Can a 12V Relay Handle a 36V Application?

  • Thread starter Jdo300
  • Start date
  • Tags
    Relay
In summary, the conversation discussed the use of a 12V automotive relay with a 30A rating in a 36V application for a motor controller project. It was mentioned that the "12V" refers to the voltage used to operate the relay and the "30A" refers to the current that the contacts can carry or break. The conversation also touched on the importance of understanding the relay's contact rating and its ability to handle voltage and fault current. Additionally, the use of a catching diode to suppress voltage spikes and the derating factor for different types of loads in DC circuits were mentioned. It was concluded that it may be necessary to over-engineer and use a larger relay to avoid potential issues.
  • #1
Jdo300
554
5
Hello All,

I'm working on a project to design and build an experimental motor controller that operates on a 36V supply. As part of my test setup, I would like to include a mechanical relay to disconnect the battery supply from the H-Bridge output stage in case of a failure during testing.

Right now, I have on hand one of the RadioShack 12V 30A automotive relays which I am considering for use in the circuit. However, I'm wondering how important the "12V" rating is to the relay's operation, and what about the maximum rated votlage of a relay limits its use at higher voltages? In my system, I'm not expecting the current to go over 20A on average, though there may be surges as high as 30-40A during startup. How concerned should I be about using a 12V relay in a 36V application?

Thanks,
Jason O
 
Engineering news on Phys.org
  • #2
The "12V" will refer to the voltage used to operate the relay. The "30A" will refer to the current that the contacts can carry or break. You would need to look at a more complete spec for the relay to decide whether it would break a circuit with 36V across it. But I can't imagine it would be a problem, unless you intend to break a very inductive circuit, in which higher voltage spikes can be generated when you break the circuit. In any case, a catching diode across the contacts can kill these spikes. The situation could be different if you were dealing with mains voltages.
 
  • #3
Hello JDO - yes you do need the spec for the relay and look at the contact rating. Since you are looking at using this for "protection" AND this is in the DC part of the circuit - the Contact rating needs to handle the Voltage and the Maximum Fault current - if this is from a battery you may have a lot of DC current available.

DC protection is a nightmare - as the current does not naturally commutate.
 
  • #4
A fuse is pretty bomb proof as long as you can afford to wait for it to blow.
 
  • #5
Hello everyone,

thanks for the comments. Yes, the relay that I am thinking to use uses 12V for the coil. My question was mainly understanding how they rate the contacts' voltage rating. for example, they have relays that say they are rated fro 120V AC or 30V DC at blah blah amps. I'm wondering what physical factors of the relay's contacts they are evaluating to come up with these numbers?

The only thing I can think of is the insulation resistance in case of an inductive spike when the circuit is broken, but if the circuit you want to use it in is adequately protected from this, I'm wondering what else they could be looking at.

- Jason O
 
  • #6
Insulation resistance wouldn't be a problem. What canes relays is arcing when the current is broken. Heavier contacts will overheat less than light contacts and a large amount of travel (big separation), coupled with fast operation can reduce the time that the arc operates each time. As I said earlier, breaking the current through an inductive load can produce hundreds of volts of spike, which is usually suppressed using a diode, connected so that it is normally reverse biased (arrow pointing towards the positive rail). Many of these issues also apply to domestic switches but AC makes life a lot easier because the volts go through zero twice every cycle and arcs are more likely to die.
 
  • #7
if you've ever arc-welded you have an intuitive feel for what goes on at a relay contact when it opens. metal from one contact gets deposited to the other and they might weld together.

DC is harder to interrupt because there's no natural "sinewave zero crossing". the current has to be brute forced to zero in the arc. that's why the relay's DC rating is so much less than its AC rating.

so find the datasheet for your relay.

Perhaps a small series resistance can limit worst case fault current to something your relay can handle, without causing much voltage drop at normal load.

fuses do an excellent job. good high current ones are filled with sand to quench the arc. little automotive fuses might be able to do your job, check their ratings.

as somebody mentioned , a diode can absorb an inductive "kick", but traditionally it is wired across the load not the contact.
 
  • #8
Hello all, first post here...

I work in the aerospace industry and work with DC circuits quite a bit. I would agree with all the posts above regarding needing the specs for the relay you are looking at.

I can offer up some generic DC relay contact rating info: typically the contact rating for a DC relay is for a resistive load. If another type of load are being used, you must de-rate. So for a relay rated at 10A (resistive), an inductive load is derated to 8A, motor load is derated to 4A and a lamp load is derated to 2A. Of course, there are many variations to these factors.

When using a DC voltage higher than the relay rating also calls out for a derating factor. I think there is a formula for this and I don't have that memorized :)

Regards,

Choppersparks
 
  • #9
Hi choppersparks

If it's really important to use the cheapest relay you can get then what you say is correct. otoh, you can just over-engineer and put in something massive and stop worrying about it.

@jdo
If you actually have one in your hand then try it (including a fuse, of course) and see how it goes when you stall the motor or give it the highest load that is likely (measuring currents all the time).
 

1. What is the purpose of relay ratings?

Relay ratings are used to determine the maximum amount of current, voltage, or power that a relay can handle without overheating or malfunctioning. This helps prevent damage to the relay and the circuit it is connected to.

2. How are relay ratings determined?

Relay ratings are determined by manufacturers through extensive testing and analysis. Factors such as the materials, design, and construction of the relay are taken into account to determine its maximum capabilities.

3. What are the different types of relay ratings?

The most common types of relay ratings are contact ratings, coil ratings, and ambient temperature ratings. Contact ratings refer to the maximum amount of current or voltage that can be switched by the relay, while coil ratings refer to the amount of current needed to energize the relay. Ambient temperature ratings indicate the maximum temperature at which the relay can operate safely.

4. Can relay ratings be exceeded?

It is not recommended to exceed relay ratings, as this can lead to overheating and potential damage to the relay. It is important to choose a relay with appropriate ratings for the specific circuit and load it will be connected to.

5. How do relay ratings affect the overall performance of a circuit?

The ratings of a relay play a crucial role in the overall performance of a circuit. If the relay is not able to handle the current, voltage, or power required, it can lead to malfunctions or even failure of the circuit. Choosing the correct relay with suitable ratings is essential for the proper functioning of a circuit.

Similar threads

Replies
10
Views
4K
Replies
7
Views
1K
  • Electrical Engineering
Replies
4
Views
6K
  • Electrical Engineering
Replies
8
Views
6K
  • Electrical Engineering
Replies
13
Views
6K
  • Electrical Engineering
Replies
7
Views
13K
Replies
2
Views
1K
Replies
2
Views
9K
  • DIY Projects
Replies
5
Views
2K
  • Electrical Engineering
2
Replies
56
Views
51K
Back
Top