Hi E,

It's 2 x 18 @ 230 Vac 160VA (I'll use one 18V secondary) @ericgibbs . So 18+Vac @ 4.44A out . I'll obviously have to replace the 24V regulator (for the fan output port) with a 15V or perhaps 18V regular but otherwise think it should give me a variable supply of circa 22-24 Vdc @ at least 2 Amp? Indeed, as long as it's at least 20+ Vdc and 2 Amps that's fine for my needs.

Erik

 

hi E.
OK,
I will run a LTSpice sim with that transformer and post the results.
By using a 18V 0v 18v centre tapped transformer you have the option of using just two rectifier diodes or making the second winding a negative supply.
E

 

Cheers E, That's great, look forward to seeing the sim and hope that the results match my, limited, expectations

Regards, E

 

hi E,
Running sims with your 18V transformer shows that the resistive sensing configuration will need modifying, the voltage and current settings are effected, I will continue checking.

If your 24v transformer is a standard E I laminated transformer, it should be possible to remove some turns from the outer secondary winding in order to to reduce the 24Vac out to say 22Vac.
Measure the cross sectional area of the transformers centre core and let me know what you measure, from that I can tell how many turns to remove.

E

 

hi E,
The PSU will require changes to the OPA biasing components etc, if you plan to use a 18Vrms transformer.
My recommendation would be to use the original 24Vrms transformer and add series diodes.
The low cost of the diodes would be the most economical and quickest way to complete the PSU build.
E

 

Thanks E,

I would like to use the original transformer but am still worried about the output. The kit specification says 24Vac (Max); It doesn't say whether that is Vrms or otherwise and as I said in post #13, I measured 30 Vac (on DMM) before rectification. I guess the question is, how many series diodes would take this output own to a safe level for this kit? To my electronically uneducated brain it would appear I need to loose about 6 volts DC to get down to the circa 34 Vdc the spec seems to be expecting post rectification (about 8 series diodes)?

Sorry to be a pain on this. I really am hugely appreciative of your help and expertise.

Erik

 

hi E,
It is not a problem.
As you can see from BitsBox the diodes are 12pence each, which makes adding say 4 or 6 diodes a cheap way of dropping the voltage.

This sim shows the +v/-v rail to rail voltage of the OPA, IMO it is acceptable.
The sim is set for 20Vout at 2Amps

E

 

Thanks again @ericgibbs. We'll see how things go when the board arrives (some weeks to go yet) and I put it all together. I'll post results here.

Regards E

 

In view of the help given by forum members, in particular @ericgibbs , I promised to post the outcome of the building of this kit. I ended up taking some windings off the over-voltage toroidal and it ended up just under 24 Vac (23.4).. Have given it a good long run with dummy loads and everything seems to be working as it should (although the max voltage I get under load is 24.4 Vdc at about 1.5A - which is just fine for my needs). Just need to tidy up the cabling a bit now. Thanks again for the help.

 

hi E,
Nicely done.!
Professional looking project.
Eric

 

Much appreciated Eric, I still have a lot to learn but this has aided the learning process....and I've enjoyed every minute!

 

Excellent work! A couple of my prototype guys could learn from you.

And I like the insulation wrapped around the AC input connector. Some companies make injection-molded boots for that, but you'd be surprised at how many "professional" products lack this kind of protection and awareness.

ak

 

Many thanks @AnalogKid . Praise indeed coming from you...cheers. Interesting (and perhaps scary ) that anyone would not consider insulating mains input, especially on pro products but then again, I probably shouldn't be surprised with some of the stuff on the market. I did actually look for some kind of rubber covert for the mains input without success...is "injection moulded boot" the 'proper name' I should search for?