Varistor (TNR) versus Zener Diode?

When the energy dissipation capability is high, the varistor is preferred since that is the major limitation of the zener diode.

When the power requirements are not to low, since the standby power consumed by varistor is greater than the one of the zener.

When the working voltage is high (>200 vdc).

In general high power circuits the varistor will be preferred, low power accurate circuits, zener diode will be preferred.

Varistor Diode

The varistor varies the current up or down,the zener diode restricts the voltage to the value of the diode break down.

Use a 5 V regulator with 4 silcon diodes in the ground return line. There are three pins to a regulator:- (1) DC volts IN (2) Ground (normally the middle pin) (3) DC volts OUT You apply your 12V to DC volts In (1). To the ground pin (2) you connect your four diodes in series (4 x IN4001s should do it) before it connects to zero volts line. You must observe the polarity of the diodes as shown. This circuit lifts the 5V normal output by 0.6V for every diode added. So, 5V + (4 x 0.6V) +12 .......................(1).. IN [5 V regulator] OUT .(3)..........................+ 7.4 V (2) I I + Diode - I + Diode - I + Diode - I + Diode - I I I ------- OV Note: you will have a 4.5 Volt drop across the regulator. If it passes a full 1Amp then the regulator will need to dissipate 4.5 Watts and so it will get hot! You will either need to use a heat sink or alternatively use a voltage IN of 9V to reduce the voltage drop and the power disipated. Of course, all this may not matter too much if the current you intend to draw is low.

zener diode (TAZ for ov protection) is more precise and has faster response time

varistors can take much higher currents, but voltage treshold and speed is not as good as withdiodes

I want to stop the transistor from blowing on a Timer used on motorcycles, would these help?

total voltage 14v current 800mA