isolation transformer ?
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Posts: 46,084
Hi All,
I am powering my unit from a 24 VAC source. that means 1.41 times
24 for a 33.8 volt DC supply.
Since I can't operate a LM7805 and/or a LM7812 ( I need 12 for op-
amps) at that voltage, I am hoping to find a small 2:1 transformer.
I have used a switching supply, but that costs quite a bit.
any suggestions ?
btw, I am using some ADC's so I need a stable power source.
Dave
I am powering my unit from a 24 VAC source. that means 1.41 times
24 for a 33.8 volt DC supply.
Since I can't operate a LM7805 and/or a LM7812 ( I need 12 for op-
amps) at that voltage, I am hoping to find a small 2:1 transformer.
I have used a switching supply, but that costs quite a bit.
any suggestions ?
btw, I am using some ADC's so I need a stable power source.
Dave
Comments
voltages from a 25-35 volt input. I have seen them surplus for half of what
they go for new.
Original Message
> I am powering my unit from a 24 VAC source. that means 1.41 times
> 24 for a 33.8 volt DC supply.
>
> Since I can't operate a LM7805 and/or a LM7812 ( I need 12 for op-
> amps) at that voltage, I am hoping to find a small 2:1 transformer.
>
> I have used a switching supply, but that costs quite a bit.
>
> any suggestions ?
and put up with the heat or better, a switching power supply. They are not
expensive when you consider all the parameters.
Jim
Original Message
From: Dave Mucha [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=PbF7IXdQXkLFue8kWMBKFssaweLA6txCp2Jn0klyefUAjOkSKsw6XXQElgv45w2OZZ-i4k2r4rA]davemucha@j...[/url
Sent: Monday, October 07, 2002 9:39 PM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] isolation transformer ?
Hi All,
I am powering my unit from a 24 VAC source. that means 1.41 times
24 for a 33.8 volt DC supply.
Since I can't operate a LM7805 and/or a LM7812 ( I need 12 for op-
amps) at that voltage, I am hoping to find a small 2:1 transformer.
I have used a switching supply, but that costs quite a bit.
any suggestions ?
btw, I am using some ADC's so I need a stable power source.
Dave
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down to something the regulator is happy with. Or you could use a voltage
regulator with a higher input voltage range (I am using one with a 125 volt
input rating and still in a TO-220 case style)...of course your regulator
will run warmer since it will now have to dissipate more power (volts X
amps). Another way would be to use a small AC transformer as you suggested
but of course you will feed the 24 VAC into it and get 12 VAC out which would
then supply your regulator.
[noparse][[/noparse]Non-text portions of this message have been removed]
>Hi All,
>
>I am powering my unit from a 24 VAC source. that means 1.41 times
>24 for a 33.8 volt DC supply.
Does the transformer have a center-tap? Use that if it does.
If not, build a small but clean supply for the noise-sensitive stuff and
power the rest of your stuff from a simple but dirty supply.
Much of the equipment that I design is for the HVAC industry. Like you,
all they have is a 24 Vac supply. I operate all my relays from a simple
half-wave power supply and then derive a clean supply for the processor and
analog stuff. This approach works well.
The other advantage to using half-wave supplies is that you can generate a
negative voltage very easily if you need it.
dwayne
--
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
Celebrating 18 years of Engineering Innovation (1984 - 2002)
.-. .-. .-. .-. .-. .-. .-. .-. .-. .-
`-' `-' `-' `-' `-' `-' `-' `-' `-'
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.
I found that the building automation systems share a common ground
with the analogue inputs. that means if your board has a 0-5 output,
that zero is tied to the 24 vac ground, no if-ands or buts.
The automation system ADC is tied to ground. therefore, so are all
the instument outputs.
An isolation xfrmr allows the board power to be seperate so the 24vac
ground can tie to the 0-5 output. I just had to get a 40 va
isolation xfmr for one board. $20 bux.
Dave
--- In basicstamps@y..., Dwayne Reid <dwayner@p...> wrote:
> At 01:38 AM 10/8/02 +0000, Dave Mucha wrote:
> >Hi All,
> >
> >I am powering my unit from a 24 VAC source. that means 1.41 times
> >24 for a 33.8 volt DC supply.
>
> Does the transformer have a center-tap? Use that if it does.
>
> If not, build a small but clean supply for the noise-sensitive
stuff and
> power the rest of your stuff from a simple but dirty supply.
>
> Much of the equipment that I design is for the HVAC industry. Like
you,
> all they have is a 24 Vac supply. I operate all my relays from a
simple
> half-wave power supply and then derive a clean supply for the
processor and
> analog stuff. This approach works well.
>
> The other advantage to using half-wave supplies is that you can
generate a
> negative voltage very easily if you need it.
>
> dwayne
>
> --
> Dwayne Reid <dwayner@p...>
> Trinity Electronics Systems Ltd Edmonton, AB, CANADA
> (780) 489-3199 voice (780) 487-6397 fax
>
> Celebrating 18 years of Engineering Innovation (1984 - 2002)
> .-. .-. .-. .-. .-. .-. .-. .-. .-. .-
> `-' `-' `-' `-' `-' `-' `-' `-' `-'
> Do NOT send unsolicited commercial email to this email address.
