How do you limit a 12VDC Molex circuit to 9A?

[NickPickerWI]

I'm planning on doing a PC mod soon, and one of the requests from the client is to include a functional car cigarette lighter (it will be for cigarette smoking, unfortunately...I tried to tell him it's a bad idea, but he's paying me to do it anyway).

 

I'm a noob to electronics. I know I can wire a 10 amp ATC fuse in series with the +12VDC leg in series to the back of the lighter socket, but what if I wanted to prevent the lighter from ever drawing more than 9 amps? It will still work, albeit the lighter will heat up slower than it maybe wants to.

 

I don't want the fuse to blow, only as a last resort. It will be very hard to get to, and I don't want the fuse to pop and melt other things in the process, or send shrapnel everywhere. I don't want to use a breaker because they're too slow, and only a few have a chrome dipswitch like P&B's (this is an aesthetic requirement).

 

Interested to know how the circuit would look. I don't know how to calculate resistors or find parts, really. I need help.

 

I just don't want this thing to start on fire or ruin itself.

[mariushm]

A molex connector is only rated for 5A.

At 9A, you may melt the housing of the molex connector.

Technically you could boost 5v @ 5A = 25w to 12v and add that to your 12v 5A , for a total of around 7A and that's about it.

 

The simplest way would be to use a dc-dc converter with configurable current limit.

 

If you want to do it with "basic building blocks" you'd start by using a very low value resistor to measure the current by reading the voltage drop on resistor then maybe use that to control a pass transistor or mosfet to limit current / drop voltage when current goes too high.

 

 

[mariushm]

The downside is that resistor is gonna have to be a high wattage one, as it's gonna dissipate a lot of heat... you also can't do more than around 1.5A with just the regulator ... the R1 must be between around 0.8 ohm and 120 ohm (maybe 240 ohm with newer chips)

 

For more basics see this : https://www.electronics-notes.com/articles/analogue_circuits/power-supply-electronics/current-limiter-circuit.php

 

 

 

 

[Unimportant]

21 hours ago, NickPickerWI said:

I'm planning on doing a PC mod soon, and one of the requests from the client is to include a functional car cigarette lighter (it will be for cigarette smoking, unfortunately...I tried to tell him it's a bad idea, but he's paying me to do it anyway).

 

I'm a noob to electronics. I know I can wire a 10 amp ATC fuse in series with the +12VDC leg in series to the back of the lighter socket, but what if I wanted to prevent the lighter from ever drawing more than 9 amps? It will still work, albeit the lighter will heat up slower than it maybe wants to.

 

I don't want the fuse to blow, only as a last resort. It will be very hard to get to, and I don't want the fuse to pop and melt other things in the process, or send shrapnel everywhere. I don't want to use a breaker because they're too slow, and only a few have a chrome dipswitch like P&B's (this is an aesthetic requirement).

 

Interested to know how the circuit would look. I don't know how to calculate resistors or find parts, really. I need help.

 

I just don't want this thing to start on fire or ruin itself.

The options are (from simplest to most complex):

• A simple fuse that pops when you go above the allowed current and needs to be replaced.
• An electronic fuse: A circuit that disconnects the load when it goes above the allowed current and then needs to be manually reset (or resets itself when the fault condition goes away).
• Linear current limiter: A circuit that does not allow more then the allowed current to flow to the load and lowers the load voltage as required. Probably not usable in this case as it would dissipate around 100W worst case here.
• Switching current limiter: A circuit that does not allow more then the allowed current to flow to the load but does so by switching current trough an inductor on and off quickly - dissipates little power.

[Unimportant]

20 hours ago, Grumpy Old Man said:

 

Unusable, 12V/9A would dissipate +100W worst case.

[Unimportant]

6 minutes ago, Grumpy Old Man said:

Bottom circuit diagram and use LM117K transistor with heatsink

Doesn't matter, it's still linear, it'll dissipate +100W worst case - that's way too much to be reasonable. (and the chip can nowhere near handle that kind of power/current).