Post your last circuit.

 

Hi,
I am using Sgt Wookies Circuit posted earlier in this threat, except that I am using it at 18VDC and I have added a pull up resistor to Q1. I think that is where my issue is... double checking my math I believe that I should have a 5142 ohms resistor. Which would mean in need I need a 5.1K ohm resistor. If all of my math is correct I got the wrong resistor by accident, 51K instead of 5.1K (after all that I realize now that I would probably need a 5.6K ohm resistor). Does that math check out? Would the 51K ohm resistor cause the problems that I described in my circuit? Thanks always!
-Eschatt

 

this circuit can be used to activate a relay via a transistor .
as first the transistor is working as an on/off switch ! the transistor is
switched on once a negative signal appearing on its base and the voltage accross the relay wil b vdd-vbe =18 -0.7 =17 volts when the transistor is off which mean that no negative signal on its base and the voltgae accross the relay is approx zero

 

my friend u cannot say that this is the schema of a comparator since u dont have a reference voltage ! so this circuit cannot be named as a comparator its an activating relay via transistor circuit u can determine the kind of transistor if its pnp or npn depending of the triggering signal on the base if its positive so u require to use npn otherwise if its negative pnp must be in use ,dont forget to reverse the polatiry supply when reversing the use of transistor cheers !!

 

Sorry, I should have clarified, I mentioned earlier in the thread that I am using a comparator as the control for the transistors/relay. I have added a pick up resistor with the help of SgtWookie because there is not enough current provided by the comparator to turn "on" the transistor. The pick up resistor is 51K ohms (I know it should be 5.1K ohms if my math is right) connected to 18VDC. Could the pick up resistor be why the voltage to the relay is only changing by 1/2-1VDC? And Zimbarak, could you explain what you mean when you say, "dont forget to reverse the polatiry supply when reversing the use of transistor" I don't quite understand fully what you are saying. Thanks!
-Eschatt

 

if the signal comming from the comparator is more then 2 volts which mean is enough to switch on the transistor otherwise if it was 2 votls and more and the TR will not switched on ! which mean that the current passing in the base of tr is not enough so u can reduce the value of R1 in order to get enough current for triggering the TR ! or if the comparator output is not enough though u can play with the resistors that can indicate the reference voltage of the comparator

 

hi,
I am stumped, I have been experimenting, looking at things again from the start. I found another circuit design that worked at first (circuit is attached). Though shortly after I set it up, it stopped working and burnt through another replacement transistor. Does anyone see any blatant problems with the circuit? Thanks.
-Eschatt

 

Sorry! I realized I made a simple mistake, the voltage from the base-emitter was too high. Thanks anyways.

 

Stick the NPN at the bottom end of the relay/solenoid and you can 'sink' the relay on.