Heated Bed PSU

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1

I used this circuit to add a 24V PSU to heated bed
k8200-wiki.de/Datei:Heizbett-Treiber.jpeg
(I don’t speak german so am using google translate)

I added a decent heatsink to the FET
The bed heats up much quicker now and I get it to 100 degrees finally.

However The LED is on when the bed is off and vice versa
(seems counter intuitive - I have checked LED polarisation and it is correct)
Is this how this should work ?

The resistor (R3) which is in series with the LED has heated up an burnt

I tried a higher wattage one also but that looks like its going the same way (based on smell).

This is obviously dangerous

I suppose I could just remove the LED and resistor as they play no functionally role other than to indicate bed status

Anybody throw any light on this ?

tks
Noel

2

Hi Noel

The resistor R3 in that diagram seems to have a wrong value.
220 Ohm is far too low. With a 24VDC PS this would result in a current of approx. 100mA, far to much for a LED.
Try a resistor with 2.4kOhm and 0.5W.

And yes, the way the LED/resistor is connected the LED is on, when the heat bed is off. Q1 shorts the LED/resistor when it turns on.
To change this, connect the LED/resistor combination in parallel with the heat bed:

Regards
Rainer

3

tks

I tried this but FET got extremely hot and heatsink fell off and FTE burnt out !!!

CAn’t see why that would have happened

I ordered this power expander http://reprap.me/heatbed/power-expander.html over 21 days ago but it still hasn’t arrived
Guess I will need to wait a bit longer for a 100degrees bed :slight_smile: