I want to control a 12V heater coil with 5KHz 5V PWM using the below circuit, but the load voltage does not goes higher than 5V. I made a similar circuit with a MOSFET and the result does not changed. What's wrong with my circuit? How can I fix it?enter image description here


1 Answer 1


Your BJT is an emitter follower - it can't have voltage gain greater than than 1 hence your 5V input produces an output that cannot be greater than 5V.

Think about how an NPN BJT works - the base has to have a voltage slightly greater than the emitter to cause collector current to flow. This means that the emitter voltage MUST be slightly less voltage than the base hence you'd probably get about 4 to 4.5 volts p-p at the emitter for 5Vp-p at the base.

If you want to switch a load connected to 12 volts, put the load in the collector and connect emitter to 0V/GND. This will also work for a MOSFET powering a heater coil. I take it the heater coil is purely resistive - if not then a slight modification may be needed.

If you definitely need a ground-tied load then consider this as an option: -

enter image description here

It should be OK for 5kHz PWM - if it were >50kHz I'd want to analyse it in more detail to see what the switching losses would be like.

  • \$\begingroup\$ And replacing Q1 with a MOSFET will give similar results because you will have 5V on the gate (from the microprocessor) and since you need some voltage to turn the FET on (Vth), R_LOAD will see 5V - Vth = something less than 5V! what you should do is put the R_LOAD in the collector (drain if MOSFET) lead. Then when the transistor turns on, the collector (drain) will go near ground and you will see almost 12V across the R_LOAD. \$\endgroup\$
    – hwengmgr
    Commented Mar 24, 2015 at 18:34
  • \$\begingroup\$ I have a NTC that has common ground with heater, is there any way rather than placing load on collector? I can't put it on collector because the NTC must connects to 5V and GND. \$\endgroup\$
    – Vahid
    Commented Mar 24, 2015 at 18:46
  • \$\begingroup\$ You can use an NPN BJT to turn on a Pch MOSFET that has its source connected to 12 volts - this will produce a ground referenced load driver but you have to watch your PWM speed. What is it? \$\endgroup\$
    – Andy aka
    Commented Mar 24, 2015 at 18:49
  • \$\begingroup\$ I have a 5KHz PWM. \$\endgroup\$
    – Vahid
    Commented Mar 24, 2015 at 18:52
  • \$\begingroup\$ I've added a circuit and here's the SE link: electronics.stackexchange.com/questions/67343/… \$\endgroup\$
    – Andy aka
    Commented Mar 24, 2015 at 18:56

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