Electronic PSU  load using heater as it's load element? - Page 1
Forum section: electronics / projects

author: dumle29
author_rank: Regular Contributor
timestamp: September 13, 2014, 03:46:46 pm
content: Hello, let me first apologize for any missing / extra spaces. My spacebar switch is dodgy and yeah :/ Waiting for a replacement switch so I can solder that in.I was watching a mailbag, where Dave got a PCB for an electronic load in the mail, and I thought to my self, that the most limiting factor of the digital loads, is the Tj max of the transistor used to burn the energy, and I was wondering if it would be possible to provide a variable load, with an external heater.The easiest way would be to PWM a heating element, but that obviously will be rather nasty, as we could be talking about PWMing high currents.My question is, would it be possible at all, or would we have to drive a fet analog, and have the same heat issue in that?

author: penfold
author_rank: Frequent Contributor
timestamp: September 13, 2014, 05:46:55 pm
content: The optimum solution is very dependant on the situation.If your circuit can tolerate the PWM switching noise and your measurements won't be affect by it, then yes, its a good idea if implemented properly.If you need something less noisy then yours options are reduced to using a transistor in linear mode or a fixed resistor

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 13, 2014, 05:56:32 pm
content: I've always wondered about that. What's wrong with switching in discrete power resistors in a binary tree with MOSFETs? So you lose resolution.

author: tom66
author_rank: Super Contributor
timestamp: September 13, 2014, 06:22:23 pm
content: Quote from: Alex Eisenhut on September 13, 2014, 05:56:32 pmPrecision high power resistors are expensive. If you need 12-bit resolution, the problem is crazy complicated.

author: saturation
author_rank: Super Contributor
timestamp: September 13, 2014, 06:33:05 pm
content: a 'heater' often exists in many eload designs, its R2 in series used as a current sensor.A problem dissipating more heat hence power in R2, is increasing the lack of control you have over programming the load since R2 is passive. Resistive elements have their own quirks which reduces the capabilities of the design: changes with temperature, inductance, noise and some capacitance. By having Q1 do most of the work, you can control the load effects optimally and produce an eload of high accuracy. Typical R2 is ~ < 0.1 ohm so it contributes negligibly to loading effects.The key to Tj management is cooling, keep case temps << 50oC, operate with the SOA of the device, specify Q1 then derate it 50% and heat will not be a concern.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 13, 2014, 06:40:28 pm
content: Quote from: Alex Eisenhut on September 13, 2014, 05:56:32 pmHow many MOSFETs and resistors would you need to accommodate every possible combination of test voltages, currents and power if you only use the MOSFETs as switches? Tons of high-power resistors which will quickly set you back hundreds of dollars.How many MOSFETs do you need if you use them as programmable current sinks? Only as many as you need to meet your peak power dissipation and these will set you back maybe $50 per 500W with arbitrary precision mostly limited by your control loop.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 13, 2014, 09:00:32 pm
content: Quote from: DanielS on September 13, 2014, 06:40:28 pmYou don't. If you're building one, just pick what you need.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: September 14, 2014, 01:16:46 am
content: Quote from: Alex Eisenhut on September 13, 2014, 09:00:32 pmIf you're just picking what you need, then you're not building a binary tree any moreBut a small selection of power resistors still won't give a stable load without a control loop. I think I would approach it like most linear power supplies, except instead of different transformer taps, it would be different power resistors. That way you still get the benefit of an analog control loop while keeping down the number of expensive sense resistors and cooled linear mosfets, and still getting decent precision and range of current draws.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 14, 2014, 02:25:53 am
content: What about a sort of buck converter where the switching speed (slew rate) is purposely kept low, with a linear stage taking out remaining ripple?

author: tom66
author_rank: Super Contributor
timestamp: September 14, 2014, 07:58:27 am
content: One idea I had would be to build a multiphase buck/boost converter, which dumps load energy into a resistor. The load voltage across the resistor would essentially determine load current, which would then determine the load current drawn from the input (multiplied by a ratio.) The problem with this design is it would be slow to deal with changing loads, so could only work for constant loads, and it would introduce a lot more switching noise than a conventional design - this could be filtered out by some L-C networks, but that might change the result of any power supply measurements (ripple voltage etc.)

