AC Current Limiter

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This may be more off-topic than "General Electronics; however, I need some help!

 A client has a product that had a specially built tester designed for it some 15 years ago. Unfortunately, both the tester and all information regarding the tester were lost in the flooding due to Hurricane Harvey. The original designer/builder has passed away...

 

I have been asked to try to replicate a part of the tester, specifically a current limited, variable AC source.

The diagram below shows (pretty much) what I need:

The variable transformer is among the items saved from the flood. It will ramp from 0 to max voltage and has a maximum output current rating of 3.5A.

 

The target device includes a logging device for the input voltage and current.

In a test mode, the logging device records the input AC voltage and current at the point at which the target system first begins to operate.

The logging device continues to record AC voltage and current through the AC voltage ramp.

 

The target device must be protected from over-current as the variable transformer continues to ramp to its max voltage...

 

So, for any voltage from the variable transformer, I must pass the AC voltage, until limited to a maximum of 25mA(rms).

 

(Fuses and/or circuit breakers are not viable)

My googlefoo has not found anything that would work

Any ideas?

 

I have also looked at programmable AC power supplies, but the current limit resolution seems to be limited to ±10mA...

If you know of a model with better resolution, I would be happy to recommend it to my client!

 

Thanks,

David (aka frog_jr)

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My first thought was a positive-temperature coefficient solid state (self resetting) fuse. The problem with these, however, is that 25mA is a pretty low trip level (they tend to be rated at trip levels above 100mA) and the trip level tends to be pretty imprecise.

 

You MIGHT be able to put a 2-transistor current limiter inside a 4-diode bridge. The maximum variable transformer output of 200V (assume this is really RMS, so that the peak would be 280V)n is a bit daunting for this method. It might work, however with suitable components. There is, however, a "power issue". Lets suppose that the load sits at 25V while the variable transformer is all the way on (200V) but under current limiting. There would be 175V across the limiter. The current of 25mA says that the power dissipated in the limiter will be close to 4.5W. This is not an impossible power level but one that, with the peak voltage, will require some careful design.

 

Jim

 

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

Last Edited: Mon. Mar 19, 2018 - 05:22 PM
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Light bulb?

Tom Pappano
Tulsa, Oklahoma

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By lost in hurricane, as in swept out to sea?  Or just damaged?  Is it still available to look at how it was done (reverse engineer)???

 

Jim

 

 

Mission: Improving the readiness of hams world wide : flinthillsradioinc.com

Interests: Ham Radio, Solar power, futures & currency trading - whats yours?

 

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Tom perhaps inadvertently raises an important question. How "hard" is this limit? And how precise does this limit have to be?

 

For example, suppose that it starts limiting at 25mA but the limit increases to 30mA at maximum voltage.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Tom perhaps inadvertently raises an important question. How "hard" is this limit? And how precise does this limit have to be?

 

For example, suppose that it starts limiting at 25mA but the limit increases to 30mA at maximum voltage.

 

Jim

Right, for example, is the current limit simply to protect the device under test from damage in case it has a problem, or is the 'limit current' a test or calibration value?

Tom Pappano
Tulsa, Oklahoma

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To answer some questions:

  @Midwest Jim, The flood waters were filled with raw sewage. Many areas (should have) required HazMat suits. In this particular case, the device (as I understand it) was among a group of boxes that were loaded into a refuse container via a front-end loader along with collapsed sheet rock, carpeting and other detritus. In his building, the flood waters only rose to about 6 feet...

 @ Tom from Tulsa, I am unsure about the light bulb, may try on a test fixture. I don't think that it will prevent high current at lower voltages. @West Coast Jim, yes the voltages are RMS (as is the current)...

 @West Coast Jim and Tom from Tulsa, the limit is to protect the DUT in case it has a faulty component or other assembly problem. My client tried to test just using the unlimited variable transformer and destroyed two devices... (He is not a happy camper). This is not a hard limit, but certainly don't want to reach a point where the device looks OK as voltage rises then fails catastrophically (e.g. 20mA, 21mA, 22mA, 1A).
 

