Cheapest / simplest high voltage dummy load - Page 1
Forum section: electronics / projects

author: dexters_lab
author_rank: Unknown
timestamp: January 19, 2016, 11:57:42 am
content: I have a HV power supply here i would like to test at full load, so need some kind of dummy load for itunfortunately it's current control is a basic trip, there is no constant current modeit's spec is 20kv at 5ma so that totals 100 watts or a load of 4megohmsfirst thought was some of these cheap hv resistors in parallelhttp://www.ebay.co.uk/itm/291551546591maybe there are some different options i'm not thinking of to use as a load?cheapest option wins!

author: bktemp
author_rank: Super Contributor
timestamp: January 19, 2016, 12:19:33 pm
content: 3x GP-5 hv triodes in parallel. Add a current sense resistor and an opamp and you have an adjustable current sink up to 30kV/6mA/112W.http://www.mif.pg.gda.pl/homepages/frank/sheets/164/g/GP5.pdf

author: Marco
author_rank: Super Contributor
timestamp: January 19, 2016, 12:45:18 pm
content: How are triodes at current sharing? Would you need a separate degeneration resistor per triode and then bring those together into a single current sense resistor?

author: Kalvin
author_rank: Super Contributor
timestamp: January 19, 2016, 12:56:30 pm
content: You may also consider using individual op amp drives for each triode instead on one common op amp driver for all triodes. Thus you won't have to worry about current distribution among the triodes because each triode will be controlled individually by its own op amp.

author: bktemp
author_rank: Super Contributor
timestamp: January 19, 2016, 01:21:28 pm
content: If you do not need the maximum possible power dissipation and don't care much about the longest possible life of the tube (GP5 tubes are really cheap, in Germany you can get them for 1.5€), it works with a single current regulation for all tubes because the tubes have a rather low transconductance, especially at lower voltages. Tubes also survive short overloads without much problems, unless you melt the glass. The anode starts to glow above 50W.But tubes generally have quite high tolerances for their current curves, so yes, the best way is to use a seperate current regulation for each tube similar to mosfet based currents sources. And then connect as many current sink modules as needed in parallel.Above 20kV it also makes sense to add a small metal shield around the tube, because it starts to produce x-rays at those voltages.

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 19, 2016, 01:34:43 pm
content: How about a series string of 50 100k/3W metal oxide resistors (e.g. - Panasonic ERG series, about 0.26USD each in qty 50) with individual spst bypass switches across 10 of them to be able to vary the load from 4M to 5M?Personally, I wouldn't mess with paralleling HV resistors, especially not ones that cost less than 4 quid each on fleabay.

author: Unknown
author_rank: Unknown
timestamp: January 19, 2016, 03:16:22 pm
content: Most ideas vanished when I read 20kv. I've used strings of domestic lightbulbs up to 2kv, but at 20 a simple string of resistors sounds like the best idea.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2016, 03:34:28 pm
content: There's always the water resistor. Use a long length of hose, filled with not-particularly-deionized water (tap water is probably fine). Anode corrosion won't be bad if you don't need to test for more than a few hours. Otherwise, you'd want to use something like a platinum or MMO anode.As for triodes, vacuum tube matching in general is extremely good. I don't know the tolerances on a 6BK4 (or any of the similar parts), but I wouldn't think it's worse than usual.The circuits they are used in feature a little cathode degeneration, which can be used here to provide more than enough help with load sharing.Grid voltage is typically -100V to the cathode, so plan for a good bit of cathode bias. As a bonus, put in a high-value divider (say, 1G into a 10M pot) to set the grid voltage, so the current is adjustable.Tim

author: HighVoltage
author_rank: Super Contributor
timestamp: January 19, 2016, 04:18:58 pm
content: I am using proper high voltage resistors, like the 1 M Ohm in this picture.You can get them cheap on ebay from time to time.They are a little large but I have a few of them in series hooked up and placed in an enclosure.

author: dexters_lab
author_rank: Unknown
timestamp: January 19, 2016, 06:58:34 pm
content: thanks for the suggestionsnot sure if i want to muck around with toobs, seems like a lot of effort for a temporary taska water resistor will probably want to kill me to i'll leave that one out toolooks like resistors are the way to go...@HighVoltage:I have seen these specialised resistors from Genvolt, they do some:http://www.genvolt.com/products_applications_information/514748ba52614/RES100-Rangebut no idea how much they cost, i might drop them an email and see before i buy anything else

author: oldway
author_rank: Super Contributor
timestamp: January 20, 2016, 07:22:38 am
content: Quote from: bktemp on January 19, 2016, 12:19:33 pmAlso 3 x PD500 HV triodes (or PD510, or ED500) used in vintage color TV set.