Turn Off Delay Circuit.

[mike49504]

Im helping a friend hook up a system in his truck..

The problem he's having if turn-off pop thru the subwoofer..

I heard i can add a cap and diode on the remote wire to solve this problem..

Or is there a better circuit i can make for this??


Is the cap and diode will work what size cap would i need and what diode would i need??


how do i wiire this up??

[mike49504]

Would this circuit work?? If so what NPN Transistor could i use??

[Perry Babin]

If you disconnect and insulate the remote wire from the head unit at the amp and use a short jumper to manually turn the amp on (B+ to remote), does holding the remote on for several seconds make the problem go away?

[Zaaphod]

this is a common problem... the source signal is being removed before the capacitors in the amp discharge.
Here's how to do it with just a diode and a cap... the diode should be a rectifier of some sort, something like 1N4004 would be fine. The Value of the cap will depend on how much current the remote sense line of your amps will be, what the dropout voltage is, and how long it takes the amp's capacitors to drain. the capacitor should be pretty large so you can get a few seconds delay on the line, something like 1000uF perhaps... just a guess to get you started.

Off Delay.jpg



A better way to do it so it doesn't matter how much current your amp sense line would be to use a relay... the cap discharges throught the relay coil, and the relay contacts provide a nice clean delayed remote signal to the amp. again exact value for the capacitor depends on the current requirement of the relay, but doing it this way, you could switch all your amps, or future amps and they will all shut off at the same time. it also provides a cleaner signal than the diode-cap only method, because in this instacne, the capacitor will be discharging at a steady rate through the amp's circuitry, not an ON/OFF signal as the input to the amp is expecting.

relay off delay.jpg

The best way to do it is with a transistor instead of a diode.. you can use a much smaller cap this way. the C1 and R1 form an RC timing circuit, the resistor limits the drain on C1. R1 should be betweeen 1K and 10K C1 probably around 20uF or so... I am just guessing at the values, this will take some expirimentation. The transistor can be any small NPN type and again the exact values will depend on the relay and transistor used.
relay off delay 2.jpg

[Zaaphod]

Originally Posted by mike49504

Would this circuit work?? If so what NPN Transistor could i use??

that would work... quite overkill if you ask me... even a 2N3904 transsisor is way overkill.. the 30AMP fuse to power relay made me laugh! if you wanted to fuse it, put just ONE fuse to run the circuit and supply the relay, personally I would not bother, the whole thing is running off milliamps, the signal both in and out is also milliamps... and the whole thing ends up getting fused from wherever you swiped your +12 feed from anyway, putting more fuses all over the place is just a waste of time and more places for things to go wrong.

you do not need that giant relay they have shown.. ANY relay would work, as long as it has a 12VDC coil.. radio shack sells some nice small ones you pretty much want the smallest 12V coil relay you can find. the remote signal going into your amp only takes a few milliamps

[Zaaphod]

oops, I forgot the relay coil should have a freewheeling diode across it to minimize the electrical noise generated when the relay de-energizes..

relay off delay 3.jpg

[Zaaphod]

Originally Posted by mike49504

Would this circuit work?? If so what NPN Transistor could i use??

This is also missing a resistor on the base of the transistor... transistors should always have a current limiting resistor on the base, and if this one had one, it would not need such large capacitors to get the desired delay... and it would burn out far less transistors.

[Zaaphod]

No no no... wait... hold on... this is ALL wrong... all of it... the problem is the amp is staying on too long as it is already... you need to delay the head unit from turning off until the caps in the amp discharge... you can't use the remote out from the head unit, it won't work. use the switched 12v line as the input to the circuit, and hook up the output to the head units switched 12V input... you can also have the reverse problem on power up.. if the amp is up and ready to go before the head unit gets it's outputs turned on, this causes a pop when you power up.. the schematic below solves both problems by delaying the amp on power up and delaying the head unit on power down.

for the power on delay, R3 should be smaller than R2, perhaps make R3 1K and R2 10K when I used to do this I would use a protoboard to get the resistors and capacitors the way I wanted them, then solder it on a breadboard once I got it the way I wanted it. Radio shack used to sell bulk packs of resistors and electrolytic capacitor and it was a great way to have a lot of different values to play around with. ah.. the good old days when radio shack had walls and walls of nothing but electronic componenets, and no over priced cell phones.

if you want just the off delay on the head unit, hook up the remote to the amps directly to the switched 12v line.. they will shut off right away while the head unit stays on until the amps caps are discharged.

Sorry it's taking me so long to get this right, it's almost 3am here


relay off delay 5.jpg

[AKHeathen]

hmm, here is an idea..... check the remote from the head unit, and compare it to the vehicle accy. power. see if the head unit is actually delaying cutting power to the remote wire. if so, then you can install a relay behind the dash, using vehicle acc as the power source, and the remote from the head unit as the trigger. this way, you know the remote power is cut off without delay. now, if there is no delay, well, you can still do exactly the same, and install a delay to the acc into the head, and delay the head from turning off until after the amp has shut off. of course, you may be barking up an imaginary tree with all this delaying buisiness, but nobody knows untill you have either a- did all this and it did not work, or b-(the easier way to find out) answer perry's question.....also, to add to his question, leave the head on with no sound playing, and manually turn the amp on and off. report what you find please.

[Zaaphod]

Originally Posted by AKHeathen

see if the head unit is actually delaying cutting power to the remote wire. if so, then you can install a relay behind the dash, using vehicle acc as the power source...

if the head unit is delaying the remote signal, you could just tie the remote signal directly to the switched power.. or acc. or whatever you use to switch on the system. there is really no reason this has to go through the head unit at all, and no reason it could not be tied directly to switched power.

However I suspect this has more to do with the amp staying on due to charged caps while the input signal is lost. if you end up needing a delay, then I would defiantly recommend a relay circuit.