# SMD vs THR Transistor for Relay

Designing a Relay control from a MCU's GPIO, there are some requirements to take into account.

• Coil Voltage: 3V
• Coil Resistance: [36-45]Ohm
• Relay Voltage: 250Vac // 30Vcc
• GPIO Voltage: 3V3
• Max GPIO Current 4mA // 20mA (depending on pin)

So, even when relay are THR, I'm looking for SMD transistor instead of THR for controlling the relay.

I would like you to share yout opinion about which would be the best option to make this work.

THR transistor:

• BC550
• 1N2222
• 1N3704

SMD transistor:

• 50C02CH

My questions are:

• Will transistor be able to dissipate power?
• MCU, would it deliver enough current to activate the base?
• Do you have any recommendations?

• EDIT: Added COIL Resistance [36-45]Ohm
• One of the most important parameters in selecting the transistor is how much current it will have to handle. In this case it is determined by the relay coil resistance which you have not included (or coil operating current). – Kevin White Jun 23 '15 at 14:16
• Edited on question. I'm playing with two relays which have a coild resistnace [36Ohm - 45Ohm]. – Yolco Jun 23 '15 at 14:29

Pick a transistor that will be able to easily handle the current (Ic(max) >> relay coil current), and check the SOA (Safe Operating Area) to ensure that it can withstand full coil current at 3*Vcc for a millisecond or so. The voltage rating should be at least 3-5x Vcc.

1. Chances are you can use a good SOT-23 transistor but you might need an SOT-89 or larger, depending on the coil current.

2. The transistor will be able to dissipate power. Whether it is enough or not, depends on how hard you drive it, the coil current and the package and mounting conditions as well as the maximum ambient temperature. Lots of factors.

3. If the coil current is low enough, then the GPIO will be able to deliver sufficient current. Think of coil current/20 or so, and ensure that the output voltage is guaranteed at that current (otherwise you cannot guarantee the base current and the transistor may not be sufficiently 'on').

4. Recommend you find the coil current- that is the missing factor in the design. The design will be quite different if the coil current is 300mA vs. 20mA.

I would also suggest considering a small MOSFET instead of the BJT. Inexpensive MOSFETs can handle lots of current in a small SOT-23 package for a very reasonable price. Eg. AO3414 which is only 0.07$\Omega$ with 2.5V drive. Even a 0.5A relay with the weakest GPIO you mention would be fine.

I think you should use whatever fits your budget, BOM, capabilities, and requirements.

• Will transistor be able to dissipate power?

Your application is not high-power in the first place, so any TO-220 or TO-263 will more than suffice, and possibly be overkill.

• MCU, would it deliver enough current to activate the base?

The 20mA should suffice for a transistor with a sufficiently high hfe.

• Do you have any recommendations?

You forgot the last specification: coil current. You should use whatever transistor has a high enough gain and current handling for the coil.

• I'm plawyingwith to relay which have a coil resistance of 36Ohm and 45Ohm, respectively. – Yolco Jun 23 '15 at 14:37