Help building a dummy load - Page 1
Forum section: electronics / beginners

author: akabek
author_rank: Contributor
timestamp: March 17, 2021, 06:50:50 pm
content: I am in the process of building a dummy load that has banana jack inputs, 2 x 200W 8ohm resistors/channel switchable for 4 or 8 Ohm, 250mA fuse, BNC output for scope or meter and a x1/x10 switch with a trim pot to fine adjust the x10 voltage. I am connect the wiring for the right side and once it is correct I will replicate it on the left side. I have tried to wire it up a couple of times but when I run a 5 dcv to the banana jack and connect my volt meter to the same banana jack the DC power supplies drops to .28dcv and it cannot be adjusted.I would like to take a step back and start over with the resistor wiring. When I disconnect everything and check the resistance of each resistor I see around 8.5 ohms. When I wire up both resistors with the 4/8ohm selector switch is there a way to measure 4 or 8 ohms with the meter depending on the switch position?

author: james_s
author_rank: Super Contributor
timestamp: March 17, 2021, 06:54:15 pm
content: I'm not sure I understand the question. If you can measure the resistors out of circuit you should be able to measure them in circuit. With the switch open you've just got one resistor across the wires into the thing. Are you sure it's wired correctly?Edit: Looking at the picture it *looks* correct. What are you getting when you try to measure resistance? What current is your power supply rated for? 5V into 8 Ohms will draw 625mA, and 4 Ohms will be over 1.3A.

author: akabek
author_rank: Contributor
timestamp: March 17, 2021, 07:20:57 pm
content: Quote from: james_s on March 17, 2021, 06:54:15 pmI am going to start over with the wiring and only include the positive and negative input banana jacks, both 8ohm 200w resistors and the 4/8 ohm 1P2T selector switch. For now disregard a power supply. My question is if I connect these items up should I be able to measure either 4 or 8 ohms with a meter depending on the selector switch position? This may be a dumb question but I just want to make sure the result I get is correct.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: March 17, 2021, 07:36:52 pm
content: Quote from: akabek on March 17, 2021, 07:20:57 pmYou should be able to get a reading accurate enough to tell you it's wired up correctly. Have you tried testing the resistor value with the banana plugs? That should read either 8-4 ohms depending on the switch position.

author: akabek
author_rank: Contributor
timestamp: March 17, 2021, 10:52:20 pm
content: I made some progress but something odd is going one. I wired up 2 resistors, 4/8 ohm switch and the negative banana terminal as per the diagram. When I connect the meter to the black banana terminal and the red lead to the red R1 terminal I can flip the switch and get either 4 or 8 ohms. If I try to connect a wire from the same red R1 terminal to the red banana terminal I only get a 0.15ohm reading and the ohm switch does not work.Attached is a pic of the red wire I disconnected from R1 to get the proper ohm readings.Any thoughts?

author: srb1954
author_rank: Super Contributor
timestamp: March 18, 2021, 01:10:15 am
content: Quote from: akabek on March 17, 2021, 06:50:50 pmYou are going to have put those resistors on a really big heatsink if you expect to run them at anywhere near their rated power. Check the resistor datasheet for the recommended heatsink configuration.Without substantial heatsinking those resistors are probably only good for 20% of the power rating stamped on them, possibly less considering their close proximity and the enclosed cabinet.

author: akabek
author_rank: Contributor
timestamp: March 18, 2021, 02:02:51 am
content: Quote from: srb1954 on March 18, 2021, 01:10:15 amI won't be working on high power amps. Just low power solid state. I will keep an eye on temperature and modify it if need be.

author: akabek
author_rank: Contributor
timestamp: March 18, 2021, 02:07:25 am
content: When I wire everything up as per the diagram and attach the meter to the inputs I only see 0.15ohms and the selector switch does not effect the reading.

author: akabek
author_rank: Contributor
timestamp: March 18, 2021, 02:12:21 am
content: If I disconnect the red input from R1 and connect the meter to the red and black points in the pic I get 4 and 8 ohms readings using the switch. Should I be able to get 4/8 ohm readings directly measured from the inputs?

author: james_s
author_rank: Super Contributor
timestamp: March 18, 2021, 02:40:07 am
content: I think you need to take a step back and spend some time studying the most basic concepts of series and parallel circuits and then you will understand exactly what is going on here.In your last drawing only one side of the input is even connected to anything, how can you expect that to work? You need a complete circuit. I mean this is not rocket science, it's first week electricity 101 stuff. You're going to need to understand it intuitively to get anywhere.

