comparator question

Audioguru

Joined Dec 20, 2007
11,249
The datasheet for the relay shows four 5V coils. One is 50 ohms so its current is 5V/50 ohms= 100mA. Another coil is 167 ohms so its current is 5V/167 ohms= 30mA.

An LM339 is made to drive low current electronic circuits not a high current mecahnical relay.
 

Thread Starter

count_volta

Joined Feb 4, 2009
435
The datasheet for the relay shows four 5V coils. One is 50 ohms so its current is 5V/50 ohms= 100mA. Another coil is 167 ohms so its current is 5V/167 ohms= 30mA.

An LM339 is made to drive low current electronic circuits not a high current mecahnical relay.
When it says a 5V relay or 5V coil, what exactly does that 5V mean? Is that the max voltage that can be applied to the coil before its damaged?

I'm not sure which one of those four I have. My relay looks like this picture exactly except it says DC-5V. And what the heck does that 5V mean?



I am planning to have a buffer stage and have the comparator drive a mosfet switch, which will then drive the relay, but then I need to know what the min current that will activate the relay is in order to properly design the mosfet stage.
 

billnow

Joined Aug 4, 2010
18
The datasheet for the relay shows four 5V coils. One is 50 ohms so its current is 5V/50 ohms= 100mA. Another coil is 167 ohms so its current is 5V/167 ohms= 30mA.

An LM339 is made to drive low current electronic circuits not a high current mecahnical relay.
Aha! I didn't notice that they had a .PDF version of the data sheet. The .PDF version makes it clear that they have a variety of relays with different coil current requirements. The most sensitive version, as Audioguru points out, requires 30 mA which is beyond the rating of the LM339.
 

Audioguru

Joined Dec 20, 2007
11,249
The relay is made in a different world. Their language and common sense are different to ours.
In most of our world, a 5V relay coil works properly with 5V but the minimum voltage might be 4V and the maximum voltage might be 7.5V which are shown on the datasheet.

Years ago I used an LM393 (a dual comparator like an LM339 quad comparator) with an American 5mA relay coil. They were inexpensive and they worked perfectly.
 

Thread Starter

count_volta

Joined Feb 4, 2009
435
I just measured the resistance of the relay coil by connecting the coil in series with a 1kΩ resistor, measuring the voltage drop across the 1k resistor and solving the voltage divider equation.

I got relay coil = 123.6Ω measured.

Looking at the data sheet for the Coil table, we have for 5V coils, 125Ω the power rating is 0.2W.

P = I^2*R

Solving for I we get 40mA. LOL no wonder the comparator can't drive it.

So now I can design the transistor buffer stage. I will report on the results of the experiment.

This whole thing is part of my binary clock project thread btw. I will show what the comparator and relay are for once I get it working.
 
Last edited:

Thread Starter

count_volta

Joined Feb 4, 2009
435
Thanks Bill but I'm still gonna design my own version. That is half the fun. The other half is seeing it work, and having the satisfaction of not electrocuting one's self.

Although this does show me how to connect the diode.
 

kingdano

Joined Apr 14, 2010
377
this was a great thread, interesting and educational.

cool driver circuit bill.

i may steal that sometime to pretend i can do analog!
 

Thread Starter

count_volta

Joined Feb 4, 2009
435
After some thought, and trying mosfets and other bjt configurations on paper, I came up with this. Yea the emitter follower seems to be the best for this particular situation. At least thats what I came up with. Now to try it on a breadboard.

 

Thread Starter

count_volta

Joined Feb 4, 2009
435
Just built the circuit on breadboard and it works. Had to replace the 10kΩ resistor with a 1kΩ, because it limits the current in the base of the emitter follower too much. The voltage at the relay coil is 4V and apparently thats enough to completely close the switch. The voltage at the base is 4.67V (i.e. the output of the comparator after its connected to the emitter follower)

That is the only downside of the emitter follower that the output voltage is decreased by Vbe, but apparently that isnt too much of a downside in this case.

Btw, I built it without the diode for my test, but I will definitely add the diode for the final design.
 
Top