what is actual voltage of 24V transformer

archibald tuttle

i know, this seems to fall in the what color is the red wire variety.



i'm not sitting next to a transformer at the moment to test (don't see independent ones as much because I've always been on the fence about zone valves and stuck with multiple pumps so i don't do a lot of 24V wiring -- except thermostat circuits themselves so i could go down and take the voltage of my SR503 and that would probably give me a reasonable idea.)



I'm looking at a custom built differential controller, pursuant to my last controls post here and apparently 24V is the upper limit for its operating voltage.  So, if a 24V transformer is actually 26 V unless under load or something, this might not be good.



and i'm going to assume it is going to vary depending on variance in input voltage so i'm really asking what the range of expected loaw side voltages would be if you have experience of this sort.



thanks,



brian

archibald tuttle

got answer one but still looking for ideas

SR503 with one circuit calling is showing 26.4 to 26.5 volts on input of 118 to 120 volts.



i've got to get with this EE and see if that is right out. i seemed to recall that they were a bit above nameplate.



don't know if they slough off a little under more serious load than a couple relays, e.g. old taco zone valve or . . . but i imagine i may have to put a resistor in line.



any other ideas for tapping for lower voltage appreciated.   i think this thing runs all the way down to 9V.  i'd just as soon figure a way to power it although it will run on DC so i probably could use a 9V battery.  that's another question for the designer.



thanks,



brian

SWEI

What are you trying to run from this?

How much current do you need?  Do you need ground isolation?

http://www.functionaldevices.com/building-automation/display.php?model=PSMN24DAS or  http://www.functionaldevices.com/building-automation/display.php?model=PSM20A12DAS might work for you.

archibald tuttle

small load

don't believe i need isolation.



i just heard from the engineer who is making these and he tells me i'm still a quarter volt below his upper limit.  he is comfortable with the approach and will warranty at that voltage and his next iteration will actually have higher voltage allowance anyway.



the load is 100 milliamps i believe.



he did give me the specs for a 20 ohm resistor to drop the current slightly if i wanted but didn't think it necessary. i might go for it anyway.

a

although i like the tunable power supply you linked. that is cute.  i just figure it would be sensible to use the 24V supply i've already got if i can.  one less piece of cost and complication.  speaking of which, that link doesn't say how much the power supply costs. do you know?



thanks



brian

SWEI

Basic regulated DC supply

Is easy enough to build.  I would not use a resistor to drop voltage because the voltage will vary with current  draw.



https://encrypted.google.com/search?q=PSMN24DAS&tbm=shop should point you in the right direction.

archibald tuttle

not much current variation in this application

and dropping a couple vots still leaves me near the top end of the range so don't worry too much.  guy who designed it, as i said, suggested i could go without so i'll look into the math curve of how much more the voltage would drop depending on range of draw.  i think his 100 milliamps was an average.  don't know if it would draw much more when closing a switch as in a magnetic relay kind of thing. the switches are rated to 5 amps so they could conceivably be solenoids of some sort that would increase draw when on.



i appreciate the link and took a look at those ebay offerings.   i can delve that way if it seems advisable in the long run but right now the manufacturer  is of the opinion that a modest resistor or even no resistor will work fine with the 24V AC i've got.



brian

SWEI

Resistor

Don't forget to figure the dissipation.  Dropping 4 Volts at 100 mA will dissipate 0.4 Watts. Leave some safety factor if it's a continuous draw -- I'd probably specify a 1W resistor for that.