Relay Question

[VegaObscura]

Will AC relays like this one:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=425-2395-5-ND

Also work for DC current?  Here's a direct link to the datasheet if anyone is interested:
http://media.digikey.com/pdf/Data%20Sheets/Sharp%20PDFs/S108,208T01.pdf

I like the look of this relay because it is very high-voltage and high current (8amps) for its price.  I just need to know if I can control DC current with it or if it will only work with AC.

[futmacl]

Will AC relays like this one:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=425-2395-5-ND

Also work for DC current?  Here's a direct link to the datasheet if anyone is interested:
http://media.digikey.com/pdf/Data%20Sheets/Sharp%20PDFs/S108,208T01.pdf

I like the look of this relay because it is very high-voltage and high current (8amps) for its price.  I just need to know if I can control DC current with it or if it will only work with AC.

Huh? There are tons of much cheaper relays with comparable ratings, e.g. this 10A / 250VAC one:

http://www.mouser.com/ProductDetail/Omron-Electronics/G5LA-1A4-DC12/?qs=sGAEpiMZZMtSzCF3XBhmWx1pIvi%2f1bNexPTEVEwydX4%3d

Triac output relay like the one you linked to should in principle work with DC, but it will latch until the *switched* voltage drops within 1.5V of ground, which is probably not what you want?

[VegaObscura]

I see.  I didn't realize it would latch like that.  I'm still inexperienced with relays and just know the basics of how they work.

I see on the datasheet to the relay you linked says that it has a coil rating of 5-48VDC.  Does this mean that I can switch it on just by sending a 5V signal from my microcontroller to the relay, without needing a transistor?

[waltr]

Look to the Data sheet in that link then again the G5LA data sheet for the details.

These relays are available with coils for different voltages. You must order the relay for the coil voltage you need.
For example:
Note: When ordering, add the rated coil voltage to the model number.
Example: G5LA-1 DC12
The DC12 means the coils is designed for 12V DC. To power from 5V look for DC5 at the end of the part number.

The next piece of the specs to look at is the coil resistance. This, the voltage and Ohm's law gives the current draw of the coil. For example for the data sheet for a 5V relay the coil resistance is 69.4 Ohm. Then I = E/R = 5/69.4 = 72mA.
This is way too much current for almost any uProcessor output so a transistor would be required.

[Soeren]

Hi,

Will AC relays like this one:

Also work for DC current?  Here's a direct link to the datasheet if anyone is interested:

I like the look of this relay because it is very high-voltage and high current (8amps) for its price.  I just need to know if I can control DC current with it or if it will only work with AC.

You don't use SSR's for DC. Instead, just use a power transistor, MOSFET or (Darlington) BJT.

[VegaObscura]

Perhaps I was mistaken in thinking that relays would be the way to go.  I need something that can handle up to 4 amps.  The purpose is to switch on a model rocket igniter.  These igniters, to my understanding, are simply a bit of highly combustible material on an uninsulated wire.  To fire them, you connect the leads of this wire to a battery, causing a short circuit.  Due to the short circuit, the wires heat up, and the combustible material combusts, thus igniting the model rocket engine.  The battery I will be using is a 9V.  I used my multimeter to test the maximum amount of current that my 9V alkaline batteries can put out when short circuited, and it read around 2 amps.  I figured I should double this number for safety, and thus I got 4amps at 9volts.  I suppose an N-channel mosfet is the best way to go for this circuit.  Not sure why I ever thought that I needed a relay.

[Soeren]

Hi,

Yes, MOSFET's comes in up to more than 100A versions, but I'd probably use something like a TIP120 for your igniter, as bipolars are much more rugged in handling. Further, some MOSFET's needs around 10V of gate voltage to open reliably, while the TIP120 needs just 1.2V to 1.4V of base voltage and have a DC current gain of more than 4000 at 2A collector current (at room temp).

[VegaObscura]

Soeren, how on earth do you remember all these part numbers?

The TIP120 should work perfectly for my application.  The datasheet I found on it (here) indicates that it has a current gain of 1000.  To switch it on at 5V base current, I would need to use a 1kOhm resistor, right?

[Soeren]

Hi,

Soeren, how on earth do you remember all these part numbers?

That's what my analog teacher on the academy asked me as well 
Probably just that I have been in the game for close to 4.5 decade - you gotta pick up a number or two in that time.

The TIP120 should work perfectly for my application.  The datasheet I found on it (here) indicates that it has a current gain of 1000. 

Look at Fig. 1 (DC current gain) on page 3 - follow the 2A vertical line (around peak gain) to where the curve intercepts it and then follow the horizontal line to get the gain.
ON semis datasheet on the TIP120 has got curves for 125°C (gain=~6500) and -55°C (gain=~2300) as well.

To switch it on at 5V base current, I would need to use a 1kOhm resistor, right?

Yes, 1k is fine. Anything between roughly 350 Ohm (assuming a 10mA output) and about 7kOhm should work.