author: German_EE
author_rank: Super Contributor
timestamp: September 14, 2014, 09:27:23 am
content: You may have a problem with your idea. If working with a 220V supply a 1KW heating element will take 4,54A and have a resistance of 48,4 ohms. If you want a 200W load on a 12V supply then you need to cater for a 16,66A current into a resistance of 0,72 ohms.So........................How are you going to connect your wires to the rest of the circuit? Soldering is out because the heat will desolder the connections. This therefore means either clamps or crimped connections to something that will expand when heated and contract when it cools.How are you going to vary the resistance and maintain a good contact on something that may be red hot?How are you going to deal with the heat? Heatsinking a length of wire may be difficult.

author: nctnico
author_rank: Super Contributor
timestamp: September 14, 2014, 10:34:20 am
content: Quote from: tom66 on September 13, 2014, 06:22:23 pmIt depends on your application. If you want to draw several amps from a PSU to test it, it doesn't matter whether it is 9.9A or 10A. As long as the readout is accurate. A while ago I build a couple of 1.2kW DC loads for one of my customers in order to discharge battery packs. It had to be compact, very rugged (able to run 24/7), cheap and simple. I choose to use resistors which can be switched in parallel to make different values.Recently I picked up my own DC load project. So far I have not been satisfied with using transistors. If you want an isolated heatsink and not use TO3 packages you can dissipate around 30W in a transistor. In order to dissipate 1.2kW you'd need 40 transistors in parallel. I ended up building another 'switched resistor' DC load:The housing is from an old UPS which I acquired decades ago. The heatsink forms an air duct together with the outer shell of the housing. I used 32 resistors which can dissipate 700W each and have a very high pulse handling capacity. 32 low cost Mosfets switch the resistors. A board with an ARM controller does the measurements and handles the user interface on an LCD display. Because the Mosfets switch I also implemented a pulsed load mode where I can program low/high current. The edges are very steep! Actually steep enough to cause spikes due to the wiring inductance; I had to include a 3W zener diode to clamp these. The ARM controller has a table in the software with all the resistor values and maximum power dissipation. Depending on the input voltage and requested current it switches the appropriate Mosfets on. The small circuit board on the left is my universal AC fan controller (which has a SOIC8 MSP430 microcontroller).BTW: Because this is more or less a hobby project I used Mosfets in a TO220 package so I can bolt a wire directly to the tab. In a commercial project I would have used surface mounted Mosfets and a connector so the board can be replaced in 5 minutes.

author: oldway
author_rank: Super Contributor
timestamp: September 14, 2014, 12:38:06 pm
content: I had already thought about this solution for a static load to test batteries or battery banks.For such an application, small changes of current in short times are acceptable.The idea was to greatly reduce the cost of the load dissipating most of the power in cheap resistors.These resistors are easy to do yourself with stainless steel wire 308LSi 0.80mm in diameter.This stainless steel wire is available for MIG welding.The wire must be annealed in order to make it malleable.I wound the wire on the central axis of a DVD's tower (15 mm diameter).The loop ends are formed on a screw 3mm.The tests I've done have shown that we can reasonably use these resistors up to 5A without forced ventilation.The resistance depends on the length of the wire and can thus be adjusted to the desired value.As stainless steel has a positive temperature coefficient, the resistance varies and increases with the heating of the wire.However, this does not matter in the scheme chosen since this change will be offset by the MOSFET in series.

author: oldway
author_rank: Super Contributor
timestamp: September 14, 2014, 12:53:27 pm
content: The project design:From 0 to a selected voltage (e.g. 5V), the electronic charge operates as a normal load. (MOSFET)We must choose the maximum load current as a multiple of 5A as this will determine the number of resistors in parallel to be provided (each resistor can be used up to 5A).Let's choose for example 10A.The maximum power dissipated by the MOSFET will be 50W.When the power dissipated by the MOSFET exceeds 50W, resistors are inserted in series to reduce this power below 50W.Switching resistors is made by relay.Cheap relays are available in high current (40A) for automotives applications.

