High Voltage! Don’t Try This at Home!

What kinds of things are done using high voltage? Some of them you already know about. They are still used for CRT visual displays, CRT displays for photo typesetting (imagine a CRT with 1200 lines per inch resolution), photomultiplier tubes, radar, X-ray generators, assassinating mosquitoes at your backyard barbecue party, and so forth. Sometimes though, the unusual need comes along.

There was this bread bakery some while ago that was using high voltage to lower their oven operating costs. They told us that when 40 thousand volts were applied from above and below in the oven chamber where loaves of bread were being passed through on a conveyer, the high voltage reduced the required natural gas usage by 20 percent.

Somebody was awfully clever about this. They had devised a high-voltage power supply using the innards of a 1950s vintage television set. There was a flyback that worked with a 6DQ6 horizontal driver tube and a 1B3GT high-voltage rectifier. How they had gotten it up to 40kV they didn't say, but since home television CRTs like the 21YP4, for example, only needed perhaps 20kV, I was very impressed.

Reliability was their issue. Although they could still produce loaves of bread without the high voltage when things broke down, the fuel savings had warranted frequent repairs. We made a “modern” 40kV supply for them and they were very happy.

Then there was this car company that made diesel engine-powered vehicles and they were trying to minimize the generation of soot. They needed to measure soot emissions so they put corotrons as soot sensors into the exhaust pipes. Simplistically, they looked sort of like this sketch, a thin wire within a conductive half-circle shroud.

The requirement for powering these things was given to us as 12kV at 1mA, so we designed the power supply to deliver just that, built a couple, and made the shipment. Shortly afterward, we heard back about the performance that had been achieved. The report was very brief.

Corotrons, as it happens, are required to be driven from a controlled current source, but we had followed the customer's specification in making a controlled voltage source.

The remedy was simple though. I studied the schematic and prepared some instructions. It was a simple modification, something like, “Remove R10 and CR4; connect a jumper from U2 pin 3 to the low end of R8…” and thereby the unit would be converted to a 1mA constant current power supply with a compliance voltage of 12kV.

It worked!

It was with great pleasure that a few months later we received a lovely letter of appreciation for that product.

12 comments on “High Voltage! Don’t Try This at Home!

    March 14, 2013

    Reminds me of a story from years ago. A fellow designer was asked to make a flash lamp power supply. When he asked about the load, they said use a resistor. The power supply worked beautifully with the simulated lamp. When the actual lamp was connected, the whole thing fried.

    Apparently they did not tell him that until that point, the lamp acted as a negative resistance. Thus the circuit had a negative-negative feedback, ie positive. And that leads to things going “pop”.

  2. Brad Albing
    March 15, 2013

    Yep – you need some way to shut the lamp off – quench it, I think….

    See whwn I was a kid, I didn't know about that or about how gas discharge lamps in general needed some sort of current limiting – like the ballast does for fluorescent lamps. So I'd haul those 8 foot lamps oyt of the trash behing the grocery store (the ones w/ the blackened ends), take them home, and hook them up to either neon-sign transformers or transformers removed from TV sets (the high-voltage winding) scavaged from the curb on trash day.

    Even those “dead” tubes would light up pretty bright. It's a wonder no one got hurt from exploding tubes.

  3. Qzz
    March 21, 2013

    Why Does 40KV reduce gas consumption when baking bread?

    (Is this like wearing magnetics for aches and pains?:-) ) It must work …but why?

    (I used to build boiler controls)

  4. John Dunn
    March 21, 2013

    The bakery never offered an explanation, only their observation and who it was who had first come up with the idea was somethng lost in the shrouds of time. My guess is that the bread dough must have been getting corona discharge effects on a grand scale which accelerated ingredients' breakdowns that the oven heat was going to do anyway, that the corona just made it happen faster.

  5. Qzz
    March 21, 2013

    I might try it……As I always say “One experiment is worth a 1000 expert opinions”


  6. WKetel
    March 21, 2013

    The benefit gained from the 40 Kv source may have altered the convection patterns and thus improved the heat transfer. Perhaps it altered then formation of the stagnation zones in the convection airflow.

    As for producing the high voltage, that description sounds a l9t like the old oscillator hich voltage system, with the single 1B3 diode. But if it is arranged a bit differently then there is a 20kV ripple on top of the 20kV dc, hence 40,000 volts.

    Presently I am looking for a similar transistorized system to provide about 10Kv for a vehicle protection device. 

  7. eafpres
    March 22, 2013

    My guess is that the energy savings must have something to do with water vapor/humidity.  I can't imagine how a potential would affect the amount of water conveted to vapor during the baking process, but I can imagine that decreasing the humidity would affect the baking time.  I found and interesting reference that indicated the thermal conductivity of moist air actually decreases with moisture content:

    Tc vs humidity of moist air

    So one theory would be the high voltage somehow de-humidifies the air, creating a better transfer of energy into the bread.

    Another thought is that tiny droplets of water might form in a cloud around the bread during baking, and the high voltage might cause them to migrate somehow, possibly removing water from the bread dough without net evaporation and consequent high waste of energy.  Since the field was AC, I'm not sure how this could occur.

    Another effect of signifcant de-humidification would be to reduce absorption of infra-red energy by water droplets, which would increase the radiative heat transfer.  I think it possible that both conduction and radiation could be involved in baking bread.

  8. Brad Albing
    March 23, 2013

    Hi John – on the topic of high voltage generation, have you ever used a piezo device to gen hi-V? Either the type you wack or a piezo-transformer?

  9. John Dunn
    March 24, 2013

    Nope. I've never had any involvement with piezo devices of any kind.

  10. Brad Albing
    March 24, 2013

    I've not heard of this process befoe this blog. Wonder if it's still used by anyone or if there is any research available….

  11. Brad Albing
    March 24, 2013

    Well, looks like it's calling out for another blog by me on the topic.

  12. Brad Albing
    March 24, 2013

    I know Jim Williams had mentioned piezo transformers some time ago in one of his app-notes (10 yeas ago? 15?).

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