Since I could not get the dual split power supply voltages I needed from one of my transformers I decided to use two "smaller" ones but with higher voltage in series to get a larger voltage to work with.
Basicaly I had only 25.2VCT@1.5A and switched to using two 40VCT@.25A in series to give me +-40 volts to work with. Basicaly I combined them so that I have like one large transformer that gives me 80VCT@.25A. I figure for now I don't need that much power but I do need atleast +-30 volts to play around with(as most of my projects will be small circuits). Later on I can worry about making a better PSU but for now I just want to get something done.
Anyways, after hooking up the transformers in "series" on the secondary side and "parallel" on the primary and putting them through a bridge rectifier and through two caps I end up measuring about +-60V at about
115Vrms(Actually about 56V).Now I ended up with a new issue ;/ The voltage regulators I plan on using all are rated at max 40V input cts.(they will be variable regulators too, not that it matters much).
So I have ~20V's(depending on the mains voltage of around 105-125Vrms) to do something with. I don't think that I can put those extra volts across the regulators? ;/ Ofcourse with a load the voltage will drop but can the regulators handle it?(I only have about 5 regulators of that type so I can't really blow them up to find out). If the max voltage is to prevent some type of heading issue then I think it might be ok since I have adaquate cooling(although maybe not).
Although I guess I could reconfigure the secondary transformers in series to lose that 20V
------ VoA
--- CTA
----- | | |--GND
----- |
--- CTB
------ VoB
Thats my configuratio now with with CT's not connected...
but I guess I could do
------ VoA
--- CTA
----- | | |--GND
----- |
--- VoB
------ CTB
which should drop some of the voltage(~20V)?
This will make it unsymmetric though and I'll have something like -40 +60 ;/
I tried to figure out how to connect the transformers in a way so that I have +-40V but I can't seem to get it to work with a common ground. i.e., if I act like I'm building two seperate power supplies I can easily get +40 on one and -40 on the other but then when I try to hook up the grounds I end up getting shorts. If I use a full wave rectifier then that cuts my voltage down to 20V or so which isn't acceptable(I need ~40V).
So my main question is if its acceptable to put a higher voltage into the regulator than it says on the datasheet when there is no load? Cause last time I posted about something like this you guys said that the transformer gives more output voltage than normal at no load conditions cause it will drop anyways. So can I expect a all those "extra" volts to drop enough not to cause problems?
Also, I'm a little confused on how to figure this stuff out. If, say, my transformer has a turns ratio of N and an input voltage in rms of VPrms then the output voltage would be VPrms/N but when I go through the bridge rectification process I get sqrt(2)*VPrms/N for my peak output. When I filter that I get ~ sqrt(2)*VPrms/N.
So say for VPrms = 115VAC with N = 2.875 which corresponds do a 115VAC to
40VAC step down transformer(?) I have a DC voltage on my filters of about 57VDC. So if I really wanted 40VDC comming out of my filters to use in my regulators I should have gotten a "smaller" transformer? Or get a regulator that can handle it but I have several such as the LM317/LM337, 78XX/79XX, LM2931 and they all have max about 40VDC input. (which means that my 40VAC transformer is to much for them?)(so my 25.2VAC transformer will give me about 35VDC which is atleast under what I need(so I can regulate to betwee 1.25n and 32VDC which is better than burning up my regulator... but unfortunately I only have 1 of those transformers so I can't get - polarity without having to cut those values in
1/2... which is not acceptable).Ultimately I guess what you guys are going to say is get the right components. Unfortunately at the moment I can't and need to work with what I've got. I know I have to compromise but I think I have to have +-40VDC and atleast 100mA(which should keep me busy for a while until I can get the right parts for the job).
Thanks for any help, Jon