AC dynamic load ideas - Page 1
Forum section: electronics / projects

author: PA4TIM
author_rank: Super Contributor
timestamp: January 27, 2013, 12:57:47 am
content: I now use a 1:1 transormer, a variac and a switchable lightbulb as current limmiter.This is not ideal.I designed and build a DC powersupply based on a dynamic DC load. It can work as load but the load also works as CC mode. ( with some circuits that keep the voltage constant as long as possible compensating the voltdrop over the load)This I would like to have for AC too. So instead of the lightbulb a load that i can set for a certain max amount of current. As long as there is enough space for the voltage it is OK. So like a current source with voltage regulation/limitation. ( or rather a voltage source with current limitation)Something with a few beefy mosfets but 230VACrms through the mosfets that need a contant Vgs of max 0 to 10-20V while the source goes up and down is not easy.It is for testing, repairing and awaking instruments, so no dimmerlike switching. I need a nice clean 50Hz 230V ( the voltage is not important, max 230 but current limmiting is what I need for protection, formating ect.) and upto 4A at least.Before I reinvent the wheel....Fred pa4tim

author: robrenz
author_rank: Super Contributor
timestamp: January 27, 2013, 03:27:14 am
content: Could you use one ofTheseBipolar operational power supplies?My reviewof the EMI Lambda version of one.

author: PA4TIM
author_rank: Super Contributor
timestamp: January 27, 2013, 09:42:56 am
content: As far as I can see these are powersupplys that can sinck and source but only DC. So I can not use that.I need a "lab-supply" ( so CV and CC mode) that can deliver 0-230VAC and 0-4A ( or better 6A) as a replacement for a variac, isolation transformer and lightbulb.The variac an isolation transformer are not the problem. This works fine but the lightbulb as current limmitter is not flexible enough.I can get 260V out the variac so I could use a switch and some beefy resistors to limmit current and adjust voltage through the variac. This will do what I want but then I get steps and swtiching so I can not increase current without transients.The main thing is a replacement for the lightbulb so I do not have to change lightbulbs for other limmits.It is for- variable limmiting inrush current while reforming the caps.- current limmit / protection, so if an instrument has a 1A current draw I choose a resistance, adjust the variac to 220-230V again and if somethings goes wrong, and the insteuments starts to draw a lot more current the voltage will drop because the voltage over the limmiting resistor Increases.I do this with a DC psu using mosfets for setting the voltage and a second set of mosfets for acting as a variable resistor. Some opamps keep the voltge between source of voltage regulating mosfets and drain of current sinking mosfets constant as long as possible. So I can change current limmit to increase or decrease current without the voltage increasing or decreasing. As long as it is within the psu rails the control circuit will hold the voltage constant.Sound maybe a bit weird but I have function for it. I use it as a dynamic load ( it can be internal or external modulated) but also as battery charger. So I can increase of decrease current or do things like pulse loading while make sure the voltage stays under a set value. ( sort of over voltage protection) just like a benchsupply but this one can be modulated and work as a load.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 27, 2013, 09:54:34 am
content: Hi,Are you looking for a source or a load?If you are looking for a source, Google 'Elgar 501'. This is a 500VA AC source. It is basically a high power amplifier with an output transformer.I designed my own oscillator plug-in which is a simple DDS based on a PIC.I used mine for testing off-line supplies and LED drivers. It will still let the 'magic smoke' out of a circuit, but it prevents the circuit board from being destroyed.Jay_Diddy_B

author: PA4TIM
author_rank: Super Contributor
timestamp: January 27, 2013, 10:11:52 am
content: I'm looking mainly for a load because I can use the variac as soruce. Combined they do what I want.But a source with CC mode is also an option.I use it for testing, repairing and awakening measurement instruments. The isolation transformer with regarding to probing. The current limmiting and voltage regulating because of safety, testing and reforming.

author: SeanB
author_rank: Super Contributor
timestamp: January 27, 2013, 10:14:17 am
content: You can do the current limit with a bridge rectifier in series with the load, and a really beefy IGBT or power transistor/mosfet capable of handling the peak voltage of the variac ( in this case at least 400V capability on the power device) running as a standard electronic limiter, using a floating power supply to power it. Adjuster will need to have insulation to full mains voltage on it, whether you use a pot or a digital means are up to you, but thet will have to have isolation as they will be at mains voltage. Will need a overtemperature monitor, and a big heatsink and good insulation between the power device and the heatsink as well.

author: nctnico
author_rank: Super Contributor
timestamp: January 27, 2013, 02:09:07 pm
content: I'd start with a PFC stage. This loads the AC with a sine-like current.

author: robrenz
author_rank: Super Contributor
timestamp: January 27, 2013, 02:54:31 pm
content: Quote from: PA4TIM on January 27, 2013, 09:42:56 amAFIK thisiswhat you want. You would drive a BOP with a signal generator at the frequency you want (up to 20kHz) and the SG amplitude controls the BOB output at whatever gain you have set on the BOP. It can either control the current CC or control the voltage CV. It also has separate current and voltage limits. These can be paralleled for higher amperage or voltage. If you take a look at my review thread I linked you will see the AC capabilities. The Kepko manual linked in that thread has all the technical info.These things are just a huge op amp. With a signal generator driving it it becomes a 0 to 20kHz, 0 to Max.V or 0 to Max.A AC power supply. It also does not have to be a sine wave, any wave you send it will be replicated within its BW capabilities.

author: robrenz
author_rank: Super Contributor
timestamp: January 27, 2013, 04:15:30 pm
content: The output is FETs and the rest of the control circuitry is opamps with a few Bjts. The two I have are 20V-10A units.

author: PA4TIM
author_rank: Super Contributor
timestamp: January 27, 2013, 05:31:15 pm
content: I found some more info and indeed they seem to able to output AC. But the info is not very clear. I need 230V output at 4-6A.But I want to build one, not buy one. Or i want better build it as an addition instead of the lightbulb to my variac with isolationtransformer, switches, protection, meters ect that is build in a 19" cabinet and can handle upto 0-260VAC and 8A ( and do 0-1200VDC at 1A)SeanB, that option is interessting. Had not thought of that.I also will study the PFC, that is new for me, but looks usefull at first glance.Also thought about some transformerlike construction in serie between variac and isolation transformer and vary its primairy impedance by loading it secundairy. But that is a wild brainfart, i not really thought through and I have the feeling I forget something important here ;-)