Smoothing voltage drops when cranking with a capacitor

Thread Starter

jawdoss

Joined Apr 25, 2023
27
Hi All,
I have a issue I am trying to resolve. I am using a timed output relay (Redarc TIM05) as a kill switch. Once a momentary button is pushed, an output is set at 30 seconds, supplying 12v to pin 86 of a 30amp relay that runs inline with the starter motor trigger. The issue I am having, is that this timed relay seems to be very sensitive to input voltage, which it requires to be present at all times whilst the timed output occurs. When the starter motor engages, the voltage at the battery drops from 12.8v to 11.3v, causing the timed relay to shut off its output.

My question is, would a capacitor be viable to smooth the voltage dops out whilst the vehicle is cranking? If so, what would be the best way to implement?

Cheers

EDIT:
Worth a mention, this is a 1960s Chev, which I have done a full ground up rewire. The customers intentions were to have a secondary start inhibit that’s not obvious to an unfamiliar user. This isn’t meant to be killing a running engine. A momentary switch with a timed output was chosen for the ease of use whilst keeping the switch hidden, so not to disrupt the immaculate fit out of the interior.
 
Last edited:

Dodgydave

Joined Jun 22, 2012
11,301
I would put a diode in series with the relay coil supply , like a 1N4007 and a 470uF capacitor across the coil supply to gnd to suppress the starter motor pulse.
 

LowQCab

Joined Nov 6, 2012
4,056
Instead of trying to fix your solution,
why not ask for alternate solutions ?

What are You trying to accomplish ?
.
.
.
 

ElectricSpidey

Joined Dec 2, 2017
2,770
It's very odd that a 12 volt relay would drop out at 11.3 volts.

You can try the suggestion in post #2.

But I would ask where in the system are you getting the power for the relay, maybe moving the connection closer to the battery would also help.
 

MrSoftware

Joined Oct 29, 2013
2,196
Addressing only your issue of trying to get an accurate timer; the two potential solutions that immediately pop to mind are (1) supply the circuit using a regulated power supply, maybe a buck/boost, so it always sees the same input voltage. Or better, (2) switch to a digital timer or some other design that is less sensitive to input voltage.
 

ronsimpson

Joined Oct 7, 2019
3,037
During cranking the voltage can drop to very little. Much lower than 11V you see. This depends on the temperature, battery age, conditions of battery wires, etc. Using a diode and large capacitor as stated above should work. The diodes catches the highest voltage and stores it on the capacitor, then keeps current from flowing from the capacitor back to the starter.

I do not know how much power the timer uses. That will determine how long the timer can run using only the capacitor as a source of power. The capacitor should only power the "brains" of the time.

Measuring with a meter, you are seeing an average voltage. The voltage probably has very fast dips and hills that range from 6V to 15V. I did automotive design years ago. As the motor turns over, on the compression stroke the starter works very hard and pulls down the voltage, on the power stroke (even when the motor has no spark) energy is pushed back into the starter. There is a short time where there is no load on the starter.

I cannot remember the RPMs during cranking. I think I wanted 1 second of storage on the auto computers I designed. But I think 0.1 sec will work.

I can't find a good data sheet on your timer.
 

crutschow

Joined Mar 14, 2008
34,403
Since the timer supply voltage must power the starter relay, it will require a large capacitor for the diode-capacitor solution to work.
A 30A automotive relay likely has a resistance of about 100Ω, so a 0.1 second time-constant requires a capacitor of 0.1s / 100Ω = 1mF.
f
 

wraujr

Joined Jun 28, 2022
160
Looking at the brief user manual for the Redarc TIM05, one would think the design should be tolerant of dips in voltage.
If I read right, pin 30 supplies both 12V to internal operation and the outputs (up to 10A), so as mentioned by crutschow, your capacitor will be supplying BOTH the internal relay and the output driving the starter relay.
It would be much better if you could provide a proper diagram showing all the connections to relay, starter relay, etc.
 

ronsimpson

Joined Oct 7, 2019
3,037
Since the timer supply voltage must power the starter relay
That is a real problem. The timer needs to be on a separate source of power.
The relay, after engaged, will hold down to 6V probably 4V. It does not need any help.
I do not know at what low voltage the timer stops working.
 

crutschow

Joined Mar 14, 2008
34,403
That is a real problem. The timer needs to be on a separate source of power.
The problem is the timer power is common with its output that must power the starter relay.

