zener relay control

Thread Starter

gombault

Joined Nov 15, 2020
6
Hi,
I want to use the voltage on a truck as a switch. when the truck is switched off its voltage is 24V I want to use the zener not to activate the relay at that voltage. when the truck star the voltage go to 28V for safety I will use a 27V zener that must pull the relay in at that voltage. Can this work, any help would be appreciated.
 

crutschow

Joined Mar 14, 2008
27,894
Note that the battery voltage does not instantly go to 24V when the motor is switched off, and charging stops with no load on the battery.
It could take several hours or more.
Is that acceptable?
 

Ian0

Joined Aug 7, 2020
3,463
They are fitted as standard on cars when you have towing electrics fitted. They are usually called "split charging relays". I suppose they make them for 24V electrics as well?
The circuit consists of a comparator with hysteresis switching at about 13.5V, controlling a relay.
 

Tonyr1084

Joined Sep 24, 2015
6,339
As far as zener's go - I think a 24V zener will conduct anything above 24V. So if your electrical system is at 27V the zener should conduct 3 volts. 27 - 24 = 3 volts. I don't think that will work. The comparator is a better solution. However, why the need to sense voltage to turn on a relay? Why not just have the relay coil connected to a 24 volt source when the truck is on? That way it's on when the truck is on and off when the truck is off.
 

Hymie

Joined Mar 30, 2018
998
It would be much simpler to use a relay that was energised when the vehicle ignition circuit is energised and operate whatever you want via the appropriate relay contacts (normally open/normally closed). But if it is a relay that is already permanently wired with the relay coil across the battery, you could add a relay energised by the ignition, and switch the first relay coil via the relevant contacts of this second relay.

This would avoid the complexity of a comparator circuit, and would switch both relays off once the vehicle ignition is off – with the advantage that no power is drawn by the relays with the ignition off.
 

Ian0

Joined Aug 7, 2020
3,463
The reason that they use voltage detection on split charge relays is that there is no convenient "12V via ignition switch" wire at the back of the vehicle.
 

Thread Starter

gombault

Joined Nov 15, 2020
6
First the voltage will drop if the time is short the voltage will drop below 24 when the engine is cranked. I don't want to use an ignition supply because of the time finding an ignition supply on different trucks. I found this video of a zener at 5V will it work on 27V see
 

Hymie

Joined Mar 30, 2018
998
If you don’t want to go with Ian0’s off the shelf solution, here is a circuit that should do what you want.

In operation, the non-inverting input of the comparator IC1 is at half the battery voltage (formed by the two resistors R2 & R3). The inverting input voltage is adjustable via VR1, between 11V and 14V (set by the two zener diodes).

With the battery voltage above twice the voltage set by VR1, the relay will be energised by the positive output of the comparator IC switching on transistor Q1. With the battery voltage less than twice the voltage set by VR1 the comparator output will be low, switching off transistor Q1 and the relay.

The comparator IC could be almost any op-amp able to operate with a supply of 30V (even a 741 should suffice). The transistor Q1 could be any NPN device capable of passing the relay coil current.

To minimise electrical noise affecting the operation of the circuit, I’d recommend you add the two capacitors shown (100µF 35V may be adequate).

With the relay in the de-energised state the circuit should draw less than 20mA, so battery drain with the circuit added should not be an issue.
 

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Tonyr1084

Joined Sep 24, 2015
6,339
First the voltage will drop if the time is short the voltage will drop below 24 when the engine is cranked. I don't want to use an ignition supply because of the time finding an ignition supply on different trucks
I had a similar issue with a face color controlled gauge. You choose the color you want. Then every time you turn it on it's that color. But when I started my truck the drop in voltage would cause the gauge to reset to the factory default color. I would then have to go through the process of resetting the color. So rather than deal with that issue I came up with a delay power on circuit that was very rudimentary but functioned well enough to do the job I wanted it to do. When power came on C1 would charge through R1. After about 4 seconds the relay would click in and apply power to the gauge. Once power was established the power circuit would hold the relay active until shut-down. Why a 24V relay? It was what i had on hand when I was experimenting. It came from a dishwasher control board; and the 13.8V was enough to pull K1 in. Why D1? That was to prevent Q1 from trying to power the gauge. Why the 4007? Because I have a snot load of them in my stock. Though not shown, I think I used another 4007 for the snubber on K1. Anyway, that was my solution to my problem. If it took longer than four seconds to start the truck then I had to reset the colors again. A larger resistor would extend that time period. So would a larger C1.
1631457663480.png
 

