Variable AC load for testing purposes - Page 1
Forum section: electronics / projects

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 31, 2018, 06:47:52 am
content: I would like to test the specs of some ups and inverters buy loading them on the AC side.So far I've been using resistor wires and adjusted their lengths to increase or decrease the load.Is there a better way to do this? Want a load between 500-1500 W that I can precisely tuned to at least 50 W increments.Output voltage range is 220-240 V AC.What can I use? Preferably something that could be found laying around the house or workshop.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 31, 2018, 06:54:59 am
content: Heavy duty light dimmer or variac + hair dryer, with a cheap power meter to get the right settings.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 31, 2018, 07:06:42 am
content: Quote from: NiHaoMike on January 31, 2018, 06:54:59 amSo the variac allows me to reduce the AC voltage which then increases or decreases the current since the load is resistive and followslaw?

author: Jeroen3
author_rank: Super Contributor
timestamp: January 31, 2018, 07:08:26 am
content: There are several method. The most common one is by a array of resistors and relays.But you can also use a water resistor.If you want to control the power factor you can add capacitors and inductors with relays to this array.But another method to get inductive is to use a stalled motor and adjust the pf by changing the rotor angle.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 31, 2018, 07:44:59 am
content: Quote from: Jeroen3 on January 31, 2018, 07:08:26 amI much prefer the first method mentioned above since it is much simpler.

author: Ian.M
author_rank: Super Contributor
timestamp: January 31, 2018, 07:58:06 am
content: A light dimmer will give a very non-sinusoidal current waveform.If you don't have a beefy enough Variac, split up the load - switch in fixed loads to get near the power you want, and trim the rest of the way with a smaller load on a Variac.Another approach is a job lot of 100W incandescent light bulbs + a switch bank to control them. You'll need some 25W bulbs as well to do the final trim, with maybe a dimmer on one of the 25W bulbs for fine adjustment. As the dimmed load is only a small part of the total, the non-sinusoidal current waveform component will be far less objectionable.However the transient startup surge drawn by multiple parallel incandescent bulbs may be an issue for your UPSes or inverters, unless you sequence switching them on.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 31, 2018, 08:13:53 am
content: Light bulbs aren't nice behaving resistors. They're cheap though.You can get better resistors by using heaters.IR heaters form good bulk without need for fans.

author: John Heath
author_rank: Frequent Contributor
timestamp: January 31, 2018, 08:22:31 am
content: With a 200 , 100 , 50 , 25 . 12.5 watt bulbs you could come up with a 5 bit , 32 count, from 0 to 382.5 watt programmable load. To overcome surge power of initial bulb turn on all bulbs could be continuously on power by UPS or house power depending on how they are switched from UPS or house power.

author: Ian.M
author_rank: Super Contributor
timestamp: January 31, 2018, 08:33:09 am
content: Light bulbs are cheap and easily handle the dissipation without fan cooling. Use tin cans with both ends cut out to make shades to eliminate most of the glare. For a fully programmable load, feed them from a full wave bridge (with no reservoir capacitor), control individual bulbs by MOSFETs controlled by a MCU, and PWM them individually for a sequenced soft start. You could PWM a low wattage bulb for fine trim. Add a power monitoring IC (+ current shunt resistor and potential divider for the voltage) so the MCU can sense the actual load power and trim the load accordingly, and you can avoid most of the deficiencies of using light bulbs as resistors. I'd suggest using an Arduino Mini for the MCU, (or a bare AVR with an Arduino bootloader) and optoisolate its UART interface, so you can easily and safely control the load and do data logging from a PC terminal.If you need a stable pure resistive load, don't need massively long run times, and don't want to heat up your workshop too much, consider water cooled heaters. You can get 500W immersion heater elements, and three of them fitted to a 25L Jerry can would give you over an hour at 1500W while still staying under boiling point. Dump it down the drain and refill with cold water for a quick reset for the. next run.

author: jbb
author_rank: Super Contributor
timestamp: January 31, 2018, 09:22:25 am
content: A kettle (or jug depending on your linguistic preferences) can also do quite nicely, but only for a couple of minutes.An oil column heater could also be used, I guess. Having no fan means it won’t worry about low voyages or strange waveforms.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 31, 2018, 10:04:36 am
content: Quote from: jbb on January 31, 2018, 09:22:25 amBut those aren't variable loads. They pull constant power from the mains

author: Ian.M
author_rank: Super Contributor
timestamp: January 31, 2018, 10:30:25 am
content: Put a Variac in front of the various fixed resistive loads.For a true variable load, Jeroen has already mentioned water resistors a.k.aliquid rheostata.k.a. 'scariac'*<EXPLETIVE>* *DANGEROUS* if poorly built or if the water temperature exceeds the softening point of a plastic tub container.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: February 01, 2018, 11:25:52 pm
content: Got myself a 2KVA Variac and very happy with my new toy! I can now test to within 5 W of overload or so

author: ocset
author_rank: Super Contributor
timestamp: February 03, 2018, 10:08:36 am
content: hia good way to do dummy loads is with those flat power resistors.......put them on a pcb, with thermal vias to bottom layer, then screw it to a heatsink(s). (using insulative material so you dont short)Assemble the heatsinks so they are like a chimney.Then mount a fan beneath it.Then you blow air up the 'chimney"Sorry i dont have a diaG of the full thing, but heres some of it as attached