Hi all, Please tell me how can I safely superimpose a 15Volt Ac signal (50Hz -
100KHz) from a signal generator onto a variable DC voltage (50V to
5000Volt). The output is 5000V DC + 15 V AC.
First I thought i wil use a transformer to isolate the function generator output from the HV and then use a diode to block Ac from getting into the HV power supply. But this can remove only a half cycle of the Ac pulse and may damage the HV output.
Please let me know if anyone has a nice idea or some circuits.. I am very much in need of this circuit...thank you.
hi luhan, thank you for ur reply. I need this for studying material properties, orientation of molecules under a static+oscillating electric field. do u have any ideas ?
I did something like that once with 700 volts of audio riding on an 800 volt dc source. This was used to create audio from 2 electrodes placed in a candle flame.
It all amounts to the insulation resistance of the transformer feeding the audio. The primary/secondary insulation resistance must be able to withstand the high DC voltage. If so, you just connect the tranformer secondary in series witht the high voltage source.
But, will this damage the output of the HV power supply ? as the secondary generates AC, it will go in to the DC unit, even if it is in series.
I planned to have a transformer with primary turns and sec turns of the same number so that no voltage amplificati> snipped-for-privacy@gmail.com wrote:
Conceptually, you have to ask the question of how the DC power supply would even know that you added this AC voltage to it's output?
The only way it can know is if either your transformer isolation breaks down (hope not) or if there is a complete circuit which allows current to flow through the supply.
Consider what the added AC component can do to change the current flow through the power supply. Can it make the current flow larger? Yes. Can it make it smaller? yes. Are the degrees to which it can do so within the current rating of the power supply?
The important question: can the AC component make the current flow negative - can it make current go through the power supply the wrong way? It would seem it can only do this if the peak AC voltage is higher than the output voltage of the supply. Can that situation occur? Sure, when the supply is turned off. Is it a problem? All depends on how the supply is built.
But if you put in a diode at the power supply output, then current can only flow through the supply in the designed direction. However, if the AC voltage ever approaches cancelling out that of the supply such that the diode comes close to cutting off, then your AC waveform will of course be distorted - but distorted signals and equipment damage are two different classes of fault.
I doubt that a 5kV DC supply would notice the 15VAC. You'll be lucky not to have 15V of ripple on the DC supply's output, at no extra charge to you.
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You might assume, hopefully, that the sample you are examining has a very high DC resistance. I assume it is placed between a pair of plates to which you apply your supply. Once these have been charged then effectively no DC current will flow.
When you apply your AC voltage, through the transformer, then much of the current flow will be capacitative and determined by the frequency and the, separation and area of the plates along with the dielectric properties of your material.
Your power supply will have a smoothing capacitance across its output terminals and, as long as the value of this capacitance exceeds that of your plates by a suitably large margin then the AC current will flow through that capacitor and bypass any active parts actually within the supply.
Things should be safe without have to add any further components.
The main issue will be a sufficient breakdown voltage between transformer primary, secondary and core. Some medical grade iso transformers go that high. If you need 50/60Hz here you might want to talk to Ulveco or companies like that.
Also, if using 50/60Hz keep in mind that what comes out of a wall outlet is hardly ever truly sinusoidal.
Some HV supplies have floating outputs, or they can be made floating trivially, e.g. by removing a strap. Putting a transformer between the cold end of the supply and ground would relax a whole lot of these difficulties, and if correctly done, would be somewhat safer. Do ground the core of the transformer, and put some transient absorbing Zeners across its output just in case something opens up without your knowing it.
Note that this suggestion assumes that you know enough not to kill yourself doing it. If the output of your power supply can supply enough current to kill you, don't mess around inside it unless you *really* know what you're doing. Some supplies do, some don't.
Absolutely, especially if there are capacitors involved. For example, even a wimpy power supply can lead to cardiac arrest and possibly death if it was used to charge up one of those microwave capacitors.
You might like to do some research on analog telephony principles since this function is commonplace in that industry. As an example, the ac ringing potential (nominally 75Vac at 20Hz) is superimposed on the -48V dc (wrt +ve gnd) line potential during ringing.
It's easier to 'superimpose' the HVDC signal on top of the LVAC signal, using a grounded transformer, or a simple voltage source in the HVDC return path.
Float the supply on the 15 V AC - i.e. the 15 source is in series with the return of the 5000 V supply. Much easier than doing it the other way round :)
Hi all, Thank you very much for the solutions and ideas. I think i wil use Frithiof's suggestion. I hope my Stanford Power supply won't mind injecting an AC to its neat clean DC output. best regards, Kishore
Agreed, injecting the AC in series with the gound side of the HV supply is a good idea... BUT BEWARE OF THE FAULT CONDITIONS...if something faults open on the ground side, all the wiring on the high side of the fault assummes a high voltage. This is dangerous because a wire that is normally near ground and not well insulated and not well respected can unknowingly be at a deadly high voltage. So put a resistor or zener or other device to limit the voltage that can appear on the "ground " side in case there is an open circuit fault.
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