Dear All,
I would like to use this kind of relay (pololu.com/product/1562). I estimate that the max driving current will be about 14A at max 8.4VDC. Could you please let me know if this switch will be safely operated under this load?
Thanks in advance.
Best regards
George
Hello George,
14A is well above the rated current for the relay we use on those carrier boards. The manufacturer rates the relay at 8A for a switching current (current that will flow as the relay is closing or is flowing as the relay is opening) and 10A for a carry current (current flowing through when the mechanical switch inside the relay is closed and stable) for low DC voltages. You can find the rated currents for those relays inside their datasheet, which we link to under the Resources tab of the relay’s product page.
Also, the stall current for each of the 6 motors in the Wild Thumper at 8.4 volts could be around 7.7A, so using 14A as a max driving current doesn’t seem to be a very conservative estimate to me. Could you explain how you arrived at that estimate?
-Nathan
Hello Nathan,
Thanks for your reply.
I understand that I have to find a relay for higher amperage. Regarding the Wild Thumper current estimation I have tested it in flat terrain at 7.4VDC at 80% of the power and I 've got so far max 11A. Probably I have to make more tests in rough terrain for a safer estimation.
Is there any other SPDT for this amperage that you may propose me?
Thanks again for your excellent support.
Best regards
George
You might try searching for automotive relays to find a low voltage high current relay, though these usually can’t be driven directly from a microcontroller. We make a carrier board with the circuitry for driving relays that you could add your own relay to if you can find one in the same form factor or with a wiring harness.
-Nathan
Hello Nathan,
Thanks for your professional help!
Best regards
George
Hello Nathan,
One more question please.
According to the data set of your relay the specifications say 10A @ 30VDC.
If I will be switching the relay when there is no load, may I have more carry current if lower than 30VDC is applied?
So for instance, if we have max 8.4VDC, is a higher carry current allowed to pass through the relay? Is there any analogy between voltage and current in the relays?
Thanks again for you support.
Best regards
George
Hello, George.
I do not expect that you could decrease the voltage in order to increase the current and still be within the rating of the relay. The rated carry current is 10A and does not change with voltage or current type.
There is a relationship between current and voltage for the switching current. The maximum switching current is 8A for 0-30V DC and decreases as a function of voltage from 30V to 125V DC. This is shown in one of the graphs on page 4 of the datasheet. If you want to learn a little bit more about why the switching current rating changes, you might find section 2.1.2.2 of this Fujitsu whitepaper interesting.
-Nathan
Hello Nathan,
Thanks for your reply and advice.
Best regards
George