Solid State DC/DC Relays

[A. G. Olphart]

My wife just burned the points off a cute little cube shaped plug in relay that we were using to switch a transistorized electric fuel pump. We have a few more relays, but... don't like using something known to fail.

 

Search didn't find much and my personal knowledge on the subject is nil. So, fellow hybridizers...

What is the best source for relatively high current DC/DC SSRs, and what sort of protection do they require, esp. on inductive loads? (Got a circuit?)

 

Google found a couple stores:

Last 2 items on this page- http://www.futurlec.com/RelSS.shtml

Third item on this page- http://www.jameco.com/webapp/wcs/stores/servlet/KeywordSearchResultView?langId=-1&storeId=10001&catalogId=10001&searchType=k&searchValue=solid%20state%20relay&categoryId=353570&rscount=32

 

Is there something better for fuel pumps and/or the high current Taurus fans?

(I promise I won't mount a solid state relay against one of my headers).

 

 

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[roger.svoboda]

try mouser.com they have a large selection of relays of all descriptions. You might try to find out the current draw of the fuel pump. Relays will burn out when you run too much current through them. Otherwise they are relatively bullet proof. try a relay that is more brutish and not so cute.

[zcarnut]

The common solution to the stress problem created when switching inductive loads is to put what is known as a flyback diode in the reverse direction across the inductive load. Place the diode so that the voltage spike at switch off will forward bias the diode creating a return path for the current:

 

http://zone.ni.com/devzone/cda/tut/p/id/4197

 

Scroll down to the section “Switching Inductive Loadsâ€

 

I have also used a capacitor across the switch (either a mechanical relay or a solid-state one) to absorb the switching energy.

 

Lots of other info out there:

 

http://www.acroname.com/robotics/info/articles/drivers/drivers.html

 

http://www.4qdtec.com/catch.html

[A. G. Olphart]

Thank you, gentlemen.

 

I think the little automotive cube relays are generally rated at 20-30 amps; it should have been sufficient... live, and learn. I had a diode to catch the voltage spike, but didn't know the trick of adding a resistor equal to the coil resistance. Slow opening may have contributed to the relay's early demise.

 

Hopefully the 10 amps my VOM will handle is enough to measure the current draw; I'll see how hard it pegs the needle, and go from there.

 

 

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[Dave]

Thank you, gentlemen.

I think the little automotive cube relays are generally rated at 20-30 amps; it should have been sufficient... live, and learn. I had a diode to catch the voltage spike, but didn't know the trick of adding a resistor equal to the coil resistance. Slow opening may have contributed to the relay's early demise.

 

Hopefully the 10 amps my VOM will handle is enough to measure the current draw; I'll see how hard it pegs the needle, and go from there.

 

Did you do the DC/DC SSR swap? I did the Honda cooling fan upgrade and burned up the fuse and fuse holder from the higher current draw of the Honda fan. I looked into the SSR's for the heater fan and the fuel pump and it looks like a better solution than the small automotive relay, and their silent.

 

I lost a couple relays for the radiator fan before I switched to a 80 amp continuous duty that looks like a Ford starter solenoid except it has a metal case. I am looking for something better than the standard relay but not as big as the ones I used on the cooling fan.

 

Are the SSR's rated as continuous duty? I did see some come attached to a heat sink.