Ideally, it should operate like the current limit on a DC power supply, allowing voltage to rise to the point where it is  then held by the setting of the current limit.

 

David (aka frog_jr)

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How about some back to back depletion mode fets?

They can act as adjustable 2 terminal unipolar current limiters  (back to back for bipolar)

 

 

you might be able to parallel (raise current), & series (higher voltage) some of these current limiting diodes (back to back for AC)

https://www.centralsemi.com/PDFs/selection/leaded/CLD_Standard.pdf

 

 

Here is a good app note I found regarding the depletion mode fets, along with some part numbers (HV parts to 500V)

Depletion mode parts are quite rare...out of 46000 fets on digikey, only 250 listing are depletion types.,,,very sad

Note, the limiter you build might need calibrated/adjusted before giving to user.

 

https://www.infineon.com/dgdl/Infineon-Application_Note_Applications_for_Depletion_MOSFETs-AN-v01_00-EN.pdf?fileId=5546d4624cb7f111014cd63d1a197d94

 

 

 

 

 

 

When in the dark remember-the future looks brighter than ever.

Last Edited: Mon. Mar 19, 2018 - 09:33 PM
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At that power level, one would think you could make a small inverter and have the micro do the ramp and current limit. The switching could be done at low voltage then fed into a transformer.

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What is the "normal" current draw of a dut?

Tom Pappano
Tulsa, Oklahoma

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Here are some details of a diode current limiting bridge that Jim alluded to :

 

https://www.radiomuseum.org/forum/ballast_tubes_solid_state_replacements.html?language_id=5

 

Item 4 ins interesting to eliminate the effect of diode drops.

 

Note, you can limit current using a plain old N-chan powerfet...for a given Vgs, that sets a current limit between drain & source (you can use a 200k pot & 9v battery to create a very long-lasting Vgs source).  However, this can be drifty/sensitive over a long time /temperature, but quick & easy to build. Two of them in opposite-series, of course current limits AC (you could even have different limits in each direction). The internal moseft parasitic diode from S to D could be paralled with a schottky for lower drop, or a more linear response when not limiting.

Image result for mosfet curve

 

Finally, from the Godfather of analog, here is a Bob Pease solution...note figure 4 pins abc get pasted into fig 3 (cut out fig 3 abc circuit).

http://www.electronicdesign.com/analog/whats-all-current-limiter-stuff-anyhow  

 

 

When in the dark remember-the future looks brighter than ever.

Last Edited: Mon. Mar 19, 2018 - 10:23 PM
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Is the variable transformer driven by a motor?  With a small DC servomotor and drive it is not difficult to build a current regulator out of the equipment you describe.

Letting the smoke out since 1978

 

 

 

 

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avrcandies reference is exactly what I was thinking of. While 25mA is not much for most transistors, the dissipation of 2W+ at maximum transformer output voltage can be a killer. The problem is that almost anything you do will exhibit the same dissipation characteristic. So, using a FET or some diode, or something else, won't "save" you. On the other hand, 2W+ is not THAT bad and use of appropriately voltage rated power transistors combined with a modest heat sink should be able to do it. You would need to be VERY careful about line voltages and isolation between the power transistor and the heat sink.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

Last Edited: Tue. Mar 20, 2018 - 12:02 AM
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A variation on this old gem might do:

 

 

... where Rs is replaced with a trim pot.  To get 25 mA, you'd want about 24 ohms.  For AC, two back-to-back.  To handle the variation of the half-sine in each direction, you could filter the gate with an RC with a TC equivalent to a few cycles, with a high R w.r.t. Rg.

 

In the above circuit the LED is the load, but you'd need a diode in series with the real load, to block current when the other phase is conducting, otherwise the body diode will parallel the the other phase and you'd lose regulation.  Similarly the other phase would also need a series diode.

 

Probably a crazy idea that won't work, though :-)

 

 

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"Read a lot.  Write a lot."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Tue. Mar 20, 2018 - 04:41 AM
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A variation on this old gem might do

This arrangement might be better suited to low supply voltage (like <15V), since many fet gates are rated for Vgs 20v max.  With small/no load (no active current limiting), the gate is pulled up to Vdd...a Vgs issue if Vdd is 200V.  Of course a zener clamp on the gate might alleviate that  issue. 

