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m_c
19-09-2011, 10:51 PM
Just a quick check before I buy too many or too few volts.

Needing a new toiroid for the lathe, and need to keep below 50V (limit of the Geckos), so aiming for 46V(ish) sounds good to me, and gives me about a 10V increase.

So, with those figures, and ye olde root2, means in a perfect world I need about 32V, but giving an allowance for mains drift, does a toiroid secondary of 30V sound good?

Also, is there any practical difference between going for 2x15, or 2x30 secondaries?

Jonathan
19-09-2011, 11:36 PM
2x15, or 2x30 both sound good to me.
2x15 gives you the option of getting half the voltage for er, something...?

Bear in mind that due to the voltage drop from the bridge rectifier, and the transformer regulation (and numerous other effects) you will get less than root 2 times the rated voltage. I think you'd get away with a 35V transformer (e.g. http://www.rapidonline.com/Electrical-Power/Toroidal-transformer-82719). Worst case scenario is you have to reduce the voltage a bit. That's easy enough - just wind a few turns of THICK wire on to the transformer and connect it in series with the output. If you put the wire the 'wrong' way round the current induced in it is 180 out of phase with the main winding, so it opposes the main current and reduces the output voltage. Side effect is that piece of wire gets a bit toasty - hence the thickness.

Web Goblin
19-09-2011, 11:52 PM
The only thing to watch out for is the VA rating. Most toroid transformers split their rating with their windings. Say a 300VA transformer with a single secondary would be rated at 300VA, but if it has two secondaries it would be 150VA per winding. This might make a bit of difference for your selection process.

Ian

Jonathan
19-09-2011, 11:56 PM
M_c will be using both windings though (in either series or parallel), so will still get the full rating of the transformer.

m_c
20-09-2011, 12:32 AM
Both windings will be getting used in series or parallel, depending on which tx I get.
I'm going to get a switched mode Din rail 12V PSU for 12V needs to simply things, so don't need any extra windings.

35V is cutting it a bit close. It theoretically gives 49.5V rectified, less 1.4V for the bridge, gives about 48V, but that's at a rated input of 230Vac, so by the time you allow 10% for mains variation, it takes you well over 50V.

Think I'll go up one size at the same time (existing TX was 160VA), to allow for future additions, so will probably go for a 225VA.
Airlink is the cheapest supplier I've found, but need to check on postage, as I need other bits they don't do.

Jonathan
20-09-2011, 12:49 AM
I'm going to get a switched mode Din rail 12V PSU for 12V needs to simply things, so don't need any extra windings.

Luke11cnc has one that might be suitable going spare. I've got a few too...


35V is cutting it a bit close. It theoretically gives 49.5V rectified, less 1.4V for the bridge, gives about 48V, but that's at a rated input of 230Vac, so by the time you allow 10% for mains variation, it takes you well over 50V.

It depends...
Clearly 30V transformer is the safest option - the margins are a bit close otherwise. I probably shouldn't recommend more but if it was me I'd get the 35V and if required do what I said in the last post to reduce the output a bit. Then maybe run the motors with oscilloscope on power supply just to be sure.

Airlink looks to have 9 postage, which makes them more expensive than Rapid electronics ... if it's some other electronics bits you need then Rapid is bound to have them which might even get you to the free postage boundary.

m_c
20-09-2011, 01:09 AM
I've just been searching for the extra bits, and have given up on the search for tonight.
Capacitor calcs work out at around 12500uF, and 63V seems to be the nearest, but neither RS or Rapid have anything suitable in stock.

Jonathan
20-09-2011, 01:15 AM
I've just been searching for the extra bits, and have given up on the search for tonight.
Capacitor calcs work out at around 12500uF, and 63V seems to be the nearest, but neither RS or Rapid have anything suitable in stock.

Farnell? They've got a good selection of transformers too...
Put two or three 4700uF in parallel to get round the lack of stock. That also gets lower ESR which is a slight advantage.

russell
20-09-2011, 11:33 AM
It's safer to go with two 15 V windings in series rather than two 30 V in parallel just in case the windings don't have exactly the same number of turns. I know they should but......

Russell.

m_c
25-09-2011, 10:09 PM
Just got the new toroid in place, and it's giving me 44V. Slightly lower than what I ideally wanted, but still 8V higher than the previous, so I should be able to eek a little bit more speed out the steppers.

russell
26-09-2011, 06:06 PM
If that's at nominal mains voltage it will give you over 48 V at the maximum of +10%; I wouldn't want to go any higher. Did you measure the mains voltage when testing?

Russell.

m_c
27-09-2011, 12:58 AM
No.
I seem to have a lack of insulated proves for the multimeter just now, so just measured to make sure the voltage wasn't too high. The Gecko's are good for 50V, so a 6V safety margin should be good enough.

russell
27-09-2011, 10:03 PM
You are probably OK. However, the reason I asked is that the UK mains voltage is 230 V with a tolerance of +10%, -6%. So, if the mains voltage was 6% low when you made the measurement, then the voltage you get at maximum mains voltage will be over 50 V. An unlikely but possible situation.

Russell.

JAZZCNC
27-09-2011, 10:23 PM
If it's for the Gecko G540 then this will do the job. http://www.airlinktransformers.com/transformer/0300233-open-style-toroidal-transformer-with-leads.asp

I've just built a control box for someone using this toroid and It is giving a steady 47V and my mains input voltage is quite high due to the fact I live bang next to the sub station.
I've had it on test know for several weeks and no issues's so far.

EDIT: Doh . . . didn't read all the post's did I.!! . See your sorted.

m_c
28-09-2011, 01:05 AM
Russell, you've got me curious now!
I'll have to go and find a multimeter with suitable clamps, just to satisfy myself.


Jazz, I'll keep a note of that, as I've got a couple other projects I'd like to do that may involve more G251s.

russell
28-09-2011, 05:52 PM
Another thing to consider is: Did you measure it under load?

A transformer usually specified under full RESISTIVE load. So, when unloaded the voltage increases by the specified regulation figure. I guess you tested it unloaded in which case you are safe. Calculating the output voltage of a transformer/rectifier under mixed capacitive and resistive load is by no means easy. For a one off, your approach of try it and see is the best way.

Russell.

stuboy
03-10-2011, 09:34 PM
If your still looking for any other caps I have a garage full! So can sort you out something appropriate for (hopefully) much less that RS or Farnell
Drop me a PM, that goes for anybody else that might be interested I guess.
Cheers
Stuart



I've just been searching for the extra bits, and have given up on the search for tonight.
Capacitor calcs work out at around 12500uF, and 63V seems to be the nearest, but neither RS or Rapid have anything suitable in stock.