Glow Plug Driver

Ábel István
(EE)  ©2002

Switching mode power supply for driving your glow plug from your car battery, based on the pulse-width modulation control circuit SG3524.
( Signetics, Texas Instruments ).

Model engine

It has some advantages against the usual designs with NE555, and it may have some disadvantages too.

You are free to build this circuit described below for yourself. Commercial usage is prohibited without my written permission. The circuit is as is. No guaranty of working and not blowing your glow plug.

GlowPlug Driver Schematic


Input power:


Click to get SG3524 Data Sheet

Download SG 3524 data sheet from
Signetics Texas Instruments


Glow Plug Driver Board Component Layout


The 5V reference voltage supplied by SG3524 is filtered with C3.
An adjustable reference voltage for the error amplifier is prepared from this 5V by the R3,R4,R5 resistor network, and by the HeatingPot.
R6 ensures keeping at GND the error amplifier input, in case of loose potentiometer.
The voltage sense network is realised with R7,R8,R9,R10. These two networks are designed to keep the input voltages in the common mode input voltage range of the error amplifier.
Output current is measured trough the RCL(1..3) resistors, and fed into the current limiting inputs of SG3524.
D4 and D5 is there for protectional purposes.
Current limiting is active when 200mV is reached at the current limiting amplifier inputs. Output current can be measured externally by sensing the voltage across these resistors ( in total 0.033 Ohm ).
R1, C1 sets the 40 kHz switching frequency.
R2, C2 are the factory defaults for the compensation input.
BDX34C is fed through R11, by the paralleled outputs of SG3524.
Heatsink ( 7 K/Watt) must be installed on BDX34C.
D1 is for input, D3 for output polarity protection.
The D2,L2,Cpuffer2 network is the main conversion loop.
Input filter is built from L1,Cpuffer1,Cpuffer3.
The purpose of R12 is to give a minimal load for the converter.

Glow Plug Driver Printed Circuit Board 

(from component side)

The circuit is easy to build, and works reliable. It has to be built into a steel case, because of EMC ( Electro Magnetic Compatibility ).

Parts List:
RM = 5 mm
Coil ( ferrit core )2L1,L21mH ( described below )
Ceramic capacitors1C21n
Electrolit capacitors3CPUFFER 1..3470µF/35V
Resistors, 2/3 W2R4,R123k3
Current sense resistors3RCL1,RCL2,RCL30.1 Ohm / 2 Watt
Fuse holder1FUSE HOLDER 
PWM controller IC1IC1SG3524
Heat sink1HEATSINK7.0 K/W
Heat sink isolator1ISOLATORTO-220

L2 is a 1..3 mH inductor custom made on ferrit core with high current handling capability. I choose a toroid ferrit core with diameter of 17 mm, and a relative permeability of approximately 1600. 34 wounds on this core from a 1.1 mm diameter enameled copper wire made it full and just fitted the needs.

Cpuffer2 has to be low ESR type if long heating periods are required. Because of the excessive amount of current flowing through it, during a longer motor heating session, heating of the capacitor occurs.

Tips for modification:

As You may noticed the circuit is basically a switching voltage regulator with all the technology's bells and whistles. This means with some modification the circuit is useable for other purposes, no only to glow plug heating.

  • For example You can use it as a stabilised current limited voltage supply. If the output voltage doesn't fits your needs, you can modify it. The output voltage is determined by the R3,R4,R5,P1 voltage dividers output voltage ( adjustable ) and the output voltage sensing R10,R8,R9 voltage divider. The SG3524 maintains the output voltage such a way that the input voltages on it's input pin 1 and 2 be equal. The circuit has some recommendations in the input voltage range on these pins, but if you change the R10,R8,R9 divider resistor values you dont't have to bother this. For example if You would like to double the output voltage, to reach around 6 V, you can connect one 10k resistor in parallel with the ( R8 - R9 serially connected ) resistors. The circuit is a step down regulator, you won't be able to get voltages out from it greater then the input voltage.
  • You can use the circuit as a constant current battery charger. If You set the P1 potentiometer to enough high output voltage, and a battery is connected to the output terminals, the SG3524 will limit the output current at the RCL1, RCL2, RCL3 determined current value. The current limit is set by these resistors in the following way: when the output current through these resistors drops a 200 mV voltage then the current limit activates ( there are 3 resistors in parallel to handle the resulting heat ). For example if You want to limit the output current to 100mA then you have to remove RCL1, RCL2, RCL3 and replace with a single 2 Ohm resistor ( 200 mV / 100 mA = 2 Ohm ). Since a 100mA current will dissipate 200 mV * 100 mA = 0.02 Watt heat only, there's no need for 2 W resistor in this case, a simple 0,6 W resistor will do the job. Warning: the current limit of the circuit with the original resistor values is set to 6 A, this means I definitely not recommend to test the circuit with a battery at this current rate, because 6 A may explode your battery.
  • Further improvements: if you don't like the heat dissipated on the circuit parts, you can make some efficiency improvements: instead of the listed rectifiers You can use MBR series of Schottky rectifiers, for example MBR 1060 is more then enough ( these kind of rectifiers may be extracted from faulty PC power supplies ). You can decrease inductor losses by applying greater sized inductor cores, with greter sized wires, or you can use 2 wires in parallel during the winding of the core ( also a good source is a PC power supply ). And if you would like to decrease heat dissipadet from the capacitors, then You can use greater capacitors ( from PC power supply ).
  • If you don't need the input and/or output polarity protection You may install a simple wire instead of D1 and D3.
  • The components C4 and R14 are usually not needed, but the place is available, if necessary ( ringing suppression ).

    The circuit currently not available from me, but You can build it yourself easily. I currently manufacture interfaces to connect your radio transmitter and your PC to use R/C flight simulators, there are 2 versions available: a COM port PIC interface for FMS, and a GAME port interface realised with digital potentiometers. You can learn more about the interfaces at the interfaces page. If you'd like to buy from me, feel free to email me. Also available from me: custom foam wing cutting, you can learn more on it ( currently only in hungarian ) at the foam page.

    The circuit is free to build for yourself in 1 piece quantity.
    Pricing information is available on request.

    If you build this circuit, and find it usefull, feel free to notify me :-)

    If you are planing to by from me, then e-mail conversation is absolutely a must.