Bed temperature problem - Pololu electronics - Fixed

Hi

I have built a version of Adrians Pololu/Mega electronics with home made PCBs using a thermistor for temperature feed back. [reprap.org]

It has been running well but I decided I would add the heated bed as per [reprap.org]

I have just started testing. Every thing seemed fine on test using the Host to
control the bed heater. I then noticed that when I turned the bed heater off
the reported bed temperature jumped 10 degrees. I also noticed my cooling fan
slows when the heater is on. I checked the supply voltage (an old computer power
supply) and it is normally 10.5 volts but this drops to 9.5 volts with
the bed heater on.

Any suggestions as to how I can establish what causes the incorrect temperature reading. The Thermistor and heater connections are on adjacent pins of a 4 pin header.

Edited 1 time(s). Last edit at 11/24/2010 02:07PM by emt.

---

Regards

Ian

Sounds like you're either:

1. Overloading the power supply
2. Power supply is faulty

Assuming you're observing a +12v rail at the supply.

The other thing to consider is how many +12v rails the PSU has, if you have everything running off one rail, but there are 2 or more in the supply, may be beneficial to move the heated bed to another rail.

I would suggest you disconnect the heated bed and put a 1meg+ resistor between the output pins. Measure the voltage at the supply again. if you're not getting IRO +12v here then the above 2 possibilities exist without the heated bed.

Another thing could be the operating current of the steppers, this is set by a pot on the pololu steppers. But how to set it I am not sure maybe there is a post here somewhere.

Have you got a dummy load on the 5V rail? Typically the 12V rail will run low without one because it can't increase the 12 without making the 5 go over voltage. Cheap PC power supplies only have one control loop for the whole supply.

---

[www.hydraraptor.blogspot.com]

To get a stable temperature reading you need to make sure none of the ground current from the heater and motors flows through the connection between the thermistor circuit and the analogue ground. I.e. the analogue power and ground pins should be connected to the 5V supply and then the thermistor components should connect to the Arduino pins without connecting to anything else. I.e. no shared ground connections.

---

[www.hydraraptor.blogspot.com]

Many thanks.

What would you use for a dummy load on the 5V. Would a resistor be suitable? Does it need a minimum current to work?

Clearly the grounds are shared. I need to seperate them some how. Will the mosfet work without a common ground?

---

Regards

Ian

Yes, a power resistor (or several) would be adequate. Use one that will pull at least 10% of the rated 5V capacity. Even better is to pull at least as much as the 12V supply does at max.

Let us assume that you have a 450W PC power supply rated for up to 21A at 3.3V, 20A at 5V, 18A on two separate +12V, and a small -12V and 5V standby connections. I pull 10% of the +5V supply, you would need to draw 2A, or have a 5V/2A = 2.5 ohm resistor. The resistor will pull 5V*2A = 10watts of power, which is easy to find. To come closer to the power drawn on one of the two +12V supplies, you want more like 10A, 0.5 ohms and 50 plus watts like this resistor. This would provide a solid drain for the 5volt switching regulator so that it can also keep the 12 volt more stable.

A second question is, does you power supply provide TWO or more +12V supplies? If so, use one for the thermistor, and the other for the heaters and fan. It does not matter much if the fan slows a little when both heaters are on, but it DOES matter if the temperature readings are affected.

Mike

Actually 10 ohms is usually sufficient load on the 5V and if you use a 10W AL clad resistor it doesn't get too hot.

The MOSFET need a good solid ground connection. The thermistor should be grounded separately. I.e. keep analogue and digital ground separate and connect them together only at one point close to the chip. That way there is no current flowing in the analogue ground so it is all at the same potential as the pin on the Arduino. Similarly for the analogue 5V. The ADC measures as a fraction of that so it doesn't matter if the 5V rail varies as long as the thermistor pullup resistor is at exactly the same voltage as the AVCC pin. I.e. not sharing a 5V connection with something that takes current.

---

[www.hydraraptor.blogspot.com]

Many thanks

I put a 10 ohm resistor on the 5 volts but the 12V is still at 10.5.

However I also connected the power legs of the mosfet and the 12 volt for the bed direct to a different 12V connector on the power supply and the temperature feed back is now stable. I will try another power supply at some time.

Edited 1 time(s). Last edit at 11/24/2010 02:06PM by emt.

---

Regards

Ian

You could try loading the 3.3V rail as well, or perhaps more load as Mike suggested, but 10 Ohms on the 5v is in the wiki and it has worked for me with about 3 different PSUs.

---

[www.hydraraptor.blogspot.com]