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m_c
07-09-2008, 06:06 PM
Just been working on how I'm going to wire up my control box, and was wondering if a solid state relay would be upto switching the 625VA toroidal transformer.

Had a quick look on RS, and part no. 346-918 is rated at 25A continous, with a surge rating of 250A.
What kind of surge rating would be needed for switching a Toroidal?

Main reason for wanting to use a Solid State Relay, is I plan on switching it from the E-stop circuit (hitting an e-stop will kill power to the main transformer, and trigger the e-stop signal to the breakout board, while the 5v control supply is kept live).
Being able to switch the transformer direct from 5v will eliminate the need for an extra booster relay to switch the transformer, as I can't find any 5v relays capable of switching the transformer direct.

irving2008
07-09-2008, 07:09 PM
Just been working on how I'm going to wire up my control box, and was wondering if a solid state relay would be upto switching the 625VA toroidal transformer.

Had a quick look on RS, and part no. 346-918 is rated at 25A continous, with a surge rating of 250A.
What kind of surge rating would be needed for switching a Toroidal?

Main reason for wanting to use a Solid State Relay, is I plan on switching it from the E-stop circuit (hitting an e-stop will kill power to the main transformer, and trigger the e-stop signal to the breakout board, while the 5v control supply is kept live).
Being able to switch the transformer direct from 5v will eliminate the need for an extra booster relay to switch the transformer, as I can't find any 5v relays capable of switching the transformer direct.A 625VA transformer has a primary current rating of 2.6A and would normally be fused with a 5A fuse so that SSR is massively overspec'd . Unless you are planning to switch the secondary? If you are using it to switch the secondary then you will need to allow for the 1.6v approx drop across the SSR. This is irrelevant for switching the primary.

I use a 24v relay for switching my transformer but it comes in a 5v equivalent, with 10A 250v AC rated contacts. Costs 1.50, much cheaper than an SSR...

m_c
07-09-2008, 08:27 PM
I know the SSR is majorly overated for continous current, but I don't want it to get cooked the first time I try turning the transformer on!

A standard relay is an option, but switching a transformer on is going to provide quite a surge, and still need a reasonably sized relay to handle the surge current.

I know the 3kw 240-415 step-up transformer I use for my 3 phase converter does on occasion trip it's 32A circuit breaker on power up, and that's without any load on it.
The Toroidal is going to be starting under slight load (it'll take a short period to charge the smoothing capacitor), so I want to make sure whatever I use to turn it on isn't going to go up in smoke.

Robin Hewitt
07-09-2008, 10:35 PM
It trips the circuit breaker "occasionally" because you "occasionally" switch it on when the mains is at peak volts.

Any SSR worth it's salt will delay switching until the mains crosses zero volts. Problem solved :D

m_c
07-09-2008, 11:09 PM
I know about the circuit breaker/peak volts, but never knew about that feature of SSRs.
You learn something new everyday!

This would be my first use of a SSR, so I just want to make sure I get it right.
I like to try and avoid puffs of smoke on the first power on!

I've been doing some calculations, and the SSR should be capable, unless the secondary side of the transformer has too big a load. It is borderline given it's going to be a 22,000uF capactitor peaking at 77V charging from zero volts, but going by my rough calculations and the datasheets, the bridge rectifier is going to die before the SSR.

If the BR does die, then I'll add an aditional relay/resistor to reduce the Bridge Rectifier load briefly until the relay closes and bypasses the resistor (a suitable relay will have a delay of around 20ms, which should be enough to overcome the initial surge)

irving2008
08-09-2008, 12:20 AM
I have been experimenting with this circuit as an alternative to my Thermistor/Relay inrush limiter

m_c
08-09-2008, 12:36 AM
That looks interesting.

I did consider something similar, but using a relay.
However, making the voltage sensitive switching is a bit beyond my electronic abilities, so that's why I was considering using a relay.

Robin Hewitt
08-09-2008, 12:41 AM
I use a 25A Crydom SSR to switch on the 2HP motor on my mill, got fed up with it pulling the RCB and killing the PC driving the steppers. It really has been a fit and forget solution, never given me a moments worry.

I liked it so much I used a small potted SSR to switch a transformer output on to a bridge for the DC suds pump.

I'd never willingly use a mechanical relay to switch AC again :D

m_c
08-09-2008, 12:54 AM
Now that I know more about SSRs, I'm already thinking about what else I can use them for.

A couple may find there way into the phase converter to reduce the surges on switch on/start up. But my priority just now is to get the wiring diagram sorted for the mill control box.

Robin Hewitt
08-09-2008, 11:06 AM
Now that I know more about SSRs, I'm already thinking about what else I can use them for.

If you want to make your own, Google for the MOC3040

An opto-isolated triac with zero crossover switching.

Add one resistor to limit the gate current and use it to drive the fat triac of your choice.

Not all SSR's feature zero cross over, but that's what data sheets are for :D

m_c
08-09-2008, 05:48 PM
I'm with you there Robin! Way too noisy :yes:

I dunno. I find it reassuring hearing a relay clicking, although I do cringe when my phase converter start-up relay chatters when a big load gets switched on.



If you want to make your own, Google for the MOC3040


I'll keep that info handy for future use, but at the moment, I'll stick to buying something that should work how it's supposed to!

irving2008
08-09-2008, 08:34 PM
If you want to make your own, Google for the MOC3040

An opto-isolated triac with zero crossover switching.

Add one resistor to limit the gate current and use it to drive the fat triac of your choice.

Not all SSR's feature zero cross over, but that's what data sheets are for :DI was of the understanding that SSR weren't TRIACS but series hi-power MOSFETS. A TRIAC wont turn on a zero crossing only off, and you usually use a diac to trigger them when the voltage reaches the trigger point, about 25v or so - thats why they work in a dimmer/speed control circuit by delaying the turn on point with an RC network. An SSR is on all the time it is enabled whatever the applied volts although there is a voltage drop across them of about 0.6v and they have an Rds(on) resistance which sounds more like a MOSFET with a series diode (the reverse diode in the second MOSFET).

Robin Hewitt
08-09-2008, 09:33 PM
I used to avoid triacs because of the need to retrigger them constantly to keep the power flowing but the opto-isolated triac makes that so easy. LED on = power flows.

That's why they made the MOC3040, a zero crossing variation of the good old MOC3020. It has no meaning other than as a switch.

I got the MOC3020 + power triac + 1 resistor circuit from an old RS catalogue which detailed an SSR they were touting. Okay they might have moved on since then but you can still build an opto isolated 25A mains switch for around a quid.

I used the circuit to switch on a half horse cap start motor which leapt convincingly until I bolted it down.

irving2008
09-09-2008, 12:19 AM
I used to avoid triacs because of the need to retrigger them constantly to keep the power flowing but the opto-isolated triac makes that so easy. LED on = power flows.

That's why they made the MOC3040, a zero crossing variation of the good old MOC3020. It has no meaning other than as a switch.

I got the MOC3020 + power triac + 1 resistor circuit from an old RS catalogue which detailed an SSR they were touting. Okay they might have moved on since then but you can still build an opto isolated 25A mains switch for around a quid.

I used the circuit to switch on a half horse cap start motor which leapt convincingly until I bolted it down.Just had a look at the data sheet for a MOC3040... its turn-on volts is 20v so not truely zero-crossing but as near as makes no difference. But at 43p, coupled with a 97p 60A continuous-rated 600V triac and a 330R resistor its a good way to switch spindle motor or similar...