One way to provide separation is to add a transistor buffer to power the starter relay coil from the timer relay.
That way a reasonably sized capacitor along with a diode should maintain the timer voltage during the cranking.
 
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Hymie

Joined Mar 30, 2018
1,282
Do as Dodgydave says – you might need to experiment with the capacitor value depending on how long the engine cranks before starting the engine.
 

wraujr

Joined Jun 28, 2022
160
So if I read your post correctly:
(1) You have momentary push button switch supplying +12V to the INPUT (pin 86) of TIM05 and you want the user to press this button briefly to start the engine.
(2) You have +12 supplying the TIM05 relay (essentially from battery) to "Battery Positive" (pin 30).
(3) You hopefully have "Negative Ground" (pin 85) of TIM05 tied directly to battery/chassis GND.
(4) You are using "Timed Positive Output" (pin 87) of TIM05 to engage ANOTHER "30amp relay that runs inline with the starter motor trigger
So you have two relays, you have a timer relay and a 30 amp relay (starter circuit)
(5) You have DIP Switch 2 as OFF as you want delay in "seconds"
(6) I believe you should have DIP Switch 1 in the OFF position to create the "Delayed Turn Off" mode of TIM05.
In this mode (see right hand "pie chart below"), the BLUE area represents the time during which the user has pressed the momentary switch to supply +12V to INPUT (pin 86). The Output (pin 87) will be active the whole time the user has the momentary switch pressed (albeit briefly). Upon release by user, the Output (pin 87) should remain active for the time period as set by the potentionmeter (30 secs you said).
In other words, the TIM05 output (pin 87) will be active (+12V) and driving/engaging the other relay for a total time of
the time user has pressed momentary switch + 30 seconds (green area).

1682451166580.png
 

MisterBill2

Joined Jan 23, 2018
18,451
I go back to post #3 and ask what is it that you are trying to achieve with the timer?? It seems that others have grabbed the concept that somehow the timer is starting the engine, which IS NOT what I think I am seeing. So instead of solving the wrong problem, I ask just exactly what is the TS trying to achieve, NO BODY ELSE ANSWER, Please. I want to know from the TS what the actual goal of the timer installation is.
 

Thread Starter

jawdoss

Joined Apr 25, 2023
27
Instead of trying to fix your solution,
why not ask for alternate solutions ?

What are You trying to accomplish ?
.
.
.
This is all part of the process first, I’d like to keep it in place as it allows a timed output.

It's very odd that a 12 volt relay would drop out at 11.3 volts.

You can try the suggestion in post #2.

But I would ask where in the system are you getting the power for the relay, maybe moving the connection closer to the battery would also help.
Yep, it puzzled me too. It’s not a standard relay and the minimum input voltage is 12V.
It sits about 4 meters from the battery, Initially I had the power supply coming from a bus at the fuse box (20cm away) and also have run independent wiring to the battery with no change.

I did check resistance from battery neg to the main couple of earth points inducing the starter, and all come back between 0.2-0.4 ohms. Main battery cable from batter to starter and battery to fuse box tested at 0.2 ohms
 

Thread Starter

jawdoss

Joined Apr 25, 2023
27
I go back to post #3 and ask what is it that you are trying to achieve with the timer?? It seems that others have grabbed the concept that somehow the timer is starting the engine, which IS NOT what I think I am seeing. So instead of solving the wrong problem, I ask just exactly what is the TS trying to achieve, NO BODY ELSE ANSWER, Please. I want to know from the TS what the actual goal of the timer installation is.
It it essentially a kill switch. The customer wanted a momentary switch that they can press giving them a set amount of time to start the vehicle. This is an old school car so it doesn’t have a starter relay at all as standard. This timed relay simply (or so it was meant to be) supplies a 12v output signal to the control pins of a starter relay I have added in.
 

wraujr

Joined Jun 28, 2022
160
I don't think your use of "kill switch" is correct as you are starting not "killing" a running engine. I think this is confusing many. Sounds like a momentary (i.e. 30 sec) start switch.
 

wraujr

Joined Jun 28, 2022
160
So you have added a 30A starter relay that engages a starter when energized? If yes, you want the TIM05 to engage the "starter relay" for 30 seconds, max?
 
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