Thread: Building an electrical control box

Originally Posted by rrobor

That is OK all except for the resstor, that should come off the bridge. C6 was fine. leave it in and where the resistor is, fit a fuse to protect against shorts

Hi rrobor,

Just a little clarification...

The resistor is intended to limit the current a bit to stop the inrush current causing the regulator to go into overcurrent shutdown. A fuse on the regulator output will do nothing. The regulator is protected against short circuits anyway. If you really want a fuse, then the place to put it would be between the transformer and the bridge, to protect the transformer against the bridge going short cct.

Regards
Ray

You need a load for your filter capacitor which is the1000uF reservoir or filter. You get hum out of the bridge it fills the filter to peak. As the cycle drops from peak to its lowest point the resistor helps maintain the voltage thet gets bled from the reservoir. Three pin regulators are short protected yes but usually by temperature. So if you are not there and a short is created for some reason, the regulator will switch off and on till it fails. If you wish to put some form of burffer there I suggest something low 1 ohm etc but as you are only needing 500ma its not needed. Also you now have 24V if you add another load your it will drop your voltage.

Another schematic

Hi .RC,

Nope... Get rid of the 1000uf on the regulator output, the regulator won't start up driving a large capacitative load, also, get rid of the resistor on the bridge output, that will just introduce more ripple on the regulator input. Finally get rid of the fuse in the regulator output cct. Three teminal regulators are protected against short circuits (as well as thermal shutdown)..

This is kindergarten stuff....

Regards
Ray

Here is the final schematic that I will do..

Hi .RC,

That looks ok, if you had some data on the contactor coil, (especially the in-rush current) you might be able to tweak the 10 ohm resistor value, preferably zero.

Regards
Ray

Thanks .RC.
I've just downloaded TinyCad looks pretty good for the price

Stuart

I wouldn't bother with capacitors and resistors but the diode across the coil terminals is ok. Just make sure the PIV is twice the supply voltage (minimum 50v).

In my approximately 10 years as a switchboard design engineer, I have never seen the warranted use of resistors or capacitors across a DC contactor coil.

On the other hand, I would be concerned about the protection of the control circuit. According to your diagram, you do not have any protection whatsoever. It appears as though you have the smallest size contactor, the contacts of which are probably rated as follows;
AC-3 2.2kW @ 415v (ranging between about 5.5 amps to about 6 amps). If the coil for this size contactor is DC, it will generally have an;
1] Inrush - 2.5W.
2] Hold in - 2.5W.

DC coils are typically used where the contactor is required to be energised for very long periods of time. DC contactor coils have a longer 'active' life than AC contactor coils. Since the coil you are using will absorb about 2.5W @ 24v DC, the current will be about 100mA. As such, your transformer (and therefore downstream wiring and switchgear) needs to be protected by;
a] One 25mA fuse on the Active of the primary and,
b] Two 400mA fuses on each leg of the secondary winding.

These sizes will allow for an inrush current of approximately 400% of the 'hold in' current of the contactor coil.

I also notice that you plan to use a 'jogging' function. I don't know the technical details about the particular contactor that you are using BUT 'jogging' in electrical terms is called 'inching and plugging'. Contactors have an AC-4 rating, which pertains to this. Basically, if your contactor is adequately rated for AC-3 but is not adequately rated for AC-4, sooner or later (usually sooner), the contacts will deteriorate badly. This can result in excessive arcing and welded contacts. If the result is welded contacts and you do not have any control circuit protection, the result could very well be 'fire'. The fire may not necessarily be restricted to your control box, unless you have a maximum 300mA (that's three hundred milliamp) RCD protecting all of your equipment.

If you think I'm pedantic, I'm not. I would very much like to see you and your machine, live a long and happy life.

If there is no resistor between the bridge and the regulator your filter capacitor is worse than useless. You talk about inrush current well your filter will look like a short circuit at switch on. Current leads in a capacitor and lags in an inductor. The value of 10 ohms yes is a bit high I would use 4,7. The 3 pin regulator will shut down on a short yes but it has no means of telling if the short has been removed so when it cools down it will switch back on. This was a TV fault when a clip broke holding a 15V 3 pin to a heatsink so not new to me. And the resistor in the output is not needed and will reduce your voltage

Originally Posted by rrobor

If there is no resistor between the bridge and the regulator your filter capacitor is worse than useless.

Sorry rrobor, that's just plain wrong. A resistor between the bridge and the filter cap will cause more ripple on the regulator input.

