Welcome to Electro-tech-online !I need some help confirming this diagram I have made. What this circuit does:
There is a switching output from an automotive ECU to the Fuel Pump ECU. It is either 2.5V or it is 5.0V. Basically a trigger to change the fuel pump voltage between 9V and 12V. I am sending this signal to a Comparator as the trigger, and I am using a 5.0V regualtor for the referance voltage. Once the ECU kicks the voltage to 5.0V the Comparator will trigger and send out the 12V signal to the NPN transistor. This NPN Amp is rated at 25 Amps, so there s/b sufficient current to trigger the relay.
Where I am not sure, is if I need to add a diode, a resistor, or a capacitor anywhere. And if I even have this diagram drawn fairly accurately.
The purpose to this is to switch over to a voltage enhancer to increase the speed of the fuel pump motor increasing its output.
TIA
Stu
Sorry, the non-inverting symbol doesn't show up very well on your schematic. Your good there. What if you went with a different comparator, say an LM339 that can deliver more current? You could then eliminate the transistor and pull-up resistor on the output. You would have to swap your inputs and use the inverting side.
View this schematic. It should work with what you need.
A snubber is usually an RC circuit used to suppress the rise of voltage or current to protect contacts of a relay from welding. Of course this depends on what load you are driving as well. They are also used with solid-state devices to protect them as well.
One thing I forgot to note on the schematic is the values of the R and C for the snubber. You can try a 1KO, 1W and .22uF cap.
That's a good point about the automotive relay audioguru. The LM339 is capable of sinking 16mA. The LP339 can sink 30mA. What is the coil resistance of the relay you are using. Automotive or not, that is what you need to know to select the proper device to drive it. The LM/LP339 is twice the size as the LM393, but as audioguru indicated, is only good for up to 6mA. You can go with a quad comparator like the LP339 if you don't mind only using one of the comparators. But if your coil resistance is less than 400 ohms (750 ohms for the LM339), you will need something beefier. The relative difference in price between a dual and quad is negligable. This is an automotive relay you are using, correct?
I have a question/comment. For the 2.5/5v signal, are you sure it is a solid 5v rather than say 4.5v? I ask because if you set your VRef for 5vdc and the signal is say 4.5vdc your device will not trigger.
Also, using unregulated 12vdc through a divider as vref may not work so well as auto 12vdc goes from ~12 to 15 V
Comarators are used with low currents. If you need a high current then the comparator drives a transistor or Mosfet.
An LM339 quad comparator has a minimum output current of only 6mA. A typical one has more output current but you get whatever they have and it might be minimum. So a weak LM339 or LM393 can't drive an automotive relay without adding a transistor or two.
The output of the comparator goes to ground with a minimum current of only 6mA. A pullup resistor from the output to the positive supply voltage causes the output to go high but it cannot drive a relay coil.
The LM311 is only capable of sinking 50mA. His relay will draw at least 133mA (90 ohm coil). He will have to use a transistor to drive the relay. A 2N3904 should work nicely (200mA continuous current). Again, going back to your original circuit, use the non-inverting input for input signal. You can use the LM311, like audioguru suggested. This will save real estate on your design.
You will want to add the things that were suggested earlier such as the snubber, flyback diode, voltage divider and pull-up resistor on the output of the LM311 (since it is open-collector). You may want to put a diode in series with your transistor base as well since the output of the LM311 will not saturate to 0V. This will require the output to be 1.4-1.5V before the transistor turns on. Otherwise, depending on things like the output current and temperature, may cause the transistor to go into conduction prematurely. Try using a 2KO resistor for the pull-up to keep the output current low. This will help in decreasing the saturation point
rezer...I have done some modifications to the circuit you drew. I added the NPN as recommended. Not specifically yours mentioned below.
2 questions, is the flyback resistor R4 in my drawing? It is now at 240 ohm, is that the one you recommend 2K ohm?
"The "diode" in series with the base".......did I draw it correctly here? If so, what is a recommended value for that? Same as the other one in paralell with the coil?
Otherwise, I think I have added everything as you said, and as given. Looks like I am ready to etch a board here if this new design works.
Stu
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?