Signal Mosfet

Signal Mosfet Switch
Mosfet Transconductance
Mosfet Signal Generator

Howto Connect your Hotbed (and or extruder) to a Mosfet

IMPORTANT: I have seen people trying to (re-)sell these Mosfet units with a HUGE profit on EBAY while referring to this page for installation schematics, go shop around, they don't need to be more expensive than around €5 to €10 ($5 to $10) incl PP.

Why

Figure 4.17 Large-signal equivalent circuit model of the n-channel MOSFET in saturation, incorporating the output resistance r o. The output resistance models the linear dependence of i D on v DS and is given by Eq. Modified Large Signal Model. The metal-oxide semiconductor field-effect transistor (MOSFET) is a semiconductor device controllable by the gate signal (g 0).

The idea is pretty simple: The high current of the hotbed brings multiple problems to your mainboard.All the current for the bed has to go through the one connector in addition to all the other current for the hotend, the steppers and the logic units, the default connectors are not rated for the current that goes through them when printing. Things like in the picture below happen to almost every printer at one time in its life ….

←– connector and corresponding plug —>

Removing the hotbed bed current from the mainboard, significantly reduces stress on your mainboard (connectors).

Another benefit, it will allow you to use a second PSU for bed power (12 or 24 volts).

What you need

a MOSFET unit like this, a 2 pin JST connector with some wire and some thicker wire(s) AWG 16 or AWG 14 (1,5 mm2 or 2,5 mm2)

3D-Printer-Parts-General-Add-on-Heated-Bed-Power-Expansion-Module-High-Power-Module-expansion-boardon banggood.com

How to install

A special bracket to mount your ANET mainboard and 1 mosfet unit, you can find here https://www.thingiverse.com/thing:1857006

remove the power-cables of the heated bed from the mainboard and attach them to the power output of the MOSFET unit
Connect 2 wires from the bed output of the mainboard to the control input of the MOSFET (polarity DOES NOT MATTER for this signalwire)
Connect 2 wires from the PSU (either the same as the printer, or a second PSU) to the power input if the MOSFET unit (Here polarity DOES MATTER)

For the signalwire there has been some confusion

some people claim polarity matters they are right sometimes some mosfets are polarity sensitive.

For the mosfet unit shown in the 2 pictures here, POLARITY DOES NOT MATTER, check them side by side with the signal wire connected both ways.

In both pictures the hotbed was set to preheat PLA, both led blue (D2) and Red (D1) light up and everything functions as it should.In the photo's below you can see the “bridge-rectifier” on the mosfet (just to the right of the white plug) wich corrects “wrong” connection of this wire!

Apply a second PSU ( 12 or 24 volts)

If you want your HOTBED to heatup faster you can add a second power supply unit to your printer.

Disconnect the wires from the PSU to the Mosfet Unit and apply them to the second PSU.

Apply a second Mosfet (for the extruder)

Eventhough in my opinion it is not really necessary, you might want to decide you want to use a 2nd mosfet for your extruder as well, here is a schematic you might find usefull

Of course you can use 1 (upgraded) PSU for all the 12 volts wires if you choose to

Increasing Voltage on the (stock) PSU

Near the power terminals, either on the left or the right of the terminals, is a small dial marked “ADJ +V”. Rotate it clockwise to increase voltage, and counter-clockwise to decrease it. Mac os x goflexhome software download.

Always disconnect your power supply before making adjustments!

Check the voltage between one of the + and one of the - terminals, using a multimeter set for voltage DC.

PSU combinations

There are a lot of combinations possible

1 x 12 volts PSU

2 x 12 volts PSU, of the one used for the heatbed you could raise the voltage to circa 14 volts

Since there are 1 million different ATX PSU's, i will give some “general” things for you to consider only. No specifics about ONE PSU.

Check the ATX PSU you plan to use for a label like the orange one in the picture and check for maximum currents it can supply. Note that using multiple voltage outputs at the same time means less available current for the separate outputs than stated as maximum sometimes.

-For easy connecting your ATX Power Supply Unit, you could use a breakoutboard like in the picture, check your PSU for maximum power (Amps) and order some extra fuses to replace the stock 5 amp fuses that come with the unit, if you plan to draw higher currents through the breakoutboard.

An other way is to just short the green (PS-ON) and black (GROUND) wire in the 20/24 connector to make the PSU be always ON. And use the wires from the wireloom as you need them. Just make sure the specific wires are designed to supply the current you need.