The first power supply I designed ( using 3 terminal regulators that is ) was in 1975, and I used a fairchild ua78CB 13.8v 3A T03 package designed for powering CB radios, I must have manufactured and sold several hundred of those. Since then, over the last 35 years then I have designed and built numerous systems (probably upwards of 500 or more), mostly embedded control systems of all types. Some of them manufactured in the 1000's of units, often with 3 terminal regulators, never have I put a resistor between the bridge and the filter cap on any design. And I've never seen anyone else do it either.

Please tell me you are kidding...

Edit: Wait... I did do a 100 amp power supply where we had 100,000uf caps and we had current limiting resistors that limited the charging current when first switched on, but a relay switched across the charging resistors when the caps were charged. So the resistor was only in circuit on initial power on... that's the only one I can call to mind..


Your comments regarding the short circuit protection are wrong as well.

Hi Elkangarito,

Excellent point about contactor rating for jogging, I don't know how much jogging RC plans to do, but a little extra cost would be well worth it. I believe RC is getting a 24Vac contactor from eskimo, so it would be worth checking the data sheets to see what the break rating is. AC4 is 6x make 6x break, whereas AC3 is 6x make 1x break. (I checked )

I agree protecting the transformer with fuses on primary and secondary windings is essential.

Regards
Ray

Originally Posted by RayG

S

I agree protecting the transformer with fuses on primary and secondary windings is essential.

Regards
Ray

This is the last item I really need to know... I have 500volt rated fuse holders coming to protect the motor (putting 6 amp 415V fuses in them)

Would 240V rated 3AG fuse holders be satisfactory for the leads into the transformer... As the voltage from phase to ground is only 240V I would think they would be safe to use.. Something like this 3AG Approved Style Fuse Holder - Jaycar Electronics

Also I would use 240V fuses but of half the size if I was using 415V ones... So it would be 500mA 240V fuses on the primary side... 500mA 3AG Quick Blow Fuse - Jaycar Electronics

Originally Posted by .RC.

This is the last item I really need to know... I have 500volt rated fuse holders coming to protect the motor (putting 6 amp 415V fuses in them)

Would 240V rated 3AG fuse holders be satisfactory for the leads into the transformer... As the voltage from phase to ground is only 240V I would think they would be safe to use.. Something like this 3AG Approved Style Fuse Holder - Jaycar Electronics

Also I would use 240V fuses but of half the size if I was using 415V ones... So it would be 500mA 240V fuses on the primary side... 500mA 3AG Quick Blow Fuse - Jaycar Electronics

Hi .RC,

I thought you already had some spare HRC fuse holders?

For protecting the primary I would use a 100ma, HRC using the same 500v rated fuse holders as you used on the motor circuits. I note elkangarito suggests 45ma, I think you could go a little bit higher just based on the transformer being 45VA

On the secondary windings the 240v holders would be fine, 1A rating would be fine for protecting the transformer. The secondary windings are rated at 1.8 amps or so.

Regards
Ray

PS.. Ive been trying to remember the brand of a small din rail mount fuse holder... I think it was Legrand... I'll check

Go to the Legrand web site and download the terminal block catalogue http://www.legrand.com.au/en/scripts...asp?P=387&L=EN
look for page 129, and they list a number of different types of lever disconnect terminal blocks with fuses, the one using 5x20 fuses is the one I would think best for the transformer protection.
The part number is 371 81 (grey), or 371 82 (blue for neutral ccts). That's for the transformer secondary, you still need the higher voltage rating holder for the transformer primary.

Originally Posted by RayG

AC4 is 6x make 6x break, whereas AC3 is 6x make 1x break. (I checked )

Ray Could you explain this a little more or point me somewhere that it explains it.

Thanks
Stuart

Originally Posted by Stustoys

Ray Could you explain this a little more or point me somewhere that it explains it.

Thanks
Stuart

Hi Stuart,

This is about the durability of the contactor, what current it can make or break, and how that affects the life (durability) of the part.

http://www.ab.com/en/epub/catalogs/1...9481/tab7.html

The key item to identify, is that AC4 category is recommended for applications involving inching, plugging, or in our terms jogging. So an AC3 contactor can break 1x rated current, whereas an AC4 contactor can break 6x rated current.

The danger is repeated inching with an AC3 rated contactor, will eventually lead to premature failure, welded contacts.. as elkangarito suggests.

For RC's application, it depends on what the ratings of the contactor that eskimo is providing, and to some extent how much inching/jogging he is doing.

The difference might be a life of (only) 200,000 operations versus maybe 1-2 million operations...

Regards
Ray