author: nctnico
author_rank: Super Contributor
timestamp: September 14, 2014, 12:54:41 pm
content: Another way would be a wide input DC-DC converter which dumps energy into a resistor with a transistor or mosfet based linear DC load parallel to it for finer regulation. The planar resistors I used are about €15 each which is a lot of value for money given that they can dissipate 700W into a forced air cooled heatsink. AFAIK Toploser still has some heatsinks for sale which are perfect for this purpose.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 14, 2014, 01:07:38 pm
content: Quote from: nctnico on September 14, 2014, 10:34:20 amI like your project. At least I'm not completely wrong. I'd guess that if you want that .1 amp accuracy you can add a transistor in parallel to servo out the error from the resistor tempco.

author: mzzj
author_rank: Super Contributor
timestamp: September 14, 2014, 01:21:02 pm
content: Quote from: nctnico on September 14, 2014, 12:54:41 pmbetter yet, use grid-tie inverter to "dump" the energy back to the supply

author: DanielS
author_rank: Frequent Contributor
timestamp: September 14, 2014, 03:44:59 pm
content: Quote from: mzzj on September 14, 2014, 01:21:02 pmI had the same idea almost a year ago.With today's fancy multi-phase PWM controllers for PC CPUs, I imagine it would be relatively simple to operate those on per-pulse current-limit, use transformers instead of buck inductors on their output to step load power up to a usable intermediate voltage and then step that up or down to whatever is needed elsewhere instead of wasting all of it as heat.

author: oldway
author_rank: Super Contributor
timestamp: September 14, 2014, 06:41:54 pm
content: it's not worth it.The investment is too high.We do not use an electronic load every day, nor for many hours.In the past, I even did such an analysis for a battery factory (plates formation) and we had arrived at the same conclusion.

author: nctnico
author_rank: Super Contributor
timestamp: September 14, 2014, 08:38:12 pm
content: Same here. Even when my customer runs the DC loads 24/7 it does not pay to invest in dumping the energy back into the mains or use it otherwise.

author: David Hess
author_rank: Super Contributor
timestamp: September 17, 2014, 06:09:33 am
content: The problem with a big heater as a load element is that its impedance will be too high for low voltage applications. You can get around that by using a boost converter to drive the heater but high impedance ratios will require a transformer to keep the duty cycle from getting out of hand.A boost converter could also drive a standard grid tie inverter.

author: mzzj
author_rank: Super Contributor
timestamp: September 17, 2014, 09:23:52 pm
content: Quote from: David Hess on September 17, 2014, 06:09:33 amAnd here we get to the point of having 2 square meters of stainless steel sheet cut to stripsHad to get inventive to test 24v 40kW motor drive for go-cart on a student budget

author: DanielS
author_rank: Frequent Contributor
timestamp: September 17, 2014, 10:08:06 pm
content: Quote from: mzzj on September 17, 2014, 09:23:52 pmA bit like in one of Mike's videos where he needed a low-value, high-power resistor but had nothing adequate on-hand so instead of wasting a fortune putting together off-the-shelf resistors, he simply dumped a bunch of enameled wire in a water bucket.

author: David Hess
author_rank: Super Contributor
timestamp: September 19, 2014, 07:04:04 pm
content: I have a couple small spools of nickel-chrome wire for this sort of thing but I have used stainless steel strips before or even copper tubing. The two problems I have had winding my own power resistors are that nickel-chrome is very difficult to solder and when cut, you get a very sharp springy edge which is perfect for puncturing fingers.

author: blackdog
author_rank: Frequent Contributor
timestamp: September 24, 2014, 11:26:23 am
content: Hi ntcnico,Can you tel me were you buy these resistos you use for your load?Kind regarts,Blackodg

author: nctnico
author_rank: Super Contributor
timestamp: September 24, 2014, 03:33:16 pm
content: Farnell has them. There are also Ebay sellers which drop-ship them from Farnell. I used the WDBR1 type resistors in my DC load.This is the datasheet with various sizes and values:http://technology.ttelectronics.com/images/uploads/productdatasheets/WDBR.PDF