I need a relay to switch a 25A load from a power supply (2 batteries in series, 12V each). I need to measure the voltage of the batteries, and there will be some difference while there is a load and there is not a load (since its 2 motors that stop and start to move some weight (1ton). They tell me the difference will be even 1V sometimes. Originally I intend to use a software filter, since the measured voltage goes to an ADC, I was thinking of taking 200 samples for the whole discharge period (about 8h) and average by dividing on 200. If the accuracy needs to be increased even further. But I think that the people who want me to do this, do not believe this method will be good. If I stop the motors while they are pulling and wait for the batteries to settle, that will be a safety hazard, I do not think it is a good idea. Do you think the method of averaging is good or not? If not can you suggest some other method and a relay with low coil resistance in order to pass 25A of current while the motors are pressed to the maximum for some time. The biggest problem comes from the fact that if there is a high voltage drop over the relay, there will be less from the batteries left for the load. This means the load will be able to drain current for a shorter time. Meaning instead of working from 24V to 20V (2 batteries), it will work from 22V to 20V (2 batteries). Also the current drain will be higher to make up for the power.

 

If not possible to use a relay (I am thinking of the mechanical finder 66 series, but I can not find its coil resistance), then a MOS or a SSR will also be usefull. The problem is how to cool it and connect it to the PCB, which is in my other post.

 

Could always use the sequencing 12 bit SAR on PSOC 4, it will do the
averaging for you in HW. Or use the 5LP family and its HW digital filter.

See attached.

Regards, Dana.

 

can you suggest some other method and a relay with low coil resistance in order to pass 25A of current while the motors are pressed to the maximum for some time.

Why is the coil resistance (rather than the contact resistance) critical? The coil won't be in series with the motors.

 

Hello,

How many threads do you need for your relays questions?
Relay for 25A load with low resistance
PCB sizing for 25A-30A
Relay power dissipation calculation
Relay mount documentation

All questions copuld have stayed in one single thread.

Bertus

 

The question in post #1 is not clear. If you are asking for recommendations of relays that can switch a 25 Adc inductive load, look into automotive relays; many are based on a Bosch design rated for 30 A. Beyond that, a DC contactor is much larger, but built with field wiring terminals and can handle much higher current with a longer operating life.

The relay coil resistance should not affect the voltage available to the load, but does affect the circuit that is driving the coil. What is that circuit?

ak

 

1. What I meant is the contact resistance. Sorry about that.
2. I am sorry for posting multiple threads, if I post it at 1 thread i get from the moderator on other forums : Multiple questions posted in 1 thread.
3. The question is just about what is the contact resistance of the relay, so I can calculate the voltage drop over it. This is a problem for me, because the batteries will be charged with 14.6V, and if the voltage drop is high, the batteries will not be charged properly.
4. I will check about the 12 bit SAR.

 

If you are worried about the contact resistance of a relay switching high DC then the arcing that is going to occur will affect the ultimate resistance.
Using a magnetic arc blow out relay like I mention in the other thread will minimize this.
The P&B PRD-1DH0 double pole series with DC magnetic Arc Blow-out goes up to around 50amps and runs around $40.00.
Max.

 

Will there be an arcing on this relay? Its electromagnetic, sealed. I am using flange mount.

 

That Finder relay appears to only quote AC rating for contacts.
I am suspect about using a High current switching DC sealed relay, the internal condition can become ionized and increase the chances of sustained arcing.
Max.

 

If I have time ot read the IPC standard, I will probably be able to make a 30A PCB and use the Finder 67 solar relay instead.

 

If you intend using these relays for a DC inductive load they are only rated for DC1 category which is non inductive.
Max.

 

Now that is bad, since I will be powering a motor. We have battery, PCB, motor.

 

Maybe I should just put a 100uF capacitor on the ADC pin, so when the voltage changes while there is a load and while there is not a load, the capacitor will charger very slowly?

 

Or use this: https://www.components.omron.com/product-detail?partId=131

 

Hello,

The 67.xx-x500 can only switch about 20 Amps inductive.
The 67.xx-x300 is even much less.
See this graph from the datasheet:



Bertus

 

Or use this:

That should do it, so would the PRD!
Max.

 

Ok whatever relay it is, its needed to stop the load so the ADC can measure properly the battery voltage. The reason for the low resistance is so the voltage drop over the relay is not taken away from the load (that way the load will work less time) and so there is low power dissipation over the relay. Also not to pass 25A through the PCB if possible. That is why I prefer to use a flange mount relay, solid state relay or in line MOS, all I need is one that is picked properly. What bothers me is what will happen if the requirements increase even more and they make me put a relay to stop the charger as well.... That way the batteries will not get charged properly because there will be a voltage drop over the relay. Otherwise this NPN MOS will do I think: SI9926CDY-T1-E3. The problem is that there will be a high current through the PCB and the NPN MOS.

EDIT: sorry I posted before I read the last post.

 

I need to buy 25 relays if they are only for stopping the load, I need to buy 50 if it will be for controlling the load and chargers. By 40 USD, 50*40 = 2000 USD.

 

Not that I am paying, but when I say it costs 2000 USD it will not end well.

EDIT: Thank you all for your help in all threads! I have 1 last question. Normally a transistor can dissipate 1W without a heatsink, how much can this relay of OMRON dissipate? It will have 6.25W at the monte carlo scenario.