Hi everyone,
I've built a circuit to control a transistor and I'm using it with an arduino board.
I don't want to damage the arduino, but I want the max possible current to flow to the transistor so it opens the gate as full as possible.
What resistor would you recommend?
I tried a 220 ohm resistor but it's not opening the gate enough.
tx,
J
What transistor?
If it is MOSFET, make certain that you are using a "logic level" device.
Well, for a NPN transistor, the voltage from Base to emitter will be around 0.7V, one diode drop.
If the Ardiuno put out 5V, and you wanted to limit current flow to 20mA, then:
(5V - 0.7V)/.02 = 215 ohm.
You could go lower, say to allow 35mA:
(5-.7)/.035 = 125 ohm
However, output voltage will start dropping as current increases, and in fact the High out may be as low as 4.2V, see Table 29-1 in the '328P datasheet.
For higher currents, it could be expected to droop even more.
(4.2-.7)/.02 = 175 ohm
So for values less then 175, measure the voltage across the resistor, and do the math to see what current you are getting:
(V1 - V2)/resistor = current.
A 220ohm would be plenty of current...
Do you have it wired correctly?
Hi everyone,
sorry for the delay in replying.
I'm using a IRF540N.
For the load circuit, I have it connected to the Gate (Negative) and Source (Positive).
The Drain is connected to the Arduino.
Gate, Source and Drain Assumed by this url http://www.sproboticworks.com/mosfets/mosfets/irf540n-100v-33a.html
From my understanding with an NPN transistor, the more voltage/current I give it, the more it will open the gate, and the opposite for PNP.
J
I think I've messed up the logic.
Gate is actually my voltage/signal to turn on/off.
Source and Drain is the flow of the circuit I wish to open
I'm going to play a little more and try get the logic correct 
J
Yes, you are not connected correctly.
Here are the connections for a 12V LED strip for example with an N-channel MOSFET.
Hi CrossRoads,
I just built a small circuit based on what you suggested.
From what I've observed, Negative Voltage to GATE = Transistor Closed, and Positive voltage = Transistor Open.
is this correct?
J
I learnt what Base collector and emitter are... what is the equivalent for NPN (i.e Gate, source and drain)?
Is this a correct analogy?
Base = Source
Collector = Gate
Emitter = Drain
This part? Gate High = On. Gate Low = Off.
NPN N-channel
Base = Gate = control signal
Collector = Drain = connects to - of load, + goes to V+
Emitter = Source = connects to Gnd.
See NPN in Reply #2 & N-channel in #6.
jgporteous:
Hi CrossRoads,
I just built a small circuit based on what you suggested.From what I've observed, Negative Voltage to GATE = Transistor Closed, and Positive voltage = Transistor Open.
is this correct?
J
In electronics in English "closed" means "on" and "open" means "off", as in "open-circuit",
"switch closed". This is different from water taps.
Use the words "on" and "off" to avoid ambiguity...
The IRF540 is not logic level and won't work from 5V.
Thanks everyone.
What logic level transistor should I use with Arduino to amplify (control) a 14.8V @ 6A circuit?
Thanks,
J
For 6A you want one with an Ron of 15 milliohms or less without a heatsink,
or 30 milliohms or less if you use a small heatsink capable of dissipating a couple of
watts.
Needs to be logic level and 20V or more Vds for resistive load, Vds 30V or more if
inductive (in which case a flyback diode maybe needed if the avalanche rating of
the MOSFET isn't up to the load or you are using PWM).
Don't be confused by the Vthr or current ratings, go by the on-resistance, Vgs
specified for a given Rds(on) and the power dissipation rating of the case. You'll
never get close to the max current without liquid nitrogen cooling anyway1
Thanks everyone for your input.
I built a small circuit with a BDW93C and an LED.
From what I understand (from datasheet) the BD93C's Emitter-Base voltage is 5V
(I think that is what is required to turn it on fully).
Input Voltage used: 5.08V
Voltage across LED is measured when on is:
I would like to get MAX voltage, i.e. 5V, when the transistor is in the ON state.
What am I missing?
Thanks,
J
MarkT:
For 6A you want one with an Ron of 15 milliohms or less without a heatsink,
or 30 milliohms or less if you use a small heatsink capable of dissipating a couple of
watts.Needs to be logic level and 20V or more Vds for resistive load, Vds 30V or more if
inductive (in which case a flyback diode maybe needed if the avalanche rating of
the MOSFET isn't up to the load or you are using PWM).Don't be confused by the Vthr or current ratings, go by the on-resistance, Vgs
specified for a given Rds(on) and the power dissipation rating of the case. You'll
never get close to the max current without liquid nitrogen cooling anyway1
Hi, thanks for this.
I tried to find the Vgs for this Transistor but I can't find it on the datasheet:
J
Need to use an N-channel MOSFET - low Rds means less voltage drop across the part with current flow:
Here's a better schematic showing basic Arduino functionality and the MOSFET/LED.
Hi CrossRoads.
That Transistor is not available in South Africa.
My local supplier recommended this one: http://www.mantech.co.za/datasheets/products/IRFU024.pdf
Does that look right?
J
That one won't fully turn on unless the Gate Voltage, Vgs, is 10V.
With 5V, it will be on a little and will likely get very hot if there is much current flowing.
What you really want to find is a Logic Level part, one that has Rds = very low when Vgs is < 4.5V.
For example, the part I suggested has:
RDS(ON) (at VGS = 4.5V) < 11.9mOhm (0.0119 ohm)
Need to find a part like that, otherwise the part will get very hot with 6A and may fail from heat damage. Even that part will likely need heatsinking.
Power dissipated in the transistor is: P = IIR, so 66.0119 = 0.43W.
Thank you, I've posted that to my supplier.
Have a great evening,
J
