Back EMF diode

[surreysparks]

What specific diode would I need for 12v DC locks, to reduce back EMF, as when you buy them on there own there is no diode unlike the paxton stuff etc that comes with one??

 

[UKMeterman]

Try a 1N4002 or 1n5402 1N4002 - -- - DIODE, 1A 100V | CPC

 

[happysteve]

1n4148

 

[netblindpaul]

How can you specify a specific diode when you don't "KNOW" what the back EMF from the coil is?
There is not enough info in the OP to tell you what the back EMF is for the units that the OP is asking about.

 

[telectrix]

agree with paul. you need to know what emf and what current the diode is likely to need to cope with.

 

[UKMeterman]

Well,
At a rough guess it is one of these Fail secure lock release, 12v dc fail secure lock release or similar, the diode is fitted in reverse parell with the supply, when the supply is turrned off circa 1 amp is going to flow for a small fraction of a second through the diode untill the energy is lost as heat in the winding of the coil. A very generous guess would be 10H for the coil so time constant of this circuit would be =10/30 0.333 seconds after 1 second the current will be virtually nill. This is totally within the capability of the 1N4002 I suspect I may have grossly exgerated the inductance of the lock and so the current will be cut off even sooner. This are typical diodes and so any silicon diode will act in about the same way.

 

[UKMeterman]

The 1N4148 might be a bit tight in this application, a 1N5402 would have a safety factor of about 3:1

 

[Engineer54]

What specific diode would I need for 12v DC locks, to reduce back EMF, as when you buy them on there own there is no diode unlike the paxton stuff etc that comes with one??

Surely the manufacturer's data that comes with the ''new'' clock will give this information??

 

[tazz]

In4148 is a signal diode???, In4001/2/ is rectifying diode, IN4001 will stop back emf but is not what you need. Whith coil locks, all manufactures supply a varistor to protect keypad. A varistor will prevent over voltage, and suppress high voltage spikes.
View attachment Varistor.pdf

 

[johnbyrne]

A voltage dependent resistor would not be polarity sensitive, 0 12 the same as 12 0. Years ago 555 timer chips were triggered by a negative pulse to pin 2. This pulse could be tiny, so small that even a silicon diode at around 0.6V when forward biased by the back elf would not stop the timer from starting, the solution was to use germanium diodes which only have 0.1 V when forward biased. The point being the back elf is of the opposite polarity to the collapsing coil voltage so diode reverse biased with a small forward voltage would deal with problem better than a voltage dependent resistor which would do better giving generalised surge protection.Cheers

 

[tazz]

so all the manufactures of door access systems, such as ACT, Cardlock, Honeywell and manufactures of coil locks MEM, Eurolocks, Ducan supply varistors just for fun.....they suppress the high voltage spikes, diodes will block reverse emf, but will not suppress forward emf

 

[Dcf]

What door entry system requires a diode for a back voltage then?
none I've come across in 27 years of dealing with them
but I await to be corrected
every days a school day n that

 

[tazz]

If you use maglocks, DCF they are not required, only used with door coil locks. Maglocks have varistors built in

 

[johnbyrne]

Hi thanks for the reply. I thought I had accepted in my last post that varistors had a role as General purpose suppressor. However because they are not polarity sensitive and also need to allow the circuits normal voltage they will not hold the reverse voltage down to 0.6V or.1V that diodes would. Cheers

 

[tazz]

As you keep pointing out reverse voltage, not high voltage suppression....this is why access manufactures prefer varistors to suppress high voltage. emf produce by a coil can only be ac, best you will achieve with a diode is high voltage...half rectified.