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JRouche
11-12-2011, 11:59 PM
I have a couple of projects that I need to use some relays.

I need to switch a DC load with a DC sig.

Im buying from ebay cause the prices are good. I see alot of DC triggered and AC load relays. I am switching DC loads with a DC trigger. Can I use an AC load SS relay to control a DC load? I would think so, I dont know why they differentiate between an AC load or DC other than maybe an AC relay has some advantages to be used with AC. But a simple DC switching relay might not have the issues that an AC load might.

And Im well within my current needs and voltages. So thats not an issue. Im switching lower voltages, higher current. So the RMS value between an AC relay and a DC relay is not a concern here. They are 230VAC relays and Im switching 24VDC. Current is rated at 40amps and Im switching 20amps max.

Im asking because the availability of DC triggered, AC load SS relays is massive. Pennies on the dollar. Dc-Dc SS relays for my current demand are much higher. Id rather save some money if the AC load relays will work.

Thanks in advance for any help... JR

ftl
11-13-2011, 12:09 AM
If the relays are not rated to switch DC, they will likely not work. Most devices that switch AC use Triacs or SCR's (two SCRs are similar to one Triac), which work in the same way. They turn on and latch on until the current goes to zero. With AC that happens with the next zero crossing (100 to 120 times a second depending on your AC line frequency (50 or 60 hz)). With DC they will go on and latch on until something else turns the current off.

With regular mechanical relays the DC rating is usually lower than the AC rating. That is for a similar reason. When the contact opens an arc will start. With AC the arc will tend to extinguish at the next zero crossing. With DC the arc will continue until the contacts are far enough apart to extinguish the arc. Once an arc starts it tends to continue because the air is ionized which gives it a relativly low resistance.

Evan
11-13-2011, 12:10 AM
AC solid state relays use a triac to control the load. In order to switch off the voltage across the triac must pass through zero after the gate signal is grounded. A triac is equivalent to a pair of SCRs connected in anti-parallel with the gates tied together. Like an SCR it won't turn off when it is conducting current.

Because of that it cannot be used to switch pure DC although it can switch pulsating DC, either half wave or full wave.

Paul Alciatore
11-13-2011, 01:14 AM
If you want to control DC with a DC control signal the device to use is called a transistor or an FET. There is no need to design a "DC Relay" when these simple semiconductor devices do it already.

JRouche
11-13-2011, 01:17 AM
Thanks guys. You nailed it.

So the triac is latched with current and with DC Ill never get the break in current, Wow, the triacs are fast to see the drop in current with 60hz.

So.. How do the DC SS relays work? A dropping circuit?

So I guess my search is on for an inexpensive SS dc relay...

Thanks for the fast reply guys, you really know yer electronics... JR

Oh, on a second thought. My switching needs are not in operation. I dont need to switch the load on and off while the system is working. I want to use the relay as a very simple DC high power relay to turn a sub-system on, not so much off.

I need to use a low current circuit to turn on a subsystem. Not to switch the power on and off. Its a very basic system where I need to energize a panel when the main power source is turned on then it will power down when the mains are turned off.

Its not a continuous switching circuit. It just needs to turn on and allow current flow when the main switch is activated and as far as the turning off part thats not a problem, the supply will be removed anyway.

So how bout that? As a simple switch, ALL power will be removed for the off time. Will the AC relay work for the on time? To switch the DC on and I can remove the power source to to shut it off.

I should have been more clear on my needs.. Thanks guys. JR

ftl
11-13-2011, 01:32 AM
An AC SSR may work for your requirements. There is another issue. Some SSRs use zero crossing switching as well. That means that they wait until an AC cycle is just past a zero crossing to turn on. They do that to reduce the switching noise. If it switches on when the AC is at only a few volts there is no switching of 150 volts in a microsecond, which causes RF interference.

The triacs are the device that is used in an incandescent light dimmer. In that case the triac turns on at different points in the AC cycle to provide a set of variable pulses to the load. If it turns on early in the cycle the lamp will be bright. If it turns on very late in the cycle, the lamp will only get very short pulses, so it will be dim. In all cases the triac turns off when the current goes to zero (and there is no gate current present). Dimmers are often electrically noisy because of how they turn on with each AC pulse.

