Powerstat and transformer for variable voltage output

[tucker@conninc.com]

Hello, I need to charge a alot of different voltage lead acid batterys
6,8,10,12, ect. I have a
old charger that only puts out 12-14vdc, I have a 10amp 120vac
powerstat (variac) im thinking
of adding to the input of the transformer to reduce output on the
secondary. Will this work?
Im not sure about the inductance & operating a transformer @ 1/2 or
2/3 the normal 120 vac
input voltage, im wandering if this will effect efficiency or if it
will work at all.

 

[Jon]

You can do it, but you can not do it safely. The problem is that
Powerstats are autotransformers. They do not provide isolation from
the mains. In order to reduce the effective voltage to the primary of
the transformer, you would need to connect the normally "Neutral" side
of the Powerstat to the "Hot" side of the mains, in order to provide a
voltage that is out of phase with the mains voltage. The normally
"Hot" side of the Powerstat would connect to neutral. The "Low" side
of the existing transformer transformer would connect to the tap of
the Powerstat. I would not do this without an additional isolation
transformer.

 

[AcidBURN]

Hello, I need to charge a alot of different voltage lead acid batterys
6,8,10,12, ect. I have a
old charger that only puts out 12-14vdc, I have a 10amp 120vac
powerstat (variac) im thinking
of adding to the input of the transformer to reduce output on the
secondary. Will this work?
Im not sure about the inductance & operating a transformer @ 1/2 or
2/3 the normal 120 vac
input voltage, im wandering if this will effect efficiency or if it
will work at all.

Build yourself a variable charger with the variac.

Add a bridge rectifier to the output of the variac, input and output fuses,
an ammeter, a power switch. You could also add a timer.

Then you have yourself a 0 to 120 volt dc unregulated charger. Charge 8x
12v in series.

For lower voltages you could use your 12volt chargers transformer but if the
charger has some kind of electronic output control you would have to bypass
it because it certainly is designed for a specific input/output voltage.
Won't hurt the charger to try though.

 

[John G]

You can do it, but you can not do it safely. The problem is that
Powerstats are autotransformers. They do not provide isolation from
the mains. In order to reduce the effective voltage to the primary of
the transformer, you would need to connect the normally "Neutral" side
of the Powerstat to the "Hot" side of the mains, in order to provide a
voltage that is out of phase with the mains voltage. The normally
"Hot" side of the Powerstat would connect to neutral. The "Low" side
of the existing transformer transformer would connect to the tap of
the Powerstat. I would not do this without an additional isolation
transformer.

Nonsense

If the powerstat is connected normally then the Charger transformer
provides the isolation just the same as when it was connected directly
to the mains.

Powerstat top end to Mains Active.
Powersat bottom end to Mains neutral and to bottom end of charger input
transformer.
Powerstat slider to top end of charger input transformer.

This will only allow slightley more than 12 volts charging if the slider
passes the Powerstat top end that the Active is connected to, but it
will allow any voltage lower than 12.

The efficiency is hardly a consideration when at full load the thing
only uses 120watts plus losses.

 

[Michael A. Terrell]

Build yourself a variable charger with the variac.

Add a bridge rectifier to the output of the variac, input and output fuses,
an ammeter, a power switch. You could also add a timer.

Then you have yourself a 0 to 120 volt dc unregulated charger. Charge 8x
12v in series.

For lower voltages you could use your 12volt chargers transformer but if the
charger has some kind of electronic output control you would have to bypass
it because it certainly is designed for a specific input/output voltage.
Won't hurt the charger to try though.

Don't listen to AcidBURN, unless you want to die. That half assed
design is a death waiting to happen.

The OP had the right idea in the first place. I've used a 3 amp
variac with a 24 V C.T. "Motor home Power Converter" transformer for
over 30 years with no problem, other than the original rubber power cord
rotting away. I have charged single 2 volt cells, up to 12 volt car
batteries with it. I like that I can set the charging current to what I
need, from a few mA, up to 30 Amps.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[AcidBURN]

Don't listen to AcidBURN, unless you want to die. That half assed
design is a death waiting to happen.

The OP had the right idea in the first place. I've used a 3 amp
variac with a 24 V C.T. "Motor home Power Converter" transformer for
over 30 years with no problem, other than the original rubber power cord
rotting away. I have charged single 2 volt cells, up to 12 volt car
batteries with it. I like that I can set the charging current to what I
need, from a few mA, up to 30 Amps.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

You may have a point about safety but what I described is exactly what I
have here that was bought commercially, a variac with a bridge rectifier in
a plastic case, and mine is a 240vac. Been using it daily for years. It
was even manufactured in Sebring, Florida. If he wants to be safe then he
shouldn't modify any equipment that was designed for a specific purpose. So
I guess all hackers and experimenters are suicidal.

 

[Rich Grise]

Hello, I need to charge a alot of different voltage lead acid batterys
6,8,10,12, ect. I have a
old charger that only puts out 12-14vdc, I have a 10amp 120vac powerstat
(variac) im thinking
of adding to the input of the transformer to reduce output on the
secondary. Will this work?
Im not sure about the inductance & operating a transformer @ 1/2 or
2/3 the normal 120 vac
input voltage, im wandering if this will effect efficiency or if it will
work at all.

