# Turning off a resistive voltage divider to save power

Hi All,

I have a rather odd question so here it goes.

I need to measure up to 60kV and I have found the right resistor divider that won’t dissipate that much power but would like to turn off this divider when it is not needed so it does not interfere with general C02 laser operation. This voltage measurement circuit is for C02 lasers up to 150W. I can reliably measure the laser tube output current but also need the voltage for the internal calibration of my system.

The input voltage of the divider is 60,005 V (Roughly 60kV). I have calculated the (R1) top resistance to be 12 gigaohms (Qty 24 500 meg ohm resistors - CRHA2512AF500MFKEF) and the (R2) bottom resistance to be 1 meg ohm. The output voltage is 5V.

Now the silly question I have is can I turn OFF the voltage divider if I put in a P-Channel mosfet between the R1 and R2 link? Here is a rudimentary picture:

60kV
|
R1 - 12G ohm (300 mW dissipation when active)
|
P-Channel MOSFET
|
------------------> 5V output to op-amp voltage buffer
|
R2 - 1 MEG ohm (0.025 mW dissipation when active)
|
------------------> Ground to voltage buffer
|
Ground

What I am hoping the above circuit will do is OPEN the link thereby no current flows essentially opening the circuit. Please let me know if this will work and if there are any pitfalls to this method. I am also open to other ideas as well.

Thanks,

Eric Norton

Greetings,

An open switch must be able to withstand the entirety of the driving voltage in the circuit, and 60kV FETs aren’t exactly commonplace…

I’d suggest evaluating how significant the losses you’re trying to avoid are in relation to the power being delivered and whether or not you can compensate mathematically for any errors introduced, because it will simplify matters greatly if you can manage to just live with the influence of your measurement apparatus.

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Hi ericn,

Welcome to the Tech Forum. To answer your question, I don’t think a standard MOSFET would do the job here, for at least two reasons.

Firstly, the leakage current of a typical MOSFET in the off-mode can be in the tens to hundreds of microamp range, which is higher than the current you are creating with your voltage divider. Therefore, it wouldn’t really turn it off.

Secondly, if they could turn off the current, then the voltage across the Vsd pins would exceed the max voltage of the device (assuming you can’t find one rated for in excess of 60kV), resulting in its destruction.

Thirdly, one would have to carefully seek out one with a package which has sufficient creepage to prevent direct arcing across the device.

I’ll have to noodle on what might work for your problem.

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I was afraid you’d say this. I see that in order for something like this to work, the MOSFET needs to be able to handle the 60kV and that makes sense. I was thinking the other way around that at that node the voltage is 5V but that is not the case because the R2 resistor is not connected so that won’t work. I would just be limiting the current, not changing the voltage. Any other ideas I could try here? I guess I could leave it in the circuit and just take a measurement when it is needed as this does not draw much current.

Yeah, I see you points here. Hmm, well ok back to the drawing board. If you have another way to approach this issue I am all ears

I’ve looked at a few alternatives (various relays, optoisolators, etc.) and have not come up with anything as of yet. I think Rick’s suggestion is likely the only valid option.

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This is likely the best option if feasible. You’re speaking of a source capable of spontaneously striking an arc across an air gap several inches in length, and adding ways for things to go wrong is not a good idea in such cases.

Speaking of things going wrong, what you think is a voltage divider will become a spark gap switch if you’re not extremely careful about things. Plan accordingly, and get some qualified mentorship if you’re not practiced in HV work.

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Yes, I agree and will not add on/off functionality because of arcing. I will design the board with all resistors in series in a single line with rounded pads and solder joints close to each other. I am also considering potting the whole high voltage portion of the board so that may take care of arcing from one resistor or a series of resistors.

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That’d probably be prudent, considering the effect a bit of surface contamination can have on a small 500M resistor. Use of larger through-hole devices to increase creepage distances and reduce entrapment risks might not be silly either.

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A relay with it’s contacts in a vacuum. That’s what I’ve seen in the power industry. I’m not sure how much money you want to spend.

Well, that sounds nice but also sounds quite expensive. In this market people want/expect things cheap so I have to design something with an attractive price tag.

Hi Rick,

I found this resistor:

MHR0422SA108F70 (1G OHM)

I am going to put twelve of them in series and call that R1 of the resistor divider and then use a 1 MEG OHM for the bottom R2 of the resistor divider. By stringing twelve of the 1G OHM resistors in series, I believe that would give me sufficient distance so that there is less chance of arcing or corona or am I wrong? I would like to put the whole divider circuit in some sort of acrylic (maybe) or some other material tube with end caps with one end having two HV 55 kV red wires and HV connectors and the other end the low voltage circuitry and current sense and that end have the earth ground, laser return and signal ground along with isolated RS-485 comms cable coming out. I want to fill the whole tube with potting compound as to seal it from outside debris and also limit the ability for the thing to arc. How does this sound? Any pointers or anything you’d recommend I do differently? I am open to suggestions…

Hi ericn,
Just wondering, how do you generate that 60kV? Perhaps the H.V. supply already has a proper feedback signal available in some form.
Cheers, heke

Hi Heke,

The C02 laser power supply up to 150W lasers can generate the up to 55kV voltage that I want to measure. This way I can get the overall power if I measure the voltage and current coming from the laser power supply so that way I can determine state of health and tube current for proper and accurate system calibration without user intervention and detect if the power supply is failing which does happen.

Thanks,

Eric

Hi Eric,
So, if I got it right, the H.V. power supply is provided by the laser supplier. Is there a chance to open wide the supply housing to see what it has eaten? Maybe there is a circuit node that holds scaled down value of the output. You could then probe that.
I guess the H.V. voltage is for an excitation tube. Perhaps you could measure the light energy output of the tube to determine indirectly the health of the PSU?
Cheers, heke

Hi Heke,

That would be great but don’t want the customer doing that. I want to make something the customer can install themselves inside their laser close to or on top of the laser tube that is reasonably “safe” to use. I know high voltage is dangerous and the high voltage area is closed off in a compartment in the rear from the user while in operation. The idea is to put this high voltage resistor string in an acrylic tube and then pot it so that it is extra safe.

Thanks,

Eric

If you haven’t already, make sure somebody runs the product idea by your liability insurance provider. Remember no matter what you say to potential customers, their relatives may sue and you want an insurance company to handle those costs.

More than once when I’ve suggested that to management over potentially lethal products the answer was, lets not go any further with this product idea.

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Yes, this has crossed my mind. I look at it this way… If a user buys a Chinese laser they are already at risk of electric shock from poor wiring and lack of proper assembly. It is up to the user to install the device per the instructions provided and I will provide a document they must sign at the time of sale that would make me not liable for any damages personal or to property. I will contact my lawyer and tie up any loose ends so this particular issue does not come up.

I think Eric brought up an extremely interesting subject to discuss about (perhaps not on this thread though… Will the counties or regions, that have the least legal burden, i.e. least hawkish lawyers that seek to benefit from scientific mishaps, advance in technology fastest?

Hi Heke,

The way I see it is the customer is already working with a dangerous product and it is up to he/she to ensure their safety while the product is being used. I am a part of some of the laser groups on Facebook and let me tell you some people shouldn’t be using a laser. Some have burned down their entire house, some have tattooed themselves (video from Russia) with it, some have burned themselves by sticking their hands in the path of the beam and some have even shocked themselves and lived to tell about it.

I want my product to be as safe as possible and while some say it is deemed unsafe to design something like this, I feel it is the other way around. The benefit the customer gets from this design would outweigh the dangers and the risk of shock and would be lower than if someone were to intentionally go in the compartment and touch something they knew they were not supposed to touch.