# ac voltage divder

Discussion in 'Electronic Design' started by jabriol, Apr 29, 2009.

1. ### jabriolGuest

All,

I need to design a voltage divider, that is going to drop 15vac.
like 120 to 105 at .385 amps. voltage is running 400Hz

Any ideas?

2. ### Guest

Voltage? Frequency? Aren't they the same??

3. ### Guest

As a 'stupid' graduate student I used a capacitor to limit the AC
current going to the heater of a diffusion pump. It seemed to work
just fine. Do you need me to calculate the value for you also?

George Herold

5. ### HardySpicerGuest

At 400Hz a normal mains transformer is not a good idea.

Hardy

6. ### Spehro PefhanyGuest

This is under the previous response for a reason. What kind of
isolation would a 120:15 tranformer give in when properly connected in
the application suggested by the OP?

Also, I might point out that most tranformers, and many
autotransformers, don't have adjustment knobs to alter the turns
ratio.

Note that these particular ones are rated for 400Hz operation, and are
not unreasonably priced (\$250 or so), but are a bit overrated for the
application.

Best regards,
Spehro Pefhany

7. ### Jan PanteltjeGuest

Small transformer 120 V primary, 15 V secudary at 385 mA, secundary in series with input
in opposite phase.

--------------------------------
| |
)|| |
120 V 120 V )||( 15 V / 385 mA
)|| |_______________
)|| 105 V
|

8. ### Ross HerbertGuest

:> All,
:>
:> I need to design a voltage divider, that is going to drop 15vac.
:> like 120 to 105 at .385 amps. voltage is running 400Hz
:>
:> Any ideas?
:
:At 400Hz a normal mains transformer is not a good idea.
:
:
:Hardy

The OP didn't say that the transformer was a "normal mains transformer". Perhaps
it is rated for 400Hz operation?

9. ### Ross HerbertGuest

:All,
:
:I need to design a voltage divider, that is going to drop 15vac.
:like 120 to 105 at .385 amps. voltage is running 400Hz
:
:Any ideas?

I wouldn't muck around with fudging a voltage dropper as it will probably not be
reliable or safe in the long run. Buy a specified 400 Hz step down transformer
115V to 100V as made by Rayco Electronics Mfg - model 5T6155-11005-X (X= S
(surface mtg) or P (plugin)
http://www.raycoelectronics.com/catalog/400Hz.pdf

10. ### Ross HerbertGuest

:On a sunny day (Wed, 29 Apr 2009 06:39:13 -0700 (PDT)) it happened jabriol
:<>:
:
:>All,
:>
:>I need to design a voltage divider, that is going to drop 15vac.
:>like 120 to 105 at .385 amps. voltage is running 400Hz
:>
:>Any ideas?
:
:Small transformer 120 V primary, 15 V secudary at 385 mA, secundary in series
with input
:in opposite phase.
:
:
:
: --------------------------------
: | |
: )|| |
:120 V 120 V )||( 15 V / 385 mA
: )|| |_______________
: )|| 105 V
: |
: --------------------------------------------------
:
:So a 6 VA transformer would do.

Good solution - provided the small transformer is also rated for 400Hz
operation. It would probably be just as costly as a new 400Hz 115V : 100V @500mA
transformer from Rayco. Also a more space consuming arrangement.

11. ### Spehro PefhanyGuest

Nice-- what do those things cost, roughly? lead time?

I'm looking at some applications that have to run at "wild frequency",
but it's centered around 400Hz.

12. ### Jasen BettsGuest

http://www.raycoelectronics.com/catalog/400Hz.asp

5T6155-1152-X (115:15 @ 1000mA) from rayco is one third the weight of
5T6155-11005-X (115:100 @500mA)

so it would also be smaller and cheaper (if done with comparable parts)

13. ### Rich GriseGuest

Sure - look up Ohm's law.

Good Luck!
Rich

14. ### Rich GriseGuest

Come on, Spehro - you _must_ knowe how to operate Google!

In the USAF (and probably commercial aircraft and ships), everything was
400 Hz; and the transformers were Mil-spec, which means "the same part at
10X the price". ;-)

Good Luck!
Rich

15. ### Spehro PefhanyGuest

Yeah, I know. This is going on an aircraft. I don't think Google is
going to find any Rayco prices -- am I wrong?

My rule of thumb is that on price..

marine: at least 3:1
aircraft: at least 10:1
space or rad-hard: could be 100:1 and up

16. ### Rich GriseGuest

Admittedly, reliability is a major consideration - it'd be a bitch to find
oneself in the middle of the ocean, suddenly lacking some vital system or
another. Also in the air, albeit if you're over land, it's possible to
make an emergency landing. Witness the "miracle landing" on the Hudson
recently. I don't remember the heroic pilot's name. )-; ) I saw one short
clip from some security cam; the rooster tails were impressive!

Cheers!
Rich