So if the SSD uses zero crossing switching it may not latch up for a DC signal.

For what you want, a SCR may be the solution. Give it a gate signal and it will turn on a DC current until the supply current goes away. A 30A scr that can handle 24V is relatively normal.

A quick search at Digikey came up with a BTW69 - 50A 200V for less than $7.

http://search.digikey.com/ca/en/products/BTW69-200RG/497-4186-5-ND/725405

Edit: How do the DC relays work?

They may use a MOSFET as the switching element. That would make them a DC only SSR. MOSFET's can switch DC with an on resistance of less than 0.05 ohms. MOSFET's are also very fast in switching. They can switch in much less than a micro-second.

Alternatively, a DC (or AC / DC) SSR may in fact still use a triac as the main current handling device. What they add is another switching device in parallel with the triac. They may add two to allow for AC. The additional device is pulsed on (when you want to turn the SSR off), so it takes the current and the current through the triac drops below its latching threshold. The triac then releases and the other switch can be released. The load is then off. The shunting device only needs to conduct for a very short pulse. A much smaller device can be used to handle this sort of pulse rather than the continious conduction of the triac.

darryl
11-13-2011, 02:18 AM
Whatever happened to the four lead scr or triac? These devices had a turn-off lead as well as the gate and main terminals. I can see the four lead scr being kind of redundant because of power transistors, but the turn-off capable triac would seem to be a handy device.

Evan
11-13-2011, 03:08 AM
One item about solid state relays that hasn't been mentioned is that they are nearly always opto isolated so there is no chance of the line power feeding back to the control circuit due to a malfunction.

Boostinjdm
11-13-2011, 03:52 AM
Two questions.....Why SS and not go mechanical? Why not just forget the relay and go with a multi pole main switch?

Black_Moons
11-13-2011, 03:55 AM
'DC switching' solid state relays are basicly an LED, photocell and mosfet.

And yes, Solid state relays like regular relays are basicly always isolated.

That said, Sometimes a AC triac can switch a DC load.. if the DC load is intermitant enough.. Mainly just brushed motors in my experiance however.

You could also just use a regular relay, Add 1 transistor (And protection diode) if needed to switch the relay with just a few mA

lakeside53
11-13-2011, 12:11 PM
Two questions.....Why SS and not go mechanical? Why not just forget the relay and go with a multi pole main switch?

1) for 1-3 poles, SSR is generally cheaper, quieter, faster, smaller, reliable, "no spark"...

2) Cheaper, smaller, no heavy contacts ... etc, and you can "turn them off" (like with a safety circuit).


yes... you can use mechanical and many still do, especially when you need multiple contacts.

lakeside53
11-13-2011, 12:23 PM
Whatever happened to the four lead scr or triac? These devices had a turn-off lead as well as the gate and main terminals. I can see the four lead scr being kind of redundant because of power transistors, but the turn-off capable triac would seem to be a handy device.


They are still made, and getting better. There are issues with turning off currents in excess of the max ratings (like with a fault ).

Unlike power "transistor", the advantage of the GTO is that it latches off and on with a single pulse.


EDIT ; here's a much better explanation! http://machinedesign.com/article/gate-turn-off-switch-1115

JRouche
11-13-2011, 01:17 PM
Thanks for ALL the help here guys.. I learned a lot and with that knowledge found a source for some inexpensive, DC switching MOSFET relays. Im back in business. Time to make a heat sink :) Thanks again..... JR

dp
11-13-2011, 01:54 PM
Thanks for ALL the help here guys.. I learned a lot and with that knowledge found a source for some inexpensive, DC switching MOSFET relays. Im back in business. Time to make a heat sink :) Thanks again..... JR

You should probably see if the spec sheet offers any information on failure mode. Ideally the device will fail open and not shorted.

hoof
11-13-2011, 06:57 PM
Fork-lift motor controls do just that. They use high current SCR's to switch DC. However there are more components to turn off the SCR then there is to turn them on. It's somewhat complex and expensive. The transistor has my vote on this as well