> This message neither grants consent to receive unsolicited
> commercial email nor is intended to solicit commercial email.
>It would be wonderful to be that simple.
>
>I found that the building automation systems share a common ground
>with the analogue inputs. that means if your board has a 0-5 output,
>that zero is tied to the 24 vac ground, no if-ands or buts.
>
>The automation system ADC is tied to ground. therefore, so are all
>the instument outputs.
Yep - what you said above is what I deal with in my HVAC designs.
The half wave power supply I mentioned in my response to you allows you to
get the power you need to operate the processor, output relays, a/d
subsystem - all with little effort and not too many problems. I find that
I have to be careful with offset caused by ground currents but I get around
that by using current loops for my analog sensors (1-5 mA & 4-20 mA). If I
need an analog output (not much of my stuff does), I also try to do that as
a current loop.
Dave - what particular problems did you have where you had to use an
isolation transformer? Perhaps we can figure out a less expensive way to
accomplish the same thing.
dwayne
--
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
Celebrating 18 years of Engineering Innovation (1984 - 2002)
.-. .-. .-. .-. .-. .-. .-. .-. .-. .-
`-' `-' `-' `-' `-' `-' `-' `-' `-'
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.
> Dave - what particular problems did you have where you had to use
an
> isolation transformer? Perhaps we can figure out a less expensive
way to
> accomplish the same thing.
>
> dwayne
Since the remote PC and power supply share the same ground, using a
common ground is a very typical application. The problme is that I
am getting about 19 volts across the PWM output and the AC power. on
a 0-5 volts signal, it just doesn't work.
I shyed away from 4-20 as the extra components and a question of
circuit stability. deffinatly an option. it would have to have
enough power to drive a 10 volt signal on the other end. typically a
500 ohm resistor as the load.
I was hoping that there was a 7824 with a max V of 40 volts or
something. That way, I could step down the voltages on the board and
still have a common ground.
Dave
>
> --
> Dwayne Reid <dwayner@p...>
> Trinity Electronics Systems Ltd Edmonton, AB, CANADA
> (780) 489-3199 voice (780) 487-6397 fax
>
> Celebrating 18 years of Engineering Innovation (1984 - 2002)
> .-. .-. .-. .-. .-. .-. .-. .-. .-. .-
> `-' `-' `-' `-' `-' `-' `-' `-' `-'
> Do NOT send unsolicited commercial email to this email address.
> This message neither grants consent to receive unsolicited
> commercial email nor is intended to solicit commercial email.
range! Did you miss that post or maybe was not clear enough when I mentioned
it to you!
I have used the part with a 90 volt input and 24 volts output. Like I
mentioned before...the regulator must be mounted on a heat sink because the
voltage difference between output (24 volts), and input (40 volts), must be
dropped across the regulator. If you are supplying any current at all the
Power (volts X amps) is appreciable. The exact part number of the regulator
is a TL783CKC. It is adjustable and is a three terminal device so you will
have to add two resistors or a pot to set the output to 24 volts. As I
mentioned before...you could "pre-regulate" the 40 volts down to 30 with a
zener diode + resistor and use your 7824. Again...all this was mentioned in
my initial posting to you!
[noparse][[/noparse]Non-text portions of this message have been removed]
I did miss your post.
That is exactly what I need.
I found that Microsemi offers a 15 volt regulator that says it can
handle 50 volts, in the data sheet is lists 50 as a transient
voltage. if it offered a 24 volt unit, the step down voltages would
produce less heat per chip and offer other voltages for use.
and your solution at $1.50 a chip is not too bad.
Thanks.
Dave
--- In basicstamps@y..., azeasi@a... wrote:
> I told you about a regulator (like the 7824) that had a higher
input voltage
> range! Did you miss that post or maybe was not clear enough when I
mentioned
> it to you!
> I have used the part with a 90 volt input and 24 volts output. Like
I
> mentioned before...the regulator must be mounted on a heat sink
because the
> voltage difference between output (24 volts), and input (40 volts),
must be
> dropped across the regulator. If you are supplying any current at
all the
> Power (volts X amps) is appreciable. The exact part number of the
regulator
> is a TL783CKC. It is adjustable and is a three terminal device so
you will
> have to add two resistors or a pot to set the output to 24 volts.
As I
> mentioned before...you could "pre-regulate" the 40 volts down to 30
with a
> zener diode + resistor and use your 7824. Again...all this was
mentioned in
> my initial posting to you!
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
you aluded to the higher input voltages on this post.
I did search for some and came across the MicroSemi site with their
50 volts units.
Thanks for the follow-up post to offer a part number.
Dave
--- In basicstamps@y..., azeasi@a... wrote:
> You might be better off using a Zener diode pre-regulator to drop
the voltage
> down to something the regulator is happy with. Or you could use a
voltage
> regulator with a higher input voltage range (I am using one with a
125 volt
> input rating and still in a TO-220 case style)...of course your
regulator
> will run warmer since it will now have to dissipate more power
(volts X
> amps). Another way would be to use a small AC transformer as you
suggested
> but of course you will feed the 24 VAC into it and get 12 VAC out
which would
> then supply your regulator.
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]