same time 11.8 on D8, another probable drop (~0.4v difference?) could be on the big yellow ptc polyfuse, and may check that it does not get too hot because those are reported to catch fire.

Perhaps the entire psu voltage is 12.4v without significant consumers, drops to 11.8 when the heatbed is on, and jumps back to 12.4 when the heatbed is off. Because the bed is a big consumer. I'd rather think its like this instead. Meaning you get different voltages, but at different times depending on the bed on/off state. Other than that, inside either state the voltage differences should not be that big.
Re: Heat Bet Voltage Question: smiling smileyRamps1.4
June 26, 2014 02:23PM
Thank you for the reply.

I see what you mean.

When I test the 16 gauge wires by connecting to D8 and testing the wire tips with multimeter , at first I get the 12.48 V. Then When I solder the bed, with + to 1 and - to 2 and 3 terminals, I get the 11.78-11.50 Vs. Noobman below mentioned how the currents vary. But could this mean I am not soldering properly. I use flux, tin the wires and terminals then flat not through the terminal hole.

Hope this makes sense.

Thanks again for this information smiling smiley
Re: Heat Bet Voltage Question: smiling smileyRamps1.4
June 26, 2014 02:24PM
Thank you for your answer. I replied above. Please advise if you see anything wrong in my process.

- Janski
Re: Heat Bet Voltage Question: smiling smileyRamps1.4
June 26, 2014 02:33PM
Attached is the pic of the solder job.

So to answer my question: This bed is drawing 11.80V and there is nothing I can do about it because of the hardware.

Let me know if I am thinking in the right direction smiling smiley
open | download - BedSolder.JPG (28.4 KB)
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