controlling 250V DC/AC with solid state

[Simon]

I need to have a control board switch DC or AC loads, due to the currents of up to 20A relays don't seem to be feasible for DC so I'm thinking solid state. SCR's don't seem to be practical at DC but I could be wrong but could a mosfet do the job ? for DC it would work fine as as the DS channel acts like a resistor and will pass current both ways will it work in AC too ?

[SeanB]

Look up some IGBT AC switches, basically a bridge rectifier, a big ass IGBT capable of handling the voltage and current, A VDR and snubber across the IGBT and bridge and finally an opto IGBT driver to provide the little wimpy voltage to turn it on and off. Handles DC, AC and anything in between.

[nctnico]

With an IGBT you'll be looking at 20 to 30W of losses. MOSFETs in series is also an option but parallel diodes may be necessary besides proper drive circuitry. I'd use a relay. Simple and reliable.

[Simon]

With an IGBT you'll be looking at 20 to 30W of losses. MOSFETs in series is also an option but parallel diodes may be necessary besides proper drive circuitry. I'd use a relay. Simple and reliable.

as i said in my op i have looked at relay's, find me one that will do 140V 20A DC (NOT AC) that does not cost the earth.

Another option may of course be to use a bridge rectifier in the case of AC systems

[nctnico]

An IGBT solution won't be cheap either. It wouldn't surprise me if a €20 relay turns out to be cheaper. Anyway, if you can use a bridge rectifier you could get away with a MOSFET. Be sure to use proper filtering and overvoltage protection 

[Simon]

find me a relay then, I have found relays that will handle 110V AC but only 28V DC

[Lurch]

Just to clarify, what is the current and voltages that you will be using?

A relay or contactor will need a different rating for AC and DC loads, but these ratings will also be dependent on the relevant voltages and load types so it isn't just a case of e.g. 250V/20A AC/DC. Last panel I assembled with 110VDC 16A switching was huge. You will need to de-rate some part of it for DC.

[Simon]

I'm powering a 1.5KW heating element and a 1 KW brushed motor both at 110-140V DC, yes as i said earlier any switching device has much lower capability at DC than AC, so to get anything that will cope on DC means humungus AC ratings so it's worth looking at solid state, I was thinking an SCR at first but was hoping to make it work for both AC and DC. Rectifying AC is not a huge obstacle and the motor only needs to run for a few seconds so we can cut out the heater when it is on to reduce the load greatly and save over designing for a few seconds of unnecessary simultaneous operation

[Lurch]

That's a decent relay you'll need there then! The ratings, and associated DC derating, will be related to inrush and arcing at switch on so you may not necessarily need to turn anything off, or rather, turning stuff off may not actually make much difference WRT switch/relay/contactor sizing.

[SeanB]

If you are using a bridge to supply the motor with DC just use a SSR in the AC feed to the bridge then, or even the AC rated relay, as it will only be switching AC. Otherwise if you have to use DC look at SCR commutation using another SCR and a capacitor to pulse the anode negative enough to switch it off.

[Simon]

If you are using a bridge to supply the motor with DC just use a SSR in the AC feed to the bridge then, or even the AC rated relay, as it will only be switching AC. Otherwise if you have to use DC look at SCR commutation using another SCR and a capacitor to pulse the anode negative enough to switch it off.

You are sort of looking at this from the wrong end, I do not have a problem with A/C control of the motor or heater they are in fact both A/C devices the problem is controlling them with DC because of the issues with mechanical contacts. But while we are solving with DC problem I am trying to ensure that the same circuitry can also switch A/C or we could go down the route of having a separate power control board so that just this part can be swapped over for A/C or DC operation or for A/C operation relays can be used and then be replaced with the control board for DC.

An SCR sound like the easiest solution I'm just a bit apprehensive about how to turn it off with DC but then at the end of the day I will not be designing the nitty-gritty of this circuit as it is being subcontracted by my employer but in order to speed things up and ensure we don't get lost in unlikely solutions I'm looking for some options to talk to our subcontractor about.

The problem we have is that the whole unit needs approving for rail and we could be getting requests for A/C or DC versions so I am trying to avoid a retest for the second batch should they be A/C units.

[Lurch]

You are sort of looking at this from the wrong end, I do not have a problem with A/C control of the motor or heater they are in fact both A/C devices the problem is controlling them with DC because of the issues with mechanical contacts. But while we are solving with DC problem I am trying to ensure that the same circuitry can also switch A/C or we could go down the route of having a separate power control board so that just this part can be swapped over for A/C or DC operation or for A/C operation relays can be used and then be replaced with the control board for DC.

