I saw lately a lot of nonsense in discussions about heated beds, the use of SSR's, current versus voltage. A large number of people seem to speak like experts about things that they do not seem to understand.
As an electromechanics engineer (for what it is worth) here are my recommendations and some background on the electrical physics behind it.
Some basic knowledge
1 - Voltage = Current times Resistance or U = I x R with units the units Volts (V) for voltage, Amps (A) for current and Ohms for resistance
2 - Power = voltage times current or P=U x I with units Watts (W) for power
and derived from this : Current = Power devided by Voltage
3 - Power is also current times current times resistance or P=I x I x R
4 - Power is also voltage times voltage divided by resistance
What does this mean in our real world?
If you have a power supply delivering 12 Volts and a heated bed rated 280Watts @ 12V then the current flowing through that bed is 280W divided by 12V equals 23,33Amps,
A 280W bed made for 24V will use 11,66 Amps (half of the current of the 12V bed since the voltage doubled) and a 280W bed for 120V will use only 2,33 Amps.
You can also calculate the resistance of the bed if you want to check if it is still ok by measuring it.
The 280W 12V bed from above would have a resistance of 280W by 23,33A by 23,33A = 0,51 Ohms
for the 24V bed this would be 280W by 11,66A by 11,66A = 2,05 Ohms
for 120V this is 51,4 ohms
AC / DC
AC stands for alternating current
Alternating means that the voltage and the current go from negative to positive and back 50 times per second or in the US 60 times per second. The wave form is a sinus. Electronics made for AC have no dedicated poles like + or -, but are sensitive to overvoltages. 120V AC electronics doesn't like to be connected to a 230V AC source.
DC stands for direct current
Direct current is the kind of current you get from a battery. One wire or pole is always positive while the other is negative. Hence the + and - marks.
Current flows from plus to minus and electronics connected to those kind of supplies don't like inverted currents. Signs of this not liking are smoke with a typical electronics 'perfume', short flash of light ...
As you saw in the previous : high power means high current or high voltage or both.
People tend to think that low voltage is safer then 'high' voltage. This is partly true. As long as you don't touch the copper 'high' voltage is ok. Everyone uses a power supply for their laptop with a wire on it carrying 230V (europe) or 120V (US) without even thinking about electrical shocks. One should indeed take some precautions to cover connectors.
Low voltage on the other hand as shown before needs high currents for the same power. So let's get more into detail on the dangers inherent to high currents.
Remember P=I x I x R
Taking the 280W example this means a 23,33 Amps current flowing through a small resistance of 0,51 Ohms. This will deliver the 280W in the heater only when you measure 12V at the connectors on the bed. But ... one needs wires and connections (soldered, screwed ...) to get the current from the controller and Power supply to the bed. Every part will add some more resistance and as we saw earlier a resistance with a current flowing through it will start heating with a power directly related to the square of the current.
have a look at the precalculated table below and check the wires you are using and the heat developed in those wires. This is heat you are not delivering in the bed and perhaps the cause of an underperforming bed. Use thick wires is the message. For a wire this may still be ok because of the 'large' surface of the wire it is capable to dissipate the heat and the temperature increase will be moderate. If not : fire
But this is also occurring in the connections you make. If one doesn't fix a cable with enough force and thus creating a good contact, one will have a resistance at that spot and the same applies. But ... since it is a spot, it cannot dissipate heat easily and temperature can increase very fast and create a fire. (see example on this forum.
Here is the table : Left side (lighter green) wire properties, right side losses for 2 meters of wire and different voltages.
Message : The thicker the wires, the lesser the losses!!
SSR, MOSFET ...
SSR stands for Solid State Relay. This is a device that uses a control signal that can vary in most cases between 3 and 32V DC and must be connected taking care of the correct polarity at the control side!!
The output connects do DC only for a DC version and AC only for an AC version.
What is the difference between those 2?
The DC version uses internally a MOSFET to switch current on and off. The output follows the input signal immediately. Since they are used for the low voltage beds, they will switch high currents and because they are not ideal switches (without resistance) they will heat up with higher currents.
to be mounted on a HEATSINK using thermal compound!! eventually adding a fan
The AC version uses TRIACS or Thyristors internally and they switch on almost immediately and off the next time the voltage goes trough the 0V (going from minus to plus or from plus to minus) this may introduce a delay of 10 milliseconds@ 50Hz or 8,3 milliseconds @ 60Hz
1 - staying with 12V or 24 DC
2 - moving to 120V or 230V AC
staying with 12V or 24 DC
You will need a DC SSR or a MOSFET switch like the TriGorilla. The TriGorilla stuff is in fact an open DC SSR with a heatsink mounted on it already. I recommend to choose a version that is designed for 150% or even higher currents. Choose at least 30 amps version for 20 amps through the bed. You don't want that this device starts a fire when you are enjoying a beer in the backyard while your printer is working on a long print in ABS with a fully charged heated bed.
A DC SSR or a MOSFET device are connected in the same way.
All the wires to and from BED-SSR-Power supply must be as thick as possible. Always try to get very flexible wire.
In the pictures below I used some example SSR. Always check the + and - marks on the device you are using.
The smaller connectors (bolts) should be the inputs, the larger ones outputs. Input and output should be clearly marked.
I wrote 'should be' as the currents are different and that should be common sense. Always check the documentation that came with the device.
moving to 120V or 230V AC
You will need an AC SSR. Although having a lower current you should choose also a high current device. I have a 230V 750W heater using 3,26 Amps with a 30Amps SSR. The SSR doesn't even get handwarm.
Trick : increasing the voltage of the power supply using the small potentiometer
If you increase the voltage to 13 V the power of the bed will increase too
With the resistance remaining 0,51 ohms for the 280W bed, the current will increase to 25,2Amps (Amps = voltage divided by resistance) and the power will increase to 328 Watt.... but also the losses in the cabling and the risk of fire on connection components. The maximum power of the FT5 power supply is 360W so be cautious when playing with that.
For those who spread the message you cannot PID an AC-SSR-ed bed : this is nonsense. Think before spreading this message.
I've put some graphs in my modifications thread about that. I'm using that all the time and it is performing excellent!
High currents are dangerous
High currents may introduce unwanted heating effects in connections and wires
Unwanted heating may cause a fire!!