 

 

When in the dark remember-the future looks brighter than ever.

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tpappano wrote:
What is the "normal" current draw of a dut?
Up to about 60VAC the current is near 0, and ramps to 5-10mA at 70VAC. The current (typically) remains fairly constant to about 170VAC, where it then starts ramping to about ~15mA at 200VAC.

digitalDan wrote:
Is the variable transformer driven by a motor?
No, by hand.

 

Unfortunately, if the DUT has a problem, the current will look fine as the voltage ramps and then ... poof, all the smoke is released!

Therefore, if the voltage can be current limited, the DUT can be repaired rather than scrapped.

 

Right now, I am considering suggesting the client move to a programmable power supply such as the BK 9801. Even though the current limit is not as refined as desired (0.01A resolution, ±(1% + 120mA) accuracy), it should allow testing without destruction.

 

David (aka frog_jr)

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frog_jr wrote:

Unfortunately, if the DUT has a problem, the current will look fine as the voltage ramps and then ... poof, all the smoke is released!

 

I take it then a good DUT presents a constant load and a bad DUT presents a variable load?  Can you simply cut the power when the abnormal situation is detected?  Or does the test require the power remain applied?

Letting the smoke out since 1978

 

 

 

 

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digitalDan wrote:
I take it then a good DUT presents a constant load and a bad DUT presents a variable load?
Having only been told of the DUT fails (and not having experienced it), it seems that the current to the DUT will change slowly (but, not necessarily as a constant load) as the voltage ramps. On a fail, the current will suddenly jump to a high value (1+ Amps) blowing components and (usually) burning board traces. Power can be cut; however, the damage is done.

 

The product has been in production for at least 12 years, so it is a known good design that works correctly, unless there is an assembly error.

With the current limited, the DUT can be examined, fault(s) located and repaired.

 

I am to examine the devices and testing system later this week to verify the various aspects of the test procedure and (hopefully) offer a plan that will allow non-destructive testing.

 

David (aka frog_jr)

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I'd be inclined to look at a high-speed circuit breaker. Not of the electromechanical type but something solid state. I have a design in production which uses such a circuit and it detects an overload in around 100us with instant off. Getting it to trip at 25mA would be trivial.

"This forum helps those that help themselves."

"How have you proved that your chip is running at xxMHz?" - Me

"If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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Characteristics for this thing are very vague.

Accuracy? Speed?

Is a temproary over current acceptable? How much? ( In case of Motor regulatation / stabilisation).

What is the voltage range for the output volgate? Is that all the way up to 120Vac, or lower?

 

In an old datasheet for LM317 I once saw an application node for an AC current limiter with 2 LM317 IC's.

Those simply worked by chopping off off the tops of the sine peak voltages.

Paul van der Hoeven.
Bunch of old projects with AVR's:
http://www.hoevendesign.com

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Vdd is 200V

 

 

Somehow I missed that!  blush

 

 

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"Read a lot.  Write a lot."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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frog_jr wrote:

Up to about 60VAC the current is near 0, and ramps to 5-10mA at 70VAC. The current (typically) remains fairly constant to about 170VAC, where it then starts ramping to about ~15mA at 200VAC.

 

Sorry, I missed this statement earlier.

Here's a crazy idea.  Replace the 200 V variable transformer with a 500 V unit.  Add a 20k, 15W series resistor and now you have a very droopy voltage source that can deliver 15mA at 200 V with a max of 25mA under short circuit conditions.  Safety aspects will certainly need to be addressed but this will very neatly cover the requirements stated above.

Letting the smoke out since 1978

 

 

 

 

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digitalDan wrote:
Here's a crazy idea.

I'm wondering if the previous design might have just been a fixed step-up transformer with a resistor...

David (aka frog_jr)

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Here's a crazy idea.  Replace the 200 V variable transformer with a 500 V unit. 

