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I am working on a project in which i will control the car fuel pump using relay to turn it on or off.

I want the relay to be suitable for keeping the fuel pump continously working for over 15 hours so that if the car goes on long journey i will be sure that the relays wont turn it off.

I know fuel pump use only 12A 12vdc

is this suitable https://www.ebay.com/itm/362076806216

and some pcb relays say max25a/1 hour .does this mean it will not be able to work more than 1 hour ?

Thanks in advance

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  • \$\begingroup\$ Can you provide a link to an example of a relay that specifies something like "25A/1 hour"? Can you go to an automotive parts store and ask them for a suitable relay? \$\endgroup\$ Jul 13 '18 at 0:41
  • \$\begingroup\$ I see this often 30A Carrying Current @ 2 Minutes Max . . . . . 25A @ 1 Hour Max . . . . 35A Switching Current \$\endgroup\$ Jul 13 '18 at 0:51
  • \$\begingroup\$ What kind of car has no battery and alternator? A 10A dc rating is too low \$\endgroup\$ Jul 13 '18 at 1:41
  • \$\begingroup\$ @ElliotAlderson google.com.eg/url?sa=t&source=web&rct=j&url=http://… \$\endgroup\$ Jul 13 '18 at 9:35
  • \$\begingroup\$ @ElliotAlderson this link will download a pdf of honga hfkw relay. In the data sheet the max current is on the left table says maximum continous current is 2*10a for 1 hr . what does this mean? \$\endgroup\$ Jul 13 '18 at 9:38
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Use a headlight relay (or a horn relay). these are typically rated at 20A or 40A continuous. not as cheap as those sugar-cube relays, but much more reliable.

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  • \$\begingroup\$ Hi, What do you think about these. Look at this on eBay ebay.co.uk/itm/171363170443 \$\endgroup\$ Jul 13 '18 at 15:35
  • \$\begingroup\$ they'll be ok: horn relays will work too. \$\endgroup\$
    – Jasen
    Jul 13 '18 at 21:41
  • \$\begingroup\$ ok great,Can you please take a look on this relay datasheet,i guess it will be suitable right? datasheetcafe.databank.netdna-cdn.com/wp-content/uploads/2016/… \$\endgroup\$ Jul 15 '18 at 2:01
  • \$\begingroup\$ says 10A onm the first page, so no! \$\endgroup\$
    – Jasen
    Jul 15 '18 at 2:44
  • \$\begingroup\$ No it is 20a at 14vdc, but it will be 10a at 125v . Kindly re read the pdf \$\endgroup\$ Jul 15 '18 at 4:49
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The problem I see, is that you are not distinguishing between two different critical parameters:
1 - The contacts carrying current capacity, and
2 - The coil's operating temperature range, and temperature rise characteristics. The current/time limitation comes from these.

The relay you selected, has a 25A/hr limit and 80 degree max temp rise.
For continuous operation, I would limit it to 40 degree rise.
It appears that at 12V 12A, it will work, but there are no safety margins for over voltage situations.

If you don't want to go through these kind of design and test steps, I recommend that you use the same kind of relay that a similar gas pump is using in your or comparable vehicle. The vehicle engineers already went through this.

Just in case you insist on doing it yourself, I recommend that at least you use one rated at 30A with 14VDC coil. Good luck!

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  • \$\begingroup\$ Hi, thanks alot for your answer,i guess engineers use the 5 leg relays not sugar cube relays because they are in the fuse box inside the car hood which is too hot in comparison to car cabin which i tend to put my device,i decided to give the songle sugar cube relays a shot and lets hope it will go fine \$\endgroup\$ Jul 21 '18 at 23:40
  • \$\begingroup\$ OK, just power the relay and monitor its temperature rise above ambient, for 24 hrs (use a recorder). If it settles at less than 40 dgr celsius, you will be OK. \$\endgroup\$
    – Guill
    Jul 23 '18 at 2:40
  • \$\begingroup\$ Keep in mind that this analysis is independent of where you place the relay (reasonably), because it is the "temperature rise above ambient" that is being used! \$\endgroup\$
    – Guill
    Jul 23 '18 at 2:45
  • \$\begingroup\$ One more thing that occurred to me, since you are going ahead with the songle relay, use two of them in parallel, and don't forget to use snubber diodes across the coils. \$\endgroup\$
    – Guill
    Jul 24 '18 at 0:14
  • \$\begingroup\$ Sorry I was sick, I just read your comments, thanks again. why should I use two in parallel? Regarding the diode i am using one piece of 1N4007 , what do you think? Take in consideration i will use the songle 20A relay but the car pump is only 12A so I guess the temperature will not be a trouble,right? \$\endgroup\$ Jul 27 '18 at 19:21

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