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I need help choosing a transistor

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m2j8

New Member
Hi,
I need some help to find a suitable transistor. I will be sending it a binary singal (0-5V) from an ATMEGA microcontroller and it needs to switch 12V on and off (for a small solenoid).

Could anyone please advise me on a transistor part that I could use?

Thank you.
 

MikeMl

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m2j8

New Member
soleniod current

On the side of the solenoid it says:
12V DC 4.8W
24V AC 50/60Hz
8.5/6.9VA
(<----what does that mean?)

On the other side of the solenoid it says:
12V DC max 6.9W
24V 50/60Hz max 6.9W


Im using DC.

I have attempted to work out the current, but my knowledge it very limited.
Is the following correct?

1W = 1V x 1A
4.8W/12V = 400mA

ANS: 400mA solenoid current ( max 575mA)
 

MikeMl

Well-Known Member
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Looks ok to me. The ATMEGA data sheet should have a spec like IOH = XmA when Vport=Y volts. Can you look that up and post it?
 

Sceadwian

Banned
Mike, a 5V signal feeding the gate to a transistor with a 12V power supply is going to act like a follower. How's it going to provide the 12V the relay needs?
 

MikeMl

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Assuming that the Atmel Vdd is 5V, this will work:

Note that any NFET with a Gate Threshold of <3V, with a Drain current rating of >1A, a drain voltage of >20V, and an on-resistance < 0.1Ω will work.

The 10k resistor is to prevent the gate of NFet floating while the Atmel is booting up and the Port Pin is yet undefined (or defaults to an input). The 10k shouldn't load the output of the Atmel while it is high.

The minimum pulse width that the solenoid can be driven on is effected by the snubber diode. Look at the plot for V(D) Green trace. Note that the diode conducts and V(D) is one diode drop above the 12V rail for the duration it takes the coil current to decay to zero. The coil inductance of 50mH is a WAG, but gives reasonable time constants consistent with what I have measured...
 

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Sceadwian

Banned
Thanks mike, I see now.
 

m2j8

New Member
IOH from the Atmega data sheets

Hi, sorry about the long delay.
For the ATMEGA32:
IOH = -20 mA, VCC = 5V (min 4.2V)
IOH = -10 mA, VCC = 3V (min 2.2V)
 

audioguru

Well-Known Member
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Assuming that the Atmel Vdd is 5V, this will work:

Note that any NFET with a Gate Threshold of <3V, with a Drain current rating of >1A, a drain voltage of >20V, and an on-resistance < 0.1Ω will work.
The Mosfet is not a logic-level one. Its threshold voltage rating is when it is barely conducting only 0.25mA. It is fully turned on when its gate is at +10V not at only +5V.
 

BrownOut

Banned
The 7201 should be fine for this app, though. According to the data sheet, there isn't much lost at 5V gate voltage for current this low.
 
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