You should also add a tight 200V clamp zener at the load. If the current draw dropped, the droopy supply might not be so droopy & suddenly apply 300 Volts to your 200V parts & blow them out with a low current "zap" (such as damaging a capacitor with an overvoltage).  

 

 

When in the dark remember-the future looks brighter than ever.

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avrcandies wrote:
You should also add a tight 200V clamp zener at the load.
Thanks, for the suggestion. I had also been considering something along those lines,  even though the input circuits "appear" to be using parts rated to 1000V. (I have not yet been NDA'd to the full schematic.)

David (aka frog_jr)

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I used to own a magnetic mains voltage stabiliser (I probably still do, but it will be at the bottom of my 25yr old junk pile). The world-wide-web wasn't around when it was manufactured but here is a page describing the technology. http://www.aelgroup.co.uk/faq/faq001.php

 

With some mods in this patent https://patents.google.com/patent/US3397361 "Magnetic amplifier utilized as a static constant current regulator" it could be possible to make something to do what you need.

 

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awneil wrote:
Does this help:
Wish they did, but at  16 and 35 A I believe they are some orders of magnitude too high!

 

I have given my client recommendations and options based upon some of the responses above (as well as some others). I am awaiting his decision on how to proceed...

 

Thanks to all that helped me brainstorm some ideas!

A very big yes to all the Freaks!

David (aka frog_jr)

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frog_jr wrote:
Does this help: Wish they did, but at 16 and 35 A I believe they are some orders of magnitude too high!
A few orders of maginude off does not nessiceraly mean that the principle behind it is not suitable for you. However:

 

I looked at their datasheets. They are all based on a very simple Resistor, Relay & timer setup.

 

It can not be very difficult to design an opamp with a discrete amplifier stage just as in an audio amplifier.

Your voltage is a bit high, but if you only need 25mA dissipation is still quite low even at those voltages.

frog_jr wrote:
I have given my client recommendations and options based upon
Guess I'm too late then...

Paul van der Hoeven.
Bunch of old projects with AVR's:
http://www.hoevendesign.com

Last Edited: Sat. Mar 24, 2018 - 12:04 AM
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I had a free time this afternoon. I designed an AC limiting circuit and simulated it on LTspice. Its screenshot is attached.

Attachment(s): 

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KerimF wrote:

I had a free time this afternoon. I designed an AC limiting circuit and simulated it on LTspice. Its screenshot is attached.

 

Have you checked the voltage rating of the components you've chosen?

"This forum helps those that help themselves."

"How have you proved that your chip is running at xxMHz?" - Me

"If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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Hi Brian,

 

Looking back at the circuit (png), I thought that forgetting to add a zener diode (about10V) between gate and source may let |Vge| > 20V at certain conditions.

Fortunately, in this configuration, -1 < Vge < 6V (approximately).

The only high voltage is on Vds. Its rating for the MOSFET I chose is 800V.

 

Do you think I missed something else?

Thank you.

 

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frog_jr wrote:
Up to about 60VAC the current is near 0, and ramps to 5-10mA at 70VAC. The current (typically) remains fairly constant to about 170VAC, where it then starts ramping to about ~15mA at 200VAC.

 

With that load profile, did you try a simple mains rated series capacitor ? (or even a power resistor)

You just want to find units that are well outside the load, so a simple load-line operating point check should be fine.

 

If you search for Depletion mode in MOSFETS, you can also find a series of parts intended for current regulation, but now you ask the active device to dissipate the power.

( eg Microchip DN2470K4-G is DPAK part, 700V - 2 in inverse-series, with a Rgs to set the current limit.)

 

KISS says a series R or C is good enough, and very easy to understand and fix in 10 years time ! 

Last Edited: Sat. Mar 24, 2018 - 11:25 PM
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The target device includes a logging device for the input voltage and current.

We've come quite a ways for logging data

 

 

When in the dark remember-the future looks brighter than ever.

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Ah, the KREMS de la KREMS.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"Read a lot.  Write a lot."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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frog_jr wrote:
The target device includes a logging device for the input voltage and current.

In a test mode, the logging device records the input AC voltage and current at the point at which the target system first begins to operate.

