[SOLVED] using 2x 1n4004 diodes for back emf suppression with 120volt mains

Thread Starter

kingArgon

Joined Mar 20, 2011
33
I want to control an US outlet with a microcontroller. I have a relay that can handle 10A @ 125VAC. I need a back emf protection diode by i only have 1n4004 diodes. Can i connect multiple 4004 diodes in series to create something equivalent to a 1n4007?
 

BillB3857

Joined Feb 28, 2009
2,570
The back EMF diodes are to protect the device driving the coil of the relay, not to protect the contacts. To protect AC contacts, it is not unusual to use a capacitor/resistor series unit. Networks such as that are called snubbers.
 

Thread Starter

kingArgon

Joined Mar 20, 2011
33
The back EMF diodes are to protect the device driving the coil of the relay, not to protect the contacts. To protect AC contacts, it is not unusual to use a capacitor/resistor series unit. Networks such as that are called snubbers.
Thats what i want to do; is protect the microcontroller thats enabling the relay.
 

jjw

Joined Dec 24, 2013
823
The voltage and and the current needed to drive the coil are important, not what happens in the contacts.
If the coil voltage is below 200V, 1N4004 is OK.
 

ian field

Joined Oct 27, 2012
6,536
The voltage and and the current needed to drive the coil are important, not what happens in the contacts.
If the coil voltage is below 200V, 1N4004 is OK.
The bare minimum back emf diode clamps the voltage spike - that means that current flows but decays in the coil, it causes slow contact release with excessive arcing on the contacts.

Tyco released an appnote covering this - last time I looked it was still on the Element14 website.
 

jjw

Joined Dec 24, 2013
823
The bare minimum back emf diode clamps the voltage spike - that means that current flows but decays in the coil, it causes slow contact release with excessive arcing on the contacts.

Tyco released an appnote covering this - last time I looked it was still on the Element14 website.
Ok.
I just answered to OP:s question if he needs two diodes in series ( maybe to increase max. value for the reverse voltage )
 

Thread Starter

kingArgon

Joined Mar 20, 2011
33
Ok.
I just answered to OP:s question if he needs two diodes in series ( maybe to increase max. value for the reverse voltage )
correct! i want to increase the reverse voltage. I can use just one 1n4004 with MAINS 120v and this will be adequate to protect against reverse voltage?
 

ian field

Joined Oct 27, 2012
6,536
correct! i want to increase the reverse voltage. I can use just one 1n4004 with MAINS 120v and this will be adequate to protect against reverse voltage?
The diode is forward biased during the emf spike and clamps it close to 0.7V - its reverse biased when you energise the relay, and only needs a reverse voltage equal to the relay coil operating voltage.
 
Top