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I know that relays are used for heating element or any electronic other device with high wattage/high current. But for my application, I am using a Nichrome wire as a heating element and only want to provide power of 100mW, for that I need a current of approximately 71mA current(which I already calculated). I will be using a microcontroller to control it and will be turning it on and off at regular intervals. I was planning to use BJTs as my current driver circuits but then someone suggested me about relays. The maximum current that the MCU pin can provide is 50mA. I could only find applications with high wattage that use relays for example in automotive applications. Can someone suggest me if using relays would work for my application? Or if they would work which I think they will, would it be better than using BJT as a current driving circuit?

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    \$\begingroup\$ At those power levels, probably. But a MOSFET would be perfect (and quiet) and a BJT would still be tolerable. \$\endgroup\$ Oct 7 '20 at 22:26
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    \$\begingroup\$ To put things in perspective, the coil of a typical e-commerce relay requires a current not unlike that which your load requires, and is more inductive. So it's almost harder to drive the relay, than to directly drive your load. \$\endgroup\$ Oct 7 '20 at 22:27
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    \$\begingroup\$ We cannot answer with certainty if you can drive the relay directly from the MCU - for that you need to check the control current (or voltage and resistance of the driving coil). Generally speaking, it can work. Is it a great idea? No. If you are looking for isolation, it would make more sense to use an optocoupler \$\endgroup\$
    – thece
    Oct 7 '20 at 22:56
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    \$\begingroup\$ 100mW is not "high wattage" It's not even "low wattage". It's "trivial wattage". You don't need to use relays unless you need electrical isolation. \$\endgroup\$
    – Kyle B
    Oct 8 '20 at 0:48
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    \$\begingroup\$ I would probably use a MOSFET. But a BJT will work fine, too, as long as you drive the base with at least 3 mA or so to get it into saturation. \$\endgroup\$
    – mkeith
    Oct 8 '20 at 2:55
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A PWM transistor switch can regulate the average current, or you can choose a better uC such as a PIC12 that supports max. current out of low 200 mA or 150 mA high.

100mW @ 71mA = 1.4V or 20 Ohms choose duty cycle from 1.4/Vdd supply voltage ratio and add a flyback diode for wire inductance.

ZTX450 NPN. 200~300 Ohm base drive from 3V, PWM 40%.

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    \$\begingroup\$ Note: Relays do not PWM very well at cycles under x minutes. The avalanche rated MOSFET is your best choice then fast PWM cycles are no problem and no diodes. It appears you are using a single cell battery at just less than 1.5V. The BJT will have a forward CE voltage drop of either 0.7 or 1.4 volts, not leaving much for the nichrome wire. \$\endgroup\$
    – Gil
    Jan 17 '21 at 21:45
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    \$\begingroup\$ I did not suggest any relays and a ZTX450 drops only 20mV @ 71 mA ($0.81). (Rce<0.3 ohms) while A low threshold FET with 1.5V needs a Vt= about 0.7V to have this resistance to conduct well at 1.5V, These ONLY come in SMD packages and cost about the same with about the same performance. Rb for the BJT needs to conduct about 7mA. or ~R= 2.3V/7m~300 ohms and with PWM controls the average current to be the correct temperature. \$\endgroup\$ Jan 18 '21 at 4:03
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My question has been answered in the comments which I am going to summarize here: As @mkeith and @KyleB have mentioned that relays won't provide any advantage unless there is a requirement of electrical isolation. Further as @ChrisStratton mentioned, it is harder to drive relays. A better solution would be to use MOSFETs/BJTs as most of the comments suggest and another way is to follow answer by @TonyStewartEE75.

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