does a "normally closed" transistor or similar exist?

Hi,

I want to control and dim (PWM) LED light strips with an Arduino. However, I want the LEDs to also work (full brightness) in case the Arduino goes down for some reason.
I’ve done PWM dimming on LEDs before with the Arduino but there the switching transistor / MOSFET would of course be off if it gets no power or no signal from the Arduino.

So I’m looking for something that behaves like a transistor but basically opens when want to but is closed without input.

1 Like

Actually, a pull-up on the Base using the LED voltage would be enough, wouldn't it. Then switch the Base LOW to switch off the LEDs.

I had a feeling that would be a silly question.... ;)

Yes. a depletion mode MOSFET has a channel that conducts current even when no gate drive is privided. A JFET is notmally conductive. See ND2410L ND2020L BST80 BS107 TI has the BF245B depletion mode transistor.

Thanks for that. I was unaware of these.
But hmmm… They need a negative voltage. I would have to generate that just for the MOSFET.

I’ve come up with the attached circuit in the meantime. Looks like that might work.

This is going to be for a home automation project. I want to be able to control a lamp (LED) from both a wall switch and an Arduino. The wall switch must always work as if it was a normal switch (for the wife :wink: )

DimSwitchLED - Schematic.pdf (13.2 KB)

Why not just use a NC solid-state relay?

http://www.mouser.com/ds/0/Ixys/CPC1219-2032.pdf

One downside is that the switching speed of these relays is not very fast--on the order of a few ms, so it may not technically be able to keep up with a 500 Hz PWM signal. But what you're doing (dimming an LED) probably doesn't require perfect transmission of the PWM signal, and anyway, you could always decrease the PWM frequency a bit to get it below the on/off time of the relay. Again: dimming an LED probably doesn't require very fast settling time, so a lower PWM frequency is probably not a big detriment.

Why buying when you can build

Pelle

yesyes wrote about depletion mode MOSFETs, " They need a negative voltage. I would have to generate that just for the MOSFET."

No they do not require negative voltages. The depletion mode mosfet has a negative threshold voltage. It has a conductive channel when the gate voltage is the same voltage as the source voltage. If the source voltage is 3 volts, the gate can be 0.0 volts to turn it off.

Connect your LED to the depletion mode MOSFET source connect the FET drain to the 5v VCC. connect a 680 ohm resistor to the led and to ground.

ON gate = 5v OFF gate = 0v

See , no negative voltages. The diode drop in the LED is about 2v , so the MOSFET is off whengate is 0 and source is +2v. The negative voltage you mentioned is only needed if the MOSFET source is near ground voltage. But in this high side switch, the source is not at ground, it is over 2 volts, so the MOSFET can be turned off.

The circuit by pelleplutt is good, using an npn to be off when power is on but Arduino is unpowered.

When OFF the LEDdrop is 0volts.

The transistor became to work in its linear area and the LEDs glow. How much depends on Vgs(off) (-1,5 to-4Volts) for ND2012L

And depletion mode MOSFETs, are not so common.

Pelle

BST80 and BS107 are enhancement transistors

AmbiLobe: No they do not require negative voltages. The depletion mode mosfet has a negative threshold voltage. It has a conductive channel when the gate voltage is the same voltage as the source voltage. If the source voltage is 3 volts, the gate can be 0.0 volts to turn it off.

The Arduino when it's low, or sourcing current, I don't think it's strictly 0v is it? -0.6v for the voltage drop of the transistor? so it get's 0.4v or so on the gate of the fet.

You could just as easily use an NPN and a small pnp to do the same thing.

Thanks for all the replies. I think for now it's good to know that depletion mode MOSFETs exists, but if the same can be done with a "normal" MOSFET and a few components, then I'll go for that as I already have some normal MOSFETs that I can use.

I'll update my circuit diagram to add the small transistor on the gate of the MOSFET and change some resistor values (according to Pelleplutt's post) and will post it again.

Pelleplutt: Why buying when you can build

Less space on the board. Less work to assemble. Proper galvanic isolation because the solid-state relay has a built-in optoisolator.

So would this work?

I've not really looked at these solid state relays in detail but at first glance they don't seem to be fast enough for PWM dimming.

yesyes: I've not really looked at these solid state relays in detail but at first glance they don't seem to be fast enough for PWM dimming.

They have a max on/off time of 5 ms, so they should be able to handle a PWM frequency of at least 100 Hz. It seems to me like that should be adequate for dimming LED lights without flicker.

The Arduino built-in PWM is around 800Hz. So that would require external PWM components, which would cancel out the advantages of the solid state relay. Thanks for the suggestion though... ;)

yesyes: The Arduino built-in PWM is around 800Hz. So that would require external PWM components, which would cancel out the advantages of the solid state relay. Thanks for the suggestion though... ;)

You can adjust the Arduino's PWM frequency.

http://playground.arduino.cc/Code/PwmFrequency

Using a divisor of 256 on pin 3, 9, 10, or 11 would give a PWM frequency of 125 Hz.

EDIT: BTW, I believe that this text is incorrect:

 *   - Changes on pins 3, 5, 6, or 11 may cause the delay() and
 *     millis() functions to stop working. Other timing-related
 *     functions may also be affected.

I'm pretty sure that only timer0 will change the output of delay() and millis(), and timer0 is only on pins 5 and 6. So you can change pins 3, 9, 10, and 11 without affecting delay() and millis().