solid state relay?

A solid state relay designed for A.C. loads will not work to switch a D.C. load on and off. The solid state relays i have worked with use a scr or a triac to turn the load on and off. If you turn on a scr or triac in a D.C. circuit it will stay on until you disconect the power from the scr or triac, in other words the SSR will latch in the on state until you remove the power from the SSR.

Best Regards Tom.

Grant, SSR's draw very little control current. A 9-12V battery will last a long time, and will fail "off", no resistor needed.

You don't say what you want to control, the generator must have some control inputs, but, I'm stumped.

More info please. Dave

sheet:

There is an idle solenoid mounted on the side of the carburetor. When it is energized, it pulls the governor arm to an idle position. When it is de-energized, it releases the governor so it goes to the weld position.

This idle solenoid has a coil resistance of 5.6 ohms, so run from 12V will draw about 2 amps DC.

This is what I want to control. The power I have available to switch on or off to a relay coil is just 12VDC, basically a car battery.

This is a gas engine coupled to a welder, a relatively crude machine.

Azotic already posted that my relay won't work to switch a DC load. Now I'm trying to find one that will, hopefully that won't break the bank.

Coil voltage: 12VDC Load voltage: 12VDC

Thanks, Grant

Mechanical Magic wrote:

sheet:

O.K.

Why the current limiting resistor? The relays can accept any voltage between 3 to 32 VDC and will draw no more than 14 mA from that -- without an external limiting resistor.

Oops -- never mind. Looking at the data sheet, this is apparently a style intended for constant current input -- and you can hook several of them in series to control them all with a single current. (The voltage input ones, which are what I am more accustomed to, would be hooked in parallel instead.

But your more serious problem is trying to switch DC as the load current.

It can happily switch *on* the DC. The problem is that it is a thyristor or triac controlled through an opto-isolator from the 3 to 32 VDC control signal.

A thyristor or triac switches on when it gets enough current through the gate.

It switches back *off* only when the current through the load drops to zero -- which happens once every 120th of a second with AC, and never with DC -- unless you provide something else to interrupt the current -- in which case you don't need this relay. :-)

It is not picky. Notice the 8.5 to 14 mA range. So -- this will probably work. You can tell by looking at the LED on the relay. The only problem might be that the input will drop a full 3V at the maximum current. This would leave you with 11 VDC to drop through the resistor -- so 11 mA which should be enough. (Note that automobile batteries, while nominally 12 VDC are usually around 14V or a bit higher when being charged. So set your resistor a bit on the high side and then see whether the LED comes on.

And really -- don't bother, because it can't switch your DC load off anyway.

Good Luck, DoN.

Grant, Forget the SSR's.

Most Auto Parts Stores have 12V relays. Used for lights and things. Simple hookup, 1/4" spade lugs. Take a look. Dave

sheet:

Most likley the control module is a simple voltage comparator which energizes the solenoid when you strike the stinger to the work and the voltage drops. They are fairley simple to make if your handy with electronics, about $10.00 worth of off the shelf parts should do it. If you don't want to build your own you can probobaly buy a ready built voltage controlled relay. You will have to measure the output voltage of the welder to buy one with the correct coil voltage. If you can find one that is SPDT that will be the easiest to set up.

Try a google search for VOLTAGE CONTROLLED RELAY.

Best Regards Tom.

sheet:

KISS - use any relay with a 12v coil, salvage a headlight relay etc from a junked car.....switching requirements are certainly not onerous....

Andrew VK3BFA.

How about a fog lights/driving lights relay? Available at about any car parts place, and can be bought with a socket & harness.

better yet, if the solenoid only draws 2 amps, just run zip cord directly to the solenoid and have a switch on the welding head - no relay, no nothing.....

znip-------

Grant, I use a lot of these devices. They are wonderful! However an AC version will turn on once, but never turn off on DC, until the power source is removed. Internally there are a pair of SCRs back to back. If you use these against an inductive load, you must use an RC filter across the output terminals and I highly recommend using one rated for twice the use voltage, because back EMF will kill'em dead. For your application, the same company makes SSRs for DC in 5, 10 and 20 Amp versions. Again, for safety, use a diode in opposite polarity across the output when using an inductive load for back EMF reasons. Steve

You need a solid state relay with a FET output stage. Crouset GF. Or transistor output. Crouzet GT.

Crouzet is one of the Schneider group.

Thanks to all, I found a nice DC-DC relay at NAPA for about nine bucks. Now I can run real skinny wire up the welding lead, doesn't have to carry 2 or 3 amps.

GWE

Grant Erw> There is an idle solenoid mounted on the side of the carburetor. When it is

Hmm ... does the generator part produce AC? If so, you could run that through a transformer to produce 12VAC, through the SSR, and into a bridge rectifier and capacitor so you would have AC to switch with the SSR. If it only produces DC, your choices would be a real relay with a coil (lots are available in 12V versions) or a circuit using a transistor, and a diode across the solenoid's coil to suppress the high voltage spikes produced when the circuit turns off.

I, personally, would go for the physical relay with a coil as the simpler device. Good ones, properly sized for the load, will last through more cycles than the rest of the mechanism of the welder will be likely to survive -- even from new.

SSRs are a poor choice for this task, I think. They require additional complication of the whole setup.

Good Luck, DoN.

SNIPPED a TON

It has been noted often in this thread that SCR's and such will turn on and then not turn off until the power is removed. That occurs by both literally "removing" power, and by zero crossing in AC. They will also "turn off" if commutated, which is effectively what the welder would do if the relay was strapped across the welding lead outputs.

I still have not figured out what the OP wants to do.

Brian Lawson.