author: beanflying
author_rank: Super Contributor
timestamp: March 18, 2021, 03:04:15 am
content: For an alternate idea here is my Dummy Load / Low Ohm decade. Consists of 1, 2, 3 and 450W resistors. In this configuration with the fan running they cope fine at full load. Without a heatsink of any sort you will be maxed out at more like 25% of rated power before you get into heating trouble. At a minimum open the case up for airflow and fit a fan!No switches involved as at high power and currents they are the weak point. Side to Side as shown the Resistance is Effectively 2 x 5resistors in parallel consisting of a 2 + 3 and a 4+1 string. To get 10I can simply break either of the center links.Internally there is a fan that draws air into the enclosure then pumps the heated air back out of the stack.

author: beanflying
author_rank: Super Contributor
timestamp: March 18, 2021, 03:19:02 am
content: So using the power Dave/Clive CAD you need to wire this configuration. Some of the wiring shown remains the same but for the 4configuration you need to switch in two links and change the tap point for your load to the middle of your stack.WHY do this? It uses ALL the resistors and spreads the load/energy across them which will help heat dissipation even in your hot box.

author: akabek
author_rank: Contributor
timestamp: March 18, 2021, 03:31:38 am
content: The culprit has been found. I was getting correct readings when the positive terminal was disconnected. I removed the positive banana jack, drilled out a larger whole in the enclosure, put a small piece of shrink wrap on the banana jack shaft and reinstalled it. The banana jack shaft was touching the enclosure causing a short. It is now reading the correct ohms when the meter is connected to the inputs. Next step I will connect the fuse, bnc, and x1/x10 switch and trim pot.I won't be repairing high power equipment but I will definitely add some heat sinks once I review the resistor spec sheet.

author: beanflying
author_rank: Super Contributor
timestamp: March 18, 2021, 04:40:23 am
content: As others have suggested you need to look seriously at your initial wiring diagram and overall concept. I cant see for example that the fourth resistor even gets used in the circuit?The Banana Jack shaft should not be in contact with the case and unless it is a speaker type designed for a wooden enclosure it should have come with plastic spacers to stop that from happening.Several below for examples. The lefthand one does not have an insulated post (I use them for chassis ground connections) but the rest have two piece plastic parts to isolate the shaft from the chassis. The drawn in lines show the breaks between the isolators and they have an internal sleeve in that area not visible.

author: Damianos
author_rank: Frequent Contributor
timestamp: March 18, 2021, 05:48:40 am
content: Quote from: akabek on March 18, 2021, 03:31:38 amThis means either that the terminals are of bad quality or you have assembled them wrongly.One more note: the black terminal is connected to the chassis, by itself (as it is now) and also via the BNC connector. So it's not possible to use the device as a floating load, for example on a bridge configured amplifier. This is a connection between the two channels but, even if it is used only one channel, it's not a good idea to have the output of an amplifier connected to a metal box, that anything can touch it. Additionally, if a grounded instrument is connected to the BNC, it connects also this terminal to ground. Study the consequences of this matter on how a DUT or an instrument can be damaged!

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 18, 2021, 06:14:01 am
content: Quote from: akabek on March 17, 2021, 10:52:20 pmSomething is wrong. You should be able to get 4/8 ohm readings directly measured from the inputs.

author: magic
author_rank: Super Contributor
timestamp: March 18, 2021, 08:30:06 am
content: Quote from: beanflying on March 18, 2021, 03:19:02 amFTFY

author: beanflying
author_rank: Super Contributor
timestamp: March 18, 2021, 08:41:31 am
content: Quote from: magic on March 18, 2021, 08:30:06 amI blame the painkillers I am on at least that is my excuse and I will stick to itVersion II below because Codeine and Beer are helping ....

author: bobbydazzler
author_rank: Regular Contributor
timestamp: March 18, 2021, 08:52:27 am
content: That case for the resistors needs top rear vents and bottom vents so it can dissipate heat. Those resistors look pretty robust and maybe they could dissipate 200w on an open bench by themselves but that case can't handle 200w let alone 800w(unless you want it to burn you and warp the plastic wiring) without proper venting and preferably a fan(probably some industrial type fan I'm not sure the nice quiet desktop pc fans can handle the high temps).

author: akabek
author_rank: Contributor
timestamp: March 18, 2021, 11:09:09 am
content: The wiring diagram is in the first post. It represents one channel. I will duplicate it for both the left and again for the right.Right now I just want to connect the 2 resistors on the right channel with the ohm switch. Than I would sort out the fuse, trim pot, bnc and x1/x10 switch. That completes the right circuit. This would be duplicate for the left channel that includes the 3rd and 4th resistor.Mounting the input jacks was my error. The banana posts do have plastic parts to isolate from the chassis. I am going to drill out the banana jack holes so they are much larger and make sure the input shaft does not contact with the chassis.I will definitely look into heat sinks, a fan or maybe an open design.