I intend to use the following flyback circuit for my relays (third circuit circled in red):

flyback ckt

Here are my relay specs:

1) G6DN - 5 A 250 VAC contact rating

Coil voltage = 5VDC

Coil resistance = 227 ohms

2) T9G series - 30 A 250 VAC contact rating

Coil voltage = 5VDC

Coil resistance = 28 ohms

I intend to use this NPN transistor to drive the relay:

Part number - MMBTA42LT3G

Max Ic = 500 mA

Max Vce = 300 V

I am going to use a 24 V zener with a series diode as shown. PCB has space constraints and hence I would like to use the tiniest packages of diodes which guarantee safety of the circuit. Below are my questions:

1) What should be the minimum reverse breakdown voltage for D2?

My guess is that Anything more than 10 V will do the job because VCC = 5 VDC.

2) What should be the packages of D1 and D2?

I believe package has to do with allowable peak current and power dissipation. More current and power calls for a bigger package. However I am clueless about how to calculate these quantities.


2 Answers 2


1: Your guess is correct, but I would just use a common 1N400x diode. They're dirt cheap and have reverse breakdown voltages measured in the hundreds or thousands of volts.

2: As for the package, the only thing that really affects what package you use is power concerns. If you need to dissipate a huge amount of power (which you don't), you would go with a TO-220 with a heatsink. Moderate power, a D²PAK would work well. But this isn't going to need that much power dissipation, because the current is going to be in very short pulses when the relay turns off, so you can probably just use whatever package you want, as long as the diode inside is rated for the current you need.

If you want a more precise answer, look for thermal resistance specifications; there should be a junction-to-ambient thermal resistance spec measured in °C/W, which tells you how much hotter than the surroundings you can expect the junction to get, and you can then look at the device's maximum temperature specs to know that it'll work.

  1. Yes, so you can use an LL4148 diode if you want. Or a 1A M5 diode.

  2. The zener is the only one of concern (because the dissipation will be 24/0.7 times larger) , and the inductance is not specified for the relay- it would be a good exercise to put an upper limit on it from the relay operating time and estimate the dissipation average and peak (average requires your maximum operating frequency). That’s a small relay so I would expect anything not pathologically tiny will do.

  3. (Not asked) I question your choice of transistor- high voltage transistors have crummy hFE and Vce(sat). At least the 2nd breakdown SOA looks acceptable. Consider a 60V or so MOSFET instead such as AON2260 (there are many possible choices).

  • \$\begingroup\$ For T9G relay, coil current will be 180 mA in steady state. Is it correct to assume that max current through zener will be 180 mA thereby requiring a power dissipation of 24*0.18 = 4.32 watts. A 5W diode comes in SMB package which can be used. OR will the actual current be much lesser than 180 mA? \$\endgroup\$ Nov 3, 2018 at 18:51
  • \$\begingroup\$ The PEAK current will be 180mA,but it will die off quite quickly. 5W would be gross overkill in this application. \$\endgroup\$ Nov 3, 2018 at 18:55
  • \$\begingroup\$ Assuming worst case switching frequency to be 1 Hz, what would be a safe wattage? Can I go ahead with a half watt zener? \$\endgroup\$ Nov 3, 2018 at 19:03
  • \$\begingroup\$ Without doing any calculations, yes. 500mW will be just fine. \$\endgroup\$ Nov 3, 2018 at 19:51

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