Help creating a circuit

[MikeMl]

There is not a coil resistance on the box for the relay I purchased, I can check with the meter when I get off work. It is a 30a 5 pole 12v relay though

That is likely to have a low coil resistance, probably less than 70 Ohms. To drive such a low coil resistance, this will effect the parts of the circuit you are asking about.

 

[thomas188]

The resistance on the coil is 90ohm. So I need to wire up my circuit identical to the schematic you have?

 

[MikeMl]

Answers to your questions:

"Can I use a 5.1V Zener?" Not sure. I'll resim the circuit to see.

About the resistor values & functions:

The LM339 has no internal pull-up. It can only sink current. R7 is there to source current to the LM339 output pin and the base of the transistor Q1. The base-to-emitter junction of Q1 conducts when the base voltage is ~0.7V, so if the LM339 output is off (not sinking current), almost all of the current that flows through R7 flows into the base of Q1. The base current of Q1 causes it to turn on, sinking current through the relay coil. The min. base current of Q1 should be ~1/15th of the relay coil current, or Ic = E/R = 12/90 = 133mA, so Ib should be ~10mA.

(Here is where I must check if Q1 can drive a 90Ω coil, especially if the Zener voltage is lowered to 5.1V)??? Now, R7 is supplied from 8.2V, while the base end is at 0.7V, so it has 8.2V-0.7V = 7.5V across it. This makes the base current 7.5/1.5K = 5mA, which is marginal for the 90Ω relay. Lowering the voltage at the Zener would be bad. To compensate for that, R7 will have to be fed from the unregulated 12V.

R2 could be eliminated; it is supposed to help turn off Q1, but the output transistor in the LM339 can do that all by itself

R3 provides a modicum of positive feedback to the non-inverting input of the LM339. It feeds back about 1/100th of the 0.7V swing at the base of Q1 to provide ~7mV of hysteresis relative to the TPS output. This should prevent the relay from chattering if the throttle is position exactly at the trip point of the circuit. This may have to be increased slightly (by making R3 ~1meg) if the circuit chatters.

 

[MikeMl]

Here is a modified circuit. 5.1V Zener, improved base drive, higher sink current for 90Ω relay coil, higher hysteresis (~ 30mV, see second plot)

 

[thomas188]

So I can use the 5.1 zener and adjust the potentiometer to the desired Vref and everything should run ok.

 

[MikeMl]

So I can use the 5.1 zener and adjust the potentiometer to the desired Vref and everything should run ok.

The pot adjusts the trip point from 0V to ~5V.

 

[thomas188]

Thanks so much! I will try to get this board made tonight and see how it goes...

 

[thomas188]

I made the circuit tonight, I just put it on the bike and I ran into a little bit of a problem. After checking over everything I noticed that I put the .1uf and 10uf in opposite places. Would this cause the maximum voltage out of the potentiometer to be .808v? When I adjust the potentiometer it goes from .808v to 5.1mv. prior the the potentiometer I am getting .808v as well, not a 5.1v. Any idea on what I did wrong?

 

[Reloadron]

Check the polarity of your zener diode. Possibly the anode and cathod are reversed.

Ron

 

[thomas188]

Ok, should the edge with the black line go to the ground?

 

[MikeMl]

Ok, should the edge with the black line go to the ground?

No, the band on the zener is the cathode (positive). If thats the way it is wired, try disconnecting the power pin at the LM339. If it is the same one you showed in your photo, it is likely blown because of the way you had it wired up...

 

[thomas188]

Ok, I will check, the good news was that since it was at .808v-5.1mv the input from the TPS was high enough to have it on and the voltage out after the transmitter was ~6v. It wasn't enough to light up the LED, but it's plenty to trigger the relay correct?

 

[MikeMl]

What LED? Where is it placed?

 

[thomas188]

I will be running a solenoid and LED off of the relay, but I didn't have the relay wired in last night so from the transistor output i connected a LED in place of the relay, but it wasn't enough V to light it up (it's a 12v led, but the output was ~6v)

 

[thomas188]

Ok, I switched the zener around and I was able to adjust the voltage as expected ... Vout from comparator was around ~.33v before turning the throttle and around ~.7ish volts after turning the throttle. The output from transistor was around 3v before and 6v after, this still didn't trigger the relay. The comparator is suppose to put out 1v when V+ is greater than V-, so should I go ahead and replace it since it is only increasing by .4v. If so it's just one more trip to Radio Shack .

I really really do appreciate all the help that everyone has provided me, especially giving thanks you Mike!

 

[Reloadron]

Ok, should the edge with the black line go to the ground?

The black edge or band is the Cathode and it goes to the (+) while the other end is the Anode and it goes to ground or common as Mike drew it. Hope that helps.

Ron

 

[thomas188]

I just got back on this project from the time off. I double checked everything and replaced the comparator and I have the results below.

0<Throttle<90 Vout=0
90<Throttle<100 Vout=1.5v

This isnt enough to trip the relay, what do I need to do to trip the relay?