1x 12 volts + 1 x 24 volts, The 12 volts one is used to power the mainboard extruder and steppermotors, the 24 volts powers the heat bed (through the mosfet)

12 volts PSU + 24 volts PSU combination

The schematic printed on the surface shows pin 1 2 and 3 representation of the element →

If wired for 12 volts, the element is doubled up, resulting in less resistance/higher current/slower heating and less poweroutput.

If wired for 24 volts, the element is connected from both ends only resulting in higher resistance/lower current/faster heating higher poweroutput.

Do the math..

HOTBED 12 Volt con. resistance betw 1.0-1.2 Ω (work with 1.1 Ω)

• I=V/R the current drawn at 12 Volts is 12/1.1= 10.9 Ampere

• P=V*I Power output at 12 Volts is 12*10.9= 130.9 Watts

HOTBED 14 Volts con. resistance betw 1.0-1.2 Ω (work with 1.1 Ω)

• I=V/R the current drawn at 14 Volts is 14/1.1= 12.7 Ampere

• P=V*I Power output at 14 Volts is 14*12.7= 178.2 Watts

HOTBED 24 volts con. resistance betw 3-3.4 Ω (work with 3.2 Ω)

• I=V/R the current drawn at 24 Volts is 24/3.2= 7.5 Ampere

• P=V*I Power output at 24 Volts is 24*7.5= 180.0 Watts

As we discussed before, the output voltage for the MOSFET amplifier is non-linear towards the input voltage: $v_{o u t}$ = $V_{S} - \frac{K (v_{D S} - V_{T h})^{2}}{2} R_{L}$ .

Figure 1 shows the MOSFET amplifier at the small-signal interpretation.

Figure 1.The MOSFET amplifier and it’s small-signal model.

This non-linearity significantly complicates design development, so linearity of the amplifier is more interesting from the designer point of view. Small-signal approximation states that at small time-varying incremental amplification, the time-changing component will be linear. Figure 2 depicts the transfer characteristics with small-signal interpretation. In this situation the incremental transconductance is $g = K (v_{G S} - V_{T h})$ , where g is the ratio between input voltage and current. Small-signal gain is $- g R_{L}$ .

Figure 2. The transfer function for the MOSFET amplifier.

Signal Mosfet Switch

In order to calculate the incremental small-signal response, we have to do a few calculations: we need to find the large-signal response for the certain DC operating point of the signal. And then we must use the Taylor approximation to obtain the small-signal response for this operating point.

Considering the circuit in terms of small-signal approximation we must:
1. Put all components to their operating value.
2. Linearise the behaviour of every circuit component at the operating point.
3. Replace orginal circuit components with their linearised components.

Some handbooks give the extensive explanation of the small-signal approximation of different components of circuits like DC voltage and current sources. In general, we need to find the small-signal approximation of the circuit component $f (x)$ so it’small signal deviation for this component is $y = \frac{d f (x)}{d x}$ at some specific value for the operation point $x_{0}$ .

For the MOSFET amplifier, small-signal approximation for the operating current is $I_{o} = \frac{K}{2} (V_{i n p u t} - V_{T h})^{2}$ , and $V_{o} = V_{S} - \frac{K}{2} (V_{i n p u t} - V_{T h})^{2} R_{L}$ . Figure 3 depicts the amplifier and its small-signal model.

Figure 3. The difference amplifier and its small signal model.

The input resistance for this model will be $r_{i} = \infty$ , the output resistance is $r_{O} = R_{L}$ . The current gain for this model will be $\frac{v_{O}}{v_{t e s t}} \frac{R_{i}}{R_{o}}$ . Power gain for this scheme will be $\frac{v_{o}}{v_{t e s t}} \frac{i_{o}}{i_{t e s t}}$ .

Let’s consider the difference amplifier AD8479 for high quality amplification. This difference amplifier consists of the operational amplifier and resistor network. The output of the difference amplifier $v_{o} = A_{D} (v_{A} - v_{B}) + A_{C} \frac{v_{A} + v_{B}}{2}$ , where $A_{D}$ is the difference-mode gain, and $A_{C}$ is the common-mode gain. These can lead us to the common mode rejection ratio for the amplifier $C M R R = \frac{A_{D}}{A_{C}}$ .[1]

Mosfet Transconductance

Figure 4. Functional diagram for the difference amplifier AD8479, Analog Devices. [2]

[1] “Foundations of Analog and Digital Electronic Circuits”, Anant Agarwal and J. H. Lang, Elsevier.

Mosfet Signal Generator

[2] AD8479 datasheet, Analog Devices.