I've got a trolling motor that pulls about 40 amps at peak, but will almost always be ran in the 10-15A range. I want to be able to reverse the prop direction. I plan on using two 40A SPDT automotive relays in parallel to act as a DPDT that is activated by a DPDT toggle switch. Is there anyway to protect the contacts from being damaged when the motor is changing directions? When I installed an electric fan in my car I used a diode to absorb the inductive load produced by the fan when it is shut off by shorting the power leads. This obviously won't work with the trolling motor since it will need to operate in both directions. Perhaps these SPDT relays have built in protection for this inductive load?
I'm also curious as to how well the relay will hold up under the desired loads? I know DC motors have a huge starting current, 3-5x the running load. This would mean a 45-75A starting load going through the relay with 15A running current. Will I need to find a bigger relay or will the 40A be able to survive this?
Thanks for any help you can offer.
Dave
you could use a capacitor with a low ESR, thats the old way to absorb inductive dump, you could also use a large transient voltage supressor (TVS) like 20 V or so and one that can handle a KW or more...
you may want to use a larger relay if its cost effective, I have seen relays that are rated at 70A or more for the 12V world and are not all that expensive, if the current is too great the contacts will weld shut...
xetmes wrote:
you could use a capacitor with a low ESR, thats the old way to absorb inductive dump, you could also use a large transient voltage supressor (TVS) like 20 V or so and one that can handle a KW or more...
you may want to use a larger relay if its cost effective, I have seen relays that are rated at 70A or more for the 12V world and are not all that expensive, if the current is too great the contacts will weld shut...
Do you know of any place where I could find a reasonably priced 75A relay? The best I've found is about $30 at Autozone for a Bosch relay. I can get two generic 40A "automotive" relays from www.allelectronics.com for $2 each.
For the contact protection would it be possible to use two diodes back-to-back (facing each other)? A book that I found on relays that was written in 1962 recommends this, but I'm guessing there would be a modern/better/cheaper way of doing this?
Also what is a transient voltage suppressor? Where could I find one?
Actually I am not sure where you could get a relay much cheaper than $30, I didnt think that was that bad a price, but compared to $2 it is quite an increase. You could try the relays in parallel and it would probably work fine if inrush current is only about 40A...
The back to back diodes you saw where zener diodes, this practice is still used today. A bidirectional transient voltage supressor is basically 2 zener diodes back to back in one package, it is also made to kill large transient voltages and can handle 500 Watts or more (for a very short period of time)...
digikey sells all kinds of voltage supressors ( digikey.com )
good luck