Can I use a 1N4004 diode across the relay coil instead of a 1N4148? Not sure what the difference is.
TONY

Actually, you should use 1N4004 instead of the 1N4148.
The 1N4148 is a switching diode designed for hi speed switching applications and has less current capacity than the 1N4148. The 1N4004, on the other hand, is slower, but has higher current capacity that is important when applied as a relay snubber. The collapsing magnetic field of the relay can reach high voltage levels, so use a diode like the 1N4004.

 

I have downloaded MulitSim 13 and found it easy to use for a beginner like me.

Where did you download? Was it free?

I downloaded Mouser's version of MultiSim 14 and it's crippled.

 

Actually, you should use 1N4004 instead of the 1N4148.
The 1N4148 is a switching diode designed for hi speed switching applications and has less current capacity than the 1N4148. The 1N4004, on the other hand, is slower, but has higher current capacity that is important when applied as a relay snubber. The collapsing magnetic field of the relay can reach high voltage levels, so use a diode like the 1N4004.

Actually the 1N4148 was sufficient. The diode will be forward biased when the relay is switched off and it just needs to handle the current built up in the coil when it's switched. Coil is 320 ohms, so only draws about 40mA. 1N4148 can handle 200mA.

 

Actually the 1N4148 was sufficient. The diode will be forward biased when the relay is switched off and it just needs to handle the current built up in the coil when it's switched. Coil is 320 ohms, so only draws about 40mA. 1N4148 can handle 200mA.

Back EMF is not the same as forward conduction. Its in the reverse direction and can reach hundreds of volts.
The 1N4004 (or 40005-4007) is better.

 

Actually, you should use 1N4004 instead of the 1N4148.
The 1N4148 is a switching diode designed for hi speed switching applications and has less current capacity than the 1N4148. The 1N4004, on the other hand, is slower, but has higher current capacity that is important when applied as a relay snubber. The collapsing magnetic field of the relay can reach high voltage levels, so use a diode like the 1N4004.

It's just the diode surge rating that has to equal the relay coil current as the diode only sees that current for a few tens of ms when the relay is turned off.
Thus, for example, the 1N4148 has a surge current rating of 1A for 1s, so it can be used for relay coil currents up to that value without a problem (unless perhaps the relay is being rapidly turned on and off several times a second).

And the diode across a relay coil only experiences a maximum voltage equal to the supply voltage. It doesn't see any high voltage from the collapsing field since it is providing a path for the current generated by the collapse, which limits the spike voltage to the forward drop of the diode.

 

Back EMF is not the same as forward conduction. Its in the reverse direction and can reach hundreds of volts.
The 1N4004 (or 40005-4007) is better.

Back EMF will be limited to a diode drop above the positive supply. Peak current will be the current in the coil when it's switched off. The diode reverse voltage rating only needs to exceed the power supply.

 

Back EMF is not the same as forward conduction. Its in the reverse direction and can reach hundreds of volts.
The 1N4004 (or 40005-4007) is better.

You have a fundamental misunderstanding of the voltage generated by the inductive kick of a coil.
It's true it's in the reverse direction, but that's the forward direction of the diode across the coil, thus the voltage is limited to the forward drop of the diode.
The diode never sees any high reverse voltage.

 

You have a fundamental misunderstanding of the voltage generated by the inductive kick of a coil.
It's true it's in the reverse direction, but that's the forward direction of the diode across the coil, thus the voltage is limited to the forward drop of the diode.
The diode never sees any high reverse voltage.

Thanks for you input crutschow, but I was in the railroad control system design industry for many years, and I understand it clearly. We always used 1N400x diodes (actually 1N4005 back then) for back emf prevention.

I found a couple of demo videos. Have a look.

Anyway...just my recommendation.

 

Where did you download? Was it free?

I downloaded Mouser's version of MultiSim 14 and it's crippled.

I have multisim 12 and 13. You've helped me a great deal. You need a copy?

 

What's the total current draw for that incandescent bulb plus the relay and LED?
You'll need to make sure Q1 has enough base current to drive all of them.

Good catch. In the discrete design there was a small signal transistor to boost base drive for the power transistor. OP removed it and it's still needed. Power transistor load is about 300mA and the flip flop will only be able to provide a fraction of the required 30mA.

I'm using a T77V1D10-12 relay, SLL-LX3044ID-12V red led and a 2w 12v miniature bulb. Not sure of part number cause its preexisting.

What do I need to do?

Tony

 

I have multisim 12 and 13. You've helped me a great deal. You need a copy?

I'd like a copy. Anything would be better than the limited/locked libraries in Mouser's version.

 

Thanks for you input crutschow, but I was in the railroad control system design industry for many years, and I understand it clearly. We always used 1N400x diodes (actually 1N4005 back then) for back emf prevention.

You don't get a large voltage spike when the relay is switched off unless you don't have a snubber diode. With the snubber diode, it never sees a large reverse voltage spike because the voltage polarity on the coil reverses when the field collapses and the diode is forward biased. The only reverse voltage the diode sees is the supply voltage when the relay isn't energized.

Using a component that greatly exceeds requirements shows a lack of understanding and finesse.

 

I'd like a copy. Anything would be better than the limited/locked libraries in Mouser's version.

Can you PM here? ... Need your email.

TONY

 

Can you PM here? ... Need your email.

It's called a Conversation here. Just started one with you...

 

It's called a Conversation here. Just started one with you...

Uploading to dropbox then I'll send you a link.

I'm using a T77V1D10-12 relay, SLL-LX3044ID-12V red led and a 2w 12v miniature bulb. Not sure of part number cause its preexisting.

What do I need to do?

Do I need something else after this 4013 output, but before the BD139?



TONY

 

You should put C1 across the switch and remove the parallel resistor.

The N.O. switch will be off board. So I can not but the C1 across the switch.

TONY

 

Do I need something else after this 4013 output, but before the BD139?

You can do something like this to increase current gain:

EDIT: R1 should be 200 ohms. Or you can connect as a Darlington if the relay will tolerate the reduced voltage.

 

The N.O. switch will be off board. So I can not but the C1 across the switch.

Put it on the flip flop clock input.

 

Thanks for you input crutschow, but I was in the railroad control system design industry for many years, and I understand it clearly. We always used 1N400x diodes (actually 1N4005 back then) for back emf prevention.
.....................

I'm afraid you don't.
And there's the old saw "We've always done it that way".

 

Put it on the flip flop clock input.

If the cap slows the rising edge of the clock pulse too much the 4013 will not trigger reliably.