Voltage reducer circuit

[Mechanical_Matt]

Hello all, This is my first post here after lurking a bit. I have been tasked with designing a circuit to drop the DC voltage of a vehicle to the fuel pump to ~9V so that the pump power drops therefore putting less heat into the fuel during idle. I've been at it for a while and feel like I'm close, but keep running into minor hiccups I am using 2 relays to switch between the 12V and the 9V setup. I have a switch in there to activate the relays to simulate my 5psi pressure switch for the full application.

The thing that I'm running into now is that when I first turn the circuit on, it'll switch and go to full speed and drop to low speed no problem. After I let the circuit run, either in full or low speed for 10 min, the relays stick and require mechanical agitation to get them to unstick. I am suspecting that possibly the coil resistance of two relay coils in series is to blame that it wouldn't want to draw the current after the relay warms up? The DC input is 13.5V @ 11Amps when in low speed, and 13.5V @ 19Amps in high speed. I'm using ACNM5112 relays, 1N5817 Flywheel diodes, a PSDF3060L1 crowbar diode, two CL5120 LED drivers for status indication, a 100uF cap for a decoupling cap for the i6A buck converter board.

I've attempted to draw the schematic online, however it didn't offer a "jump over" feature for the wires, only nodes are actual connections, otherwise treat it like a jump over.

Forgive me if the circuit is a bit "hammer tech" as I'm classically trained as a mechanical engineer and circuits is not my strong suit, so analog and electromechanical are my go-to's

Any insight on why the relays might be sticking would be super helpful!

https://files.engineering.com/getfi...5ea-4af7-bf90-e48263683dc6&file=schematic.PNG

 

[MacGyverS2000]

Plenty of things wrong with that schematic, and some may or may not be your issue... right off the bat I see both of your LED drivers are wired incorrectly (you connect power to the chip input and output), so the top LED is always on, and the bottom is at some floating voltage. I see no load of any kind between the battery and ground, except the top LED, which can't be the actual case.

If this is truly how the circuit is wired, I'm amazed it works at all without letting out hte magic smoke. If it's not, fix the schematic to show the way the circuit is actually wired and get back to us.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Mechanical_Matt]

Hi,

Thank you for the quick reply. I wired the CL520 like the datasheet suggested, please see attached where the LED was to be ground referenced. I also uploaded a photo of the diagram in hopes that it might be clearer. In the hand schematic, "hobb" is the signal from the pressure switch with the switch only lightly drawn in.

https://files.engineering.com/getfi...d2c-971994b4e67d&file=IMG_20220207_132621.jpg

https://files.engineering.com/getfi...6d-bbf1-4c0a-a7d4-f4efa77ce71d&file=CL520.PNG

 

[IRstuff]

I wired the CL520 like the datasheet suggested, please see attached where the LED was to be ground referenced.

Check your schematic connection by connection, because you didn't even come close to doing that.

Can't see any reason to have two relays; you only need 12 V or 9 V, so it seems to me that only one of your relays is really needed.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[LionelHutz]

I don't see any reason to have any relays. Find the right PWM module and you just control the voltage out of it.

 

[MacGyverS2000]

I thought the same, Lionel, but this method doesn't require any programming (often a good thing). IR is correct, though, this doesn't need two relays... the circuit is overly complicated for the intended purpose.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

If you want any level of remote control, then you'd need either a relay or remote controlled switch like a Shelly or just brute force run some wires to the passenger compartment and switch there.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[Mechanical_Matt]

I finally figured out why it wasn't working properly, I had the two relays with 12V coils wired in series instead of parallel. That's why they'd work as first until they got a little warm from running and then would stop.

IR and MacGyver,
"Can't see any reason to have two relays; you only need 12 V or 9 V, so it seems to me that only one of your relays is really needed."

I have the second one because I was afraid of sending the 12V to Vout's pin when I was running in high speed. I have the first one to break sending the 12V to Vin of the board so that it's not wasting the energy trying to convert it to 9V.

"If you want any level of remote control, then you'd need either a relay or remote controlled switch like a Shelly or just brute force run some wires to the passenger compartment and switch there."

I wanted to have it so the pumps are staged, running the low voltage at idle where I'm not putting too much heat into the fuel (For example at a car show idling with low fuel level), and then a pressure switch for when boost goes to 4-5 psi to turn them onto full speed.

Lionel,
"I don't see any reason to have any relays. Find the right PWM module and you just control the voltage out of it."

I'm way more mechanical in nature and the the fuel pumps we're running are brushed, so reducing the voltage with the i6A board seemed the way to go since it allowed for continuous duty. I'm open to suggestions in it so that I can be ready for the inevitable switch to brushless fuel pumps.

 

[LionelHutz]

I'm sick of seeing the "you can't PWM control a brushed fuel pump, it needs to be voltage controlled" BS claims. Don't get sucked in my that.

Your power supply is PWM, so you're not avoiding PWM by using it.

As for a brushless motor, the method for controlling speed is totally dependent on the controller.

 

[bradrs]

There are some vehicles that have a circuit like you are describing. The first one that comes to mind is the Misubishi 3000GT. It switches to a higher voltage under load, controlled by the ECM. This page shows the circuit, and an inline resistor that is sized to create the voltage drop:

http://www.stealth316.com/2-fuelpumprelaybypass.htm

It's not as efficient, but the heat goes into the resistor, instead of the fuel. So depending on the application, this may work for you.

 

[salmanshu322]

you can use voltage divider circuit for voltage divide.