I need a TRUE delay of relay / timer

Thread Starter

mamba76

Joined Apr 28, 2022
4
Hi all. I need a TRUE 'delay off' relay / timer for switching off car audio equiptment. Iv got some from ebay but it requires a constant power supply (50ma) but I can't have any current draw. Im not an expert and I can't decipher from an advert whether its 'true' delay off relay. Im only switching half an amp. If I can't find a cheap relay to do this I may have to make one! Any tips or diagram for that would be much appreciated as Iv never built any electronic gadgets before! Thank you.
 

Ya’akov

Joined Jan 27, 2019
5,659
Welcome to AAC.

Could you more clearly explain how you want the relay to operate so there is no ambiguity?

One of the problems you are going to have is that something has to power the relay, so anything that doesn’t power itself will be in a fatal embrace with the power and have to be manually reset to operate.

You could do something with a largish capacitor and the proper RC network which would hold the relay on after power was removed from it.
 

MrChips

Joined Oct 2, 2009
26,087
You could have the relay latch on its own power.
When the delay period is over it releases all power. It does require a manual reset to operate.
 

LowQCab

Joined Nov 6, 2012
2,043
50ma is insignificant to a Car-Battery.
The Memories in your Radio and Fuel-Injection-Computer probably pull more than that.
If 50ma worries You, because your Car sits un-driven for long periods of time,
what You really need is a Battery-Maintainer/Charger,
and leave it connected permanently.
.
.
.
 

sghioto

Joined Dec 31, 2017
3,271
Looks pretty good to me. But wondering about when the MOSFET goes through the resistance range. As the voltage of C1 drops, wouldn't that cause heating of Q1?
Wouldn't be significant as the current is also dropping . The relay I had in mind draws only 30 ma at 12 volts.
 

Thread Starter

mamba76

Joined Apr 28, 2022
4
Wow, thanks for all the replies and all are helpful.
I know 50ma isn't a lot I just rather have no drains to worry about, in some situations/circumstances it can be a hinderence.
I need the car amps to go off first when switched power is removed then 3 seconds later the DSPs, etc.
As mentioned a suitable sized capacitor to keep it latched is what I had in mind but the ones on ebay all seem to require a constant power, I always have a need for a product thats not available lol.
 
Last edited:

LowQCab

Joined Nov 6, 2012
2,043
A Relay is probably overkill and kinda clunky.

There are FET-Gate-Drivers that have a Logic-Level-Input, plus hysteresis,
and can easily supply 2-Amps continuously,
and it's all contained in a TO-220-Package,
just add a Resistor and a Capacitor.

And, it will also work as a Delay-On-Timer, so build one of each to get the sequence You want.

Better put those Batteries on a Maintainer, or You will ruin them by leaving them in a sate of discharge.
Even "just overnight" will shorten their Life expectancy.
.
.
.
 

Thread Starter

mamba76

Joined Apr 28, 2022
4
Yeah battery health is a constant issue with a 2.5kw rms audio system, luckily the alternator is rated to 180amp in my 3.0lt diesel. I sort of understand what your saying but adding a resistor and a cap is about my level at the moment tho with a detailed diagram like sghioto suggested and some reading I think I could do it on a bread board first.
I actually have two MiniDSP miniDC isolators that do all that I need except they also require constant power, maybe its best if I just use them and monitor the battery drain. Iv been in the automotive industry for 16 years and anything thats a permanent drain unnerves me lol although I will be testing later to see exactly what my ECUs are using when locked, I thought it was zero.
 

Thread Starter

mamba76

Joined Apr 28, 2022
4
Iv been a bit short sighted since even the blinking door lock LED and the central locking radio scanning must constantly draw some current. To save face here Ill try the easy method with the miniDCs and use a maintainer as suggested but thanks for the replies.
 
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