Probably not. Ordinary household battery chargers are designed to put out
a specific voltage. There's a possibility that your charger uses a
"ferroresonant" transformer, that's got regulation built in - what this
does is, when you decrease the input voltage, the input current actually
_increases_, so that it can still put out the rated output. BTDT - it's
weird watching the voltmeter and ammeter on the primary of a new ferro
xformer, and watching it draw, like, 7 amps when the input is at, like,
3 volts. ;-)

If your battery charger is just an ordinary linear transformer/rectifier,
then maybe, if you're very careful, have at least a voltmeter and ammeter,
and know what you're doing.

How hard is it to open the charger and see exactly what kind of circuit
we're dealing with?

Thanks,
Rich

 

[tucker@conninc.com]

Its just a simple high current 100AMP shop charger, full wave
rectifier/ct transformer configuration. The main thing i was worried
about
was efficiency at 1/2 the input voltage but i should have figured that
out because the Powerstat says its 120 or 240 volt input, so i guess
it was designed for 240 but will also work on 1/2 the input (120v) just
as well.

 

[Paul E. Schoen]

Its just a simple high current 100AMP shop charger, full wave
rectifier/ct transformer configuration. The main thing i was worried
about
was efficiency at 1/2 the input voltage but i should have figured that
out because the Powerstat says its 120 or 240 volt input, so i guess
it was designed for 240 but will also work on 1/2 the input (120v) just
as well.

Powerstats designed for 240 VAC input can be used on 120 VAC in boost mode,
where you get 0-240/280 VAC out, but at only about 1/2 the nominal current
rating. If you connect 120 VAC to the high end of the coil, you can get
0-120 or 0-140 at the full current rating. It will actually be more
efficient than a unit designed for 120 VAC, because it will not saturate at
the peaks. The only loss is that you have a unit twice as big as you need.

Simple battery chargers generally are current limited by impedance of the
transformer, and voltage limited by a zener diode and maybe a series pass
transistor, with some voltage adjustment to set the float charge voltage. It
is risky to use such a charger for batteries of different voltage, because
the float charge limiter won't work. Also, unless it has settings for
different currents, you could overcharge smaller batteries.

It should be possible to add an external current limiter and float voltage
limiter to match the battery you need to charge.

Paul E. Schoen
www.pstech-inc.com

 

[Michael A. Terrell]

You may have a point about safety but what I described is exactly what I
have here that was bought commercially, a variac with a bridge rectifier in
a plastic case, and mine is a 240vac. Been using it daily for years. It
was even manufactured in Sebring, Florida. If he wants to be safe then he
shouldn't modify any equipment that was designed for a specific purpose. So
I guess all hackers and experimenters are suicidal.

Is this made to charge an enclosed, connectorized high voltage
battery pack, or does it have exposed connectors where the user can be
exposed to 170 volts pulsating DC?

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[AcidBURN]

Is this made to charge an enclosed, connectorized high voltage
battery pack, or does it have exposed connectors where the user can be
exposed to 170 volts pulsating DC?

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

It is made to charge from 1 to 20 x 12V or 40 x 6 volt lead acid batteries
at up to 6 amps. The connections are made with standard copper clamps.
Obviously not 100% safe but it works very well for charging.

I also have a couple of more sophisticated chargers that are 600vac 3ph
input and 350vdc 50A output for charging industrial and auto motive
batteries. Sometimes charging 300 batteries in series/parallel. These I
have been using for 20 years. Connections are made with copper clamps.

This is a work environment so maybe it would not be a good idea in a home
environment unless the proper precautions are taken.

AcidBURN

 

[DaveM]

It is made to charge from 1 to 20 x 12V or 40 x 6 volt lead acid batteries
at up to 6 amps. The connections are made with standard copper clamps.
Obviously not 100% safe but it works very well for charging.

I also have a couple of more sophisticated chargers that are 600vac 3ph
input and 350vdc 50A output for charging industrial and auto motive
batteries. Sometimes charging 300 batteries in series/parallel. These I
have been using for 20 years. Connections are made with copper clamps.

This is a work environment so maybe it would not be a good idea in a home
environment unless the proper precautions are taken.

AcidBURN

The variac/rectifier arrangement may be totally safe.... Remember that
there ISOLATED variacs... those that have a secondary that is isolated from
the primary, making it a plain old transformer with variable secondary
voltage.
Cheers!!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!

 

[Michael A. Terrell]

The variac/rectifier arrangement may be totally safe.... Remember that
there ISOLATED variacs... those that have a secondary that is isolated from
the primary, making it a plain old transformer with variable secondary
voltage.
Cheers!!!!

It wouldn't make any difference that it has isolation if you
accidentally touched both leads with over 100 VDC at a couple amps.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[DaveM]

Well... duhhh Obviously there is the risk of injury or damage if someone
touched something of that voltage. What I was referring to was the risk of
getting the hot/neutral mixed up and making the output live WRT the power
line. The isolated variac indeed removes that risk unless the internal
wiring has been altered.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!