Car Light Project (Review Needed)

[weegee]

Hi guys, any chance that you could have a quick check over my schematic to see if ive made any obvious mistakes. This one is for a customer so i wanna get it right first time.

I appologise in advance for the mess on the right hand side of the diagram, but i wanted to get it posted before i had to go out.

Thanks

Graham

 

[weegee]

Just a quick question to add, as this is for a customer would i be better to add a protection diode to the 12v input (next to the 3 amp fuse) to prevent damage from connecting in reverse polarity (its possible becuase i have to use faston style connectors for this project) or do i let them fry it and sell them another?

 

[phil7890]

1. Caution... U2 should be an automotive-grade LDO or DC-DC chip/circuit, and not a 7805. Some/most automotive-grade chips have a load dump protection (required) and some have reverse polarity protection too.

2. Caution... GP3 (pin 4) of the PIC doesn't seem to have an internal/weak pull-up - may need an external pull-up/resistor here.

3. Note... Probably should add a 0.1uF or 0.01uF ceramic cap near pin 1 of the PIC unless the C2 cap is nearby.

4. Note... Diodes D1 and D2 may not be needed as some 2N7000 have built-in diodes. But anyway, just leave it in there... can bill customer for more

5. Note... 10A current (based on the F2 fuse) on this board requires careful routing of the PCB and adequate width traces or use a power plane for all this (design PCB for 20A).

6. Optional... Maybe it's easier to select a different PIC chip with more pins so that you can do in-circuit programming and reprogramming until you're happy with it. (Maybe add an ICSP pin header if needed.)

7. Optional... Suggest to use some polyswitch instead of fuse (3A 30V for F1, and 9A 30V for F2)

Probably got other stuff that I missed. That was just a casual look at the circuit. Have fun. Good luck. Bye.

 

[weegee]

Hi phil, thanks for the advice.

1. Can you suggest an auto grade regulator that would be suitable, i have searched and i cant seem to find anything that is thru-hole, and i would preferably have just 3 pins, as all the chips i found had at least 6 pins.

2. I shall sort that now.

3. Done

4. I will keep them just in case.

5. I expect the board will be subject to around 6 or 7 amps, but i want to be safe and over engineer the pcb. If i cant fit the 20A tracks on my board i will design for 15A at the very least.

6. I used this pic purely because i have a box full of them, and i was planning to socket it. I normally add ICSP headers to my project, but as this is a very simple program, i really doubt i will ever have to re-program the chip after i have set it up. I will program it on breadboard and then transfer the chip to the pcb socket.

7. is this the type of thing? its rated for automotive use https://uk.farnell.com/jsp/Electric...AYCHEM/RGEF300/displayProduct.jsp?sku=1345947

 

[phil7890]

Hi Graham,

Re: 1. LDO / DC-DC. Maybe try the spec sheets for the following items and one of them will do... National Semi LM2937, LM2940, LM2672, and Micrel MIC2950, MIC2951. The LM2672 is a DC-DC type (low/no heat but requires more parts).

Re: 7. Fuse/Polyswitch. Yes, you've got the right idea. The link you provided is an RGEF300 and this item is rated for 16V. We really should use the 30V version in case of load dump and this would be the RUEF300 (and RUEF900 for the other). Higher than 30V is even better but is sometimes difficult to find... so this 30V version is the next choice.

Phil

 

[weegee]

i have attached an updated schematic, do you think this is better?

I have made the suggested changes, and will use the polyswitch's for the in place of the fuses (i havn't found polyswitches in my schematic capture program)

can you have a look and see if you think its ready to go. i will obviously build and test it, but im waiting on my 2n7000's arriving, and want to get as much done as possible so i can start building when they get here.

thanks again

Graham

 

[phil7890]

It's looking better!

We use the same symbol for a polyswitch (it's a fuse)... so that's already correct there in your schematics.

U1 may get kinda warm - maybe reserve some board space to add a heatsink later (if necessary). The quiescent current for this LM2940 is rated around 15 or 20 mA - if you don't mind this ~high current draw, then it's ok.

The two relays on the lower side of the schematics look suspicious - maybe double-check the circuit where the 2 coils are getting its voltage to trigger these relays(?), or maybe one dot/connection is missing in this schematics(?)

Since there are a number of relays here, you may need to analyze the make-before-break or break-before-make characteristics of all the relays. This is to avoid any split second short circuit but this may be negligible in certain cases - so it may be perfectly OK the way it is here.

Bye.

 

[weegee]

oh yeah, i see what you mean about the bottom relays. there fed directly from the 12v supply (after the fuse), so basically they energise as soon as the circuit is powered up.

Thanks ever so much for your help, im gonna build a prototype over the next few days, and if all goes well i'll make a proper pcb next week.

 

[phil7890]

Hey you're very welcomed. Good luck and have fun..

 

[Hero999]

. Note... Diodes D1 and D2 may not be needed as some 2N7000 have built-in diodes. But anyway, just leave it in there... can bill customer for more

The built in diodes are too slow and in the wrong place to protect the MOSFETs.

Another thing you should check is that the gate voltage is high enough to get the drain current high enough to turn on the relays.