The logging device continues to record AC voltage and current through the AC voltage ramp.

 

In 2018, you could automate that testing with a load-line locus test.  - a simple MCU and solid state relays taping a resistor chain.

eg

250V drive into 35k, intersects 5mA at 75V load.

250V drive into  12.5k, intersects 10mA at 125V

250V drive into  3.3K, intersects 15mA at 200V 

etc.

If 250V exceeds the DUT, you can clamp that, and if you need lower than 75mA worst case short circuit, you can add a couple of depletion mode mosfets

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250V drive into 35k, intersects 5mA at 75V load.

250V drive into  12.5k, intersects 10mA at 125V

250V drive into  3.3K, intersects 15mA at 200V 

Not 100% sure I'd go that route...that would feed the load with a high impedance source, which it might not like, or cause it to operate differently than when provided a normal voltage source ("zero" impedance).     While the resistors do set an ultimate current limit, the supplied voltage will rapidly decline or oscillate as the load draws varying currents.   It could be "ok" depending on what exactly the load is.  A good current limiter looks like a zero impedance driver (a voltage source) until the limit is hit, then jumps into action.

When in the dark remember-the future looks brighter than ever.

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avrcandies wrote:

250V drive into 35k, intersects 5mA at 75V load.

250V drive into  12.5k, intersects 10mA at 125V

250V drive into  3.3K, intersects 15mA at 200V 

Not 100% sure I'd go that route...that would feed the load with a high impedance source, which it might not like, or cause it to operate differently than when provided a normal voltage source ("zero" impedance).     While the resistors do set an ultimate current limit, the supplied voltage will rapidly decline or oscillate as the load draws varying currents.   It could be "ok" depending on what exactly the load is.  A good current limiter looks like a zero impedance driver (a voltage source) until the limit is hit, then jumps into action.

 

Perhaps, (anything is possible) but it's simple enough to try, and for a test fixture, this approach is quite common.

The load reads like a fairly normal low power switcher, that only needs a few points checked, and if there are any parts of the load-line that really do not like a load line intersect test, those can be avoided.

We have also used light bulbs (as suggested above) for more coarse testing, but I'm not sure you can buy a 25mA 200V lightbulb  :)

 

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but it's simple enough to try

Perhaps, but a bit dangerous.  If the load DUT was a transistor radio its current draw would go up & down with the sound....with a few K in series at the poer connection, the input voltage supply would be all over the map.  That was the point I was considering.

If the load is constant, of course, less of an issue. 

When in the dark remember-the future looks brighter than ever.

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avrcandies wrote:

but it's simple enough to try

Perhaps, but a bit dangerous. 

To a novice maybe, but the OP already tests these systems - they just want to replace lost test equipment easily, with a practical solution, & (safely) catch outliers...

 

avrcandies wrote:

If the load DUT was a transistor radio its current draw would go up & down with the sound....with a few K in series at the poer connection, the input voltage supply would be all over the map.  That was the point I was considering.

If the load is constant, of course, less of an issue. 

 

The OP has already given these numbers above :  

Up to about 60VAC the current is near 0, and ramps to 5-10mA at 70VAC. The current (typically) remains fairly constant to about 170VAC, where it then starts ramping to about ~15mA at 200VAC.

 

(clearly not 'a transistor radio')

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We have also used light bulbs (as suggested above) for more coarse testing, but I'm not sure you can buy a 25mA 200V lightbulb  :)

There are a couple to experiment with:

 

4W 120v 'miniature candelabra' (commonly available 'night light' bulb)

 

"120MB" 25 ma 120V min. bayonet "switchboard" lamp.

 

May (or may not) need to use two in series for 200V testing.

 

I have a similar situation with a product that uses a switching regulator that requires several external components to configure it.  If a part is missing, a bad solder connection, bridge, etc. the chip can *instantly* self destruct if full voltage is applied, so I always do a reduced voltage current limited test.  A scope on the output displays a particular signature if everything is in place, then full voltage can be applied.

 

 

Tom Pappano
Tulsa, Oklahoma