I would expect that if it is rated for 20A@120VDC then it will most certainly not be a problem to also be used for 20A@400V AC

[SeanB]

OK, so your incoming supply will go to a bridge rectifier, and then the 2 loads will need power switching. Then you will need a heatsink at least for the bridge ( 15A units will need it for sure, and if you are doing rail 40A would not be overkill) and then 2  IGBT or power mosfet, or even bipolar transistors will do, select on voltage drop at the current of 10A or so to get the lowest cost device that runs the coolest. Those get a heatsink with the bridge, and if you are willing to make the bridge out of individual TO220 or TO247 diodes then all fit on one heatsink on edge.

[nctnico]

I'd let the subcontractor deal with the problem. Maybe he/she already has a clever solution but goes with the (more expensive) one you are providing. There is nothing more irritating than a customer with an idea on what the solution should look like.

[Simon]

I'd let the subcontractor deal with the problem. Maybe he/she already has a clever solution but goes with the (more expensive) one you are providing. There is nothing more irritating than a customer with an idea on what the solution should look like.

So far anything that they have produced i have had to fix and taking up a discussion about something that needs doing generally involves them making it sound like it needs to be more complicated than really need be. I know a customer with an idea can be a pain, but a supplier with no idea or too big an idea is worse when we are paying.

[Simon]

You are sort of looking at this from the wrong end, I do not have a problem with A/C control of the motor or heater they are in fact both A/C devices the problem is controlling them with DC because of the issues with mechanical contacts. But while we are solving with DC problem I am trying to ensure that the same circuitry can also switch A/C or we could go down the route of having a separate power control board so that just this part can be swapped over for A/C or DC operation or for A/C operation relays can be used and then be replaced with the control board for DC.

I would expect that if it is rated for 20A@120VDC then it will most certainly not be a problem to also be used for 20A@400V AC

So go and find me a relay rated for 200V DC that will switch a 20A load..........

[nctnico]

I'd let the subcontractor deal with the problem. Maybe he/she already has a clever solution but goes with the (more expensive) one you are providing. There is nothing more irritating than a customer with an idea on what the solution should look like.

So far anything that they have produced i have had to fix and taking up a discussion about something that needs doing generally involves them making it sound like it needs to be more complicated than really need be. I know a customer with an idea can be a pain, but a supplier with no idea or too big an idea is worse when we are paying.

Why not hire someone else? There are dozens of small companies to choose from. Just put an add on a forum.

[Kremmen]

Well, for example the GA 75 type DC contactor from my onetime employer ABB can do that and more. The series has much larger ones as well.
If you google 'ga dc contactor site:au' the first link will be to the relevant product page. As a general note you will find that ABB can provide a solutio  for almost any electrical or power electronics application. No idea about the prices though - other than that the Chinese will be cheaper

[Lurch]

So go and find me a relay rated for 200V DC that will switch a 20A load..........

DC contactor brochure;

http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/f25bae3eb7bde2e0c12578cd0029dca9/$file/1SFC101004B0201.pdf

Specifically;

http://www.ng.abb.com/product/seitp329/12daed844051a599482570f30045c9e5.aspx?tabKey=2&gid=ABB1SFL557025R7011&cid=9AAC100109

[AlfBaz]

It's mostly about the breaking capacity get a 3 phase contactor run one leg through one pole and series the other through two poles

[Lurch]

It's mostly about the breaking capacity get a 3 phase contactor run one leg through one pole and series the other through two poles

Bad idea.

[Kremmen]

Series connection is actually a technique to use but you won't turn a regular AC contqctor into a DC one quite that easily. A proper DC contactor is qualitatively different in that there are special arrangements to extinguish the arc when contact breaks.  You won't find those in a regular contactor.
Here is a link to the ABB contactors catalog where you will find the DC ones from page 207 onwards: http://search-ext.abb.com/library/Download.aspx?DocumentID=1SXU000023C0203_01&LanguageCode=en&DocumentPartId=&Action=Launch

[Simon]

So go and find me a relay rated for 200V DC that will switch a 20A load..........

DC contactor brochure;

http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/f25bae3eb7bde2e0c12578cd0029dca9/$file/1SFC101004B0201.pdf

Specifically;

http://www.ng.abb.com/product/seitp329/12daed844051a599482570f30045c9e5.aspx?tabKey=2&gid=ABB1SFL557025R7011&cid=9AAC100109

and how much do they cost ? probably hundred of pounds ? so cheaper to use solid state devices

[Lurch]

So go and find me a relay rated for 200V DC that will switch a 20A load..........

and how much do they cost ? probably hundred of pounds ? so cheaper to use solid state devices

See, now you're just being awkward!

[Simon]

No I have a product to get designed, I can't justify spending money on solutions that have cheaper alternatives, at the moment for all I can we can use a mosfet for the 110-150V DC and then use relays for ac if it's the cheapest solution and the customer will have to pay for testing two solutions although I'm trying to avoid it.