load bank puzzle

I've been asked to design an 8-channel dummy load board. It's not very challenging but somebody's got to do it. It will be used to simulate small loads like solenoids or relays or torque motors. It needs some inductance too, because the drivers often PWM. Maybe 10 or so watts per channel.

I could do this electronically, but it would a lot easier and more rugged if I use wirewound resistors. I was thinking of making a conductance DAC, namely resistors R 2R 4R etc switched in parallel across the inputs with an SSR per resistor.

But there is a history of clever load banks. When I was an EE student at Tulane, two semisters of Electrical Machinery (with lab) was mandatory. It was a pain but I learned a lot. We had a big load bank in the machinery lab, a string of giant series resistors with a

3-position knife switch at each node. That made me think about using series-parallel combinations to hit some target value.

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The upper pic is the ancient Tulane load bank as I remember it.

I was thinking about the lower circuit for my gadget. I could use wirewould resistors and kink the leads to space them maybe 3/4 inch above my PCB, in the air stream. The higher value resistors might be

2512 surface mounts.

I think there is a tool to bend and kink resistor leads. Or we could send a bunch out to a service maybe.

Reply to
John Larkin
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Plenty of power rheostats available. eBay has loads, many new. Inductance is a separate matter (though they will have some of course - get tubular rheostats and bung a ferrite rod down the centre?).

Reply to
Clive Arthur

We want 8 channels on a PC board, programmable resistance.

Reply to
John Larkin

So far, a Variac and big resistor on the output is a possibility. Or, a few paint cans full of oil, with rotary switch and immersed lower-watt resistors (Cantenna style), could work.

But, 'load' for even just a battery test can be anything from 1 ohm to hundreds, just for the batteries in my household; the only easy way to make the load you want, is to specify it first, then build one. That means the 'load board' is a connector plenum, and the loads would be plugins to that, plucked from a bucket of prebuilt options.

Eight-channel is easy: design one, build eight, and strap 'em together.

Reply to
whit3rd

fredag den 17. marts 2023 kl. 17.58.53 UTC+1 skrev John Larkin:

8*10W is going to require quite a bit of cooling

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;)

Reply to
Lasse Langwadt Christensen

I was thinking of kink-leading some axial wirewound resisors like they do. We'll have a lot of air flow so the cooling should be OK. Wirewound resistors can get very hot.

Reply to
John Larkin

Not on a PC board. Variacs are AC devices anyhow. Big and expensive.

This will be a product, a plugin board to a rackmount instrument.

Reply to
John Larkin

Board assembly houses should have the lead crimpers. I believe it only works with tape reel packaging. Could be wrong but check with the vendor.

Cheers

Reply to
Martin Rid

Active I=kV with a delay would give you an inductance.

Does the driver really care?

Where'd the word 'dummy' originate in the spec? If it reflects the attitude of the buyer, I'd say 'No Bid'. I've had it with idiots like that.

RL

Reply to
legg

A real inductor stores energy, which a synthesized inductor usually doesn't.

Stored energy pumps current into flyback diodes or equivalent.

It certainly might.

Oh, they are just a biggish aerospace company.

No bid? Is that a good business model?

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Reply to
John Larkin

fredag den 17. marts 2023 kl. 22.13.01 UTC+1 skrev legg:

sure it can slow the current rise like an inductor but it won't keep the current flowing after turnoff

Reply to
Lasse Langwadt Christensen

sometimes it is.

Reply to
Tabby

Not good idea. Rheostats get hot. and ferrite rods tend to have a low Curie temperature.

Manganese-zinc ferrites have Curie temperatures from 120°C – 300°C.

Reply to
Anthony William Sloman

"Dummy load" seems a terminology common enough not to suggest anything about the buyer.

Sylvia.

Reply to
Sylvia Else

A dummy load is usually invariable and requires no operator adjustment.

A 'dummy' can screw up even simple switches or rheostats.

RL

Reply to
legg

The minute you considered variable, multicase and especially inductive, it ceased to be idiot-proof.

Use sockets and real loads, if it's that important.

RL

Reply to
legg

Then offer them burn-in hardware as well as 'dummy' loads.

Reply to
legg

If your power source fits the obvious constraints, an SMPS with a fixed load resistor and a programmable output voltage could work.

Reply to
Clive Arthur

The customer has an existing design where they select and solder a selected set of resistors and inductors per channel, unique to each unit. We don't want to be in the business of doing that for them, with every unit having its own dash number and BOM and test limits.

I'd like to design a programmable dummy load board that we can manufacture and stock and ship when we get an order.

It's looking like a straight conductance DAC is the way to go: parallel N resistors (R, 2R, etc) with a solid-state switch per. Given a binary control code K, net conductance is proportional to K so resistance goes as 1/K. N=5 maybe; we're not simulating RTDs. That's nice and simple and adds an open-circuit case for free. Two more SSRs can add short and ground fault cases, selling points.

I like to add little goodies to products when it's not hard and doesn't interfere with the base function. You never know if something will appeal to someone and tip a basically emotional buy decision, as in "That thingie might be useful some day, let's buy theirs."

Colors matter too.

Reply to
John Larkin

Why? A programmable dummy load can be very useful.

They could wire anything wrong.

A good dummy load includes measurements and waveform acquisition and overload protections. If you see 17 volts and ask for 130 ohms and don't get 130 mA, you should know that something's wrong.

The load is a dummy but my customers aren't. You trust your life to them fairly often.

Reply to
John Larkin

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