Ian0

Joined Aug 7, 2020
3,463
If you don’t want to go with Ian0’s off the shelf solution, here is a circuit that should do what you want.
I'd recommend a bit of hysteresis. Otherwise, when the load is applied, the voltage will dip and it will switch off again. The Durite product tells you what voltages to use: "Switch on at 27.4V and off at 25.6V" is printed on the case
 

Thread Starter

gombault

Joined Nov 15, 2020
6
Hi All,
I send a You Tube video that did what I want by just using a zener and relay. but it seems like it would not attach it. I just want you guys views on it. Did you receive a You Tube adders with my last post? I sending it again but removed the h please just ad a h in front. I also send it as an attach. ttps://www.youtube.com/watch?v=VG_7LHW9SS8
 

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MisterBill2

Joined Jan 23, 2018
9,464
What is the purpose of the relay that is supposed to operate whan the engine is started? There is another concept entirely that I have seen. It uses the electrical noise from the alternator to control a relay. Jusy a simple high-pass filter and the amplified electrical noise triggers a circuit that operates a relay.
AND, once again, what is the purpose of this device??
 

Ian0

Joined Aug 7, 2020
3,463
. It uses the electrical noise from the alternator to control a relay.
Now there’s an idea. The usual voltage sensing circuit has a snag: with vehicles with a battery at the front and a large load in the trailer such as a 10A fridge, there is enough voltage drop in the wiring for it to switch off again, then it switches on-off-on-off until it wears out the relay.
My present car has the battery in the boot, so it isn’t a problem, but it would be an interesting circuit to try. Does anyone know how many poles an alternator usually has?
 

MisterBill2

Joined Jan 23, 2018
9,464
The alternator noise is primarily a tone that varies from maybe 300 to over 800 Hz, depending on engine speed. And in some vehcles thee is also electric fuel pump noise. With the engine off the voltage is almost pure DC. So the filter is simple. The unit that I investigated used an LM324 quad op-amp with 2 gain stages and a rectifier stage and thelast amp driving an NPN transister to operate a relay. That was a 12 volt model. For use on a 24 volt system you will need to add a voltage limiter to keep from over-voltage damaging the op-amp and any capacitors not rated for 30 volts. The voltage will get higher when the system is charging the batteries in cold weather, up to 30 volts, possibly 31.
 

Juhahoo

Joined Jun 3, 2019
215
Just an idea, perhaps not for the TS: Measure when the motor is rotating >nnn RPM, and you know your motor is running/able to charge.
Then detect the voltage on your primary battery, when its voltage reaches a certain level, secondary battery is connected. This ensures that your main battery is at least charged first. When you shut off the motor (motor rpm <yyy RPM) battery disconnects immediately.
But this isn't a simple solution to do therefore existing ready made system is the best approach for non hobbyist willing not use time for this.
 
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Ian0

Joined Aug 7, 2020
3,463
Just an idea, perhaps not for the TS: Measure when the motor is rotating >nnn RPM, and you know your motor is running/able to charge.
Then detect the voltage on your primary battery, when its voltage reaches a certain level, secondary battery is connected. This ensures that your main battery is at least charged first. When you shut off the motor (motor rpm <yyy RPM) battery disconnects immediately.
But this isn't a simple solution to do therefore existing ready made system is the best approach for non hobbyist willing not use time for this.
You would need to know the ratio of the engine pulley to the alternator pulley, and the number of poles the alternator has. Then it’s just a matter of setting the filter cutoff frequency at the right place.
 

MisterBill2

Joined Jan 23, 2018
9,464
HEY, WAIT!! The TS never mentioned anything about charging a battery! This was for ssomething in the back of a truck with a 24 volt system. So why presume that it is for battery charging? And charging a battery at the back of a big truck would not make sense any way because of the resistance of all the long wires. The alternator noise (in the system) does not get loud until it is delivering current, and so there is no reason to worry about engine speed.
And with the engine not running the frequency is zero, so the filter frequency is not so very critical.
 
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