On Sun, 19 Mar 2023 05:49:06 -0700 (PDT), Fred Bloggs <bloggs.fredbloggs.fred@gmail.com> wrote:>On Saturday, March 18, 2023 at 6:57:33?PM UTC-4, John Larkin wrote: >> On Sat, 18 Mar 2023 15:23:20 -0700 (PDT), Fred Bloggs >> <bloggs.fred...@gmail.com> wrote: >> >On Saturday, March 18, 2023 at 1:48:27?PM UTC-4, John Larkin wrote: >> >> On Sat, 18 Mar 2023 07:44:01 -0700 (PDT), Fred Bloggs >> >> <bloggs.fred...@gmail.com> wrote: >> >> >On Friday, March 17, 2023 at 11:21:37?AM UTC-4, John Larkin wrote: >> >> >> 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. >> >> >> >> >> >> https://www.dropbox.com/s/x3e7xxi13n6wd1o/Load_Banks_1.jpg?raw=1 >> >> >> >> >> >> 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. >> >> > >> >> >It's called standoff lead forming. There are bunches different tools, but it looks like the handheld plier types are more universal. Low cost too. >> >> > >> >> >https://forum.digikey.com/t/bending-and-forming-leads/8184 >> >> > >> >> >How is any of this a puzzle? >> >> A good design is most always a puzzle, which makes design interesting. >> >> >> >> One issue is, how much power can you dissipate in 4 square inches of >> >> PC board with some given air flow? Big heat sink with mosfets? Small >> >> CPU cooler? I'm thinking of using axial-lead wirewound resistors >> >> spaced off the board enough that the leads have enough thermal >> >> resistance to not scorch the board when the resistor gets very hot. >> > >> >Keep in mind people tend to lose respect for a product that smells like it's burning. You should be determining the threshold case temperature at which outgassing becomes noticeable, via olfactory sensing, and see if reducing temps by half, with proper derating for ambient, does anything for you. >> Vitreous enameled resistors would be best for not outgassing. Silicone >> or whatever would be worse. >> >> Wirewound resistors tend to run at crazy temps, like 270C, at full >> power. I wouldn't want to scorch the PCB. >> >> There are some nice DPAK power resistors that could be clamp mounted >> on heat sinks, which would provide a lot of surface area. >> >> There are also TO-220 power resistors, which could be attached to heat >> sinks with nuts and bolts. > >Setting off smoke detectors would be the absolute worst.I plan to digitize actual load voltage and current on every channel. An FPGA can calculate power dissipation and protect the parts. A polyfuse might be prudent too.
load bank puzzle
Started by ●March 17, 2023
Reply by ●March 19, 20232023-03-19
Reply by ●March 19, 20232023-03-19
On 17/03/2023 17:22, John Larkin wrote:> > I was wondering if anyone had cute ideas. I considered PWM-ing a big > resistor, but that has complications. >Aha, yes I recall offering you that for a similar task in Aug 2016 piglet
Reply by ●March 19, 20232023-03-19
On 19/03/2023 15:24, John Larkin wrote:> > There are some very cool 2 and 3-channel isolated ADCs intended for > use in electric meters. Cheap but not super fast; good for average > measurements but not for closing fast control loops. >So use the cool ADCs for metrology and do the control loop with op-amp? piglet
Reply by ●March 19, 20232023-03-19
søndag den 19. marts 2023 kl. 16.13.07 UTC+1 skrev John Larkin:> On Sun, 19 Mar 2023 09:55:06 -0400, legg <le...@nospam.magma.ca> wrote: > > >On Sat, 18 Mar 2023 08:11:29 -0700, John Larkin > ><jla...@highlandSNIPMEtechnology.com> wrote: > > > >>On Sat, 18 Mar 2023 07:13:33 -0400, legg <le...@nospam.magma.ca> wrote: > >> > >>>On Fri, 17 Mar 2023 14:41:51 -0700, John Larkin > >>><jla...@highlandSNIPMEtechnology.com> wrote: > >>> > >>>>On Fri, 17 Mar 2023 17:14:59 -0400, legg <le...@nospam.magma.ca> wrote: > >>>> > >>>>>On Fri, 17 Mar 2023 08:21:23 -0700, John Larkin > >>>>><jla...@highlandSNIPMEtechnology.com> wrote: > >>>>> > >>>>>> > >>>>>>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. > >>>>>> > >>>>>>https://www.dropbox.com/s/x3e7xxi13n6wd1o/Load_Banks_1.jpg?raw=1 > >>>>>> > >>>>>>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. > >>>>>> > >>>>>> > >>>>>> > >>>>> > >>>>>Active I=kV with a delay would give you an inductance. > >>>> > >>>> > >>>>A real inductor stores energy, which a synthesized inductor usually > >>>>doesn't. > >>>> > >>>>Stored energy pumps current into flyback diodes or equivalent. > >>>> > >>>>> > >>>>>Does the driver really care? > >>>> > >>>>It certainly might. > >>>> > >>>>> > >>>>>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. > >>>> > >>>>Oh, they are just a biggish aerospace company. > >>>> > >>>>No bid? Is that a good business model? > >>>> > >>>>> > >>>>>RL > >>>> > >>>>https://en.wikipedia.org/wiki/Dummy_load > >>>> > >>> > >>>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 > >> > >>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. > >> > > > >Overv time you've talked about all sorts of low voltage loads > >that swotted the heat in mosfets and a heatsink. > > > >Doesn't that have a placxe here? > > > >RL > I'm planning a separate big single-channel electronic load board that > uses mosfets on a CPU cooler and can do programmed constant-current > and constant-resistance ac or dc over a huge range. That topology > would be complex for a small 8-channel load that has onboard > inductors. The mosfet thing needs isolated fast DACs and ADCs per > channel and closed loop control in an FPGA. Resistors and SSR are > sure easy. > > A CPU cooler is a possibility on the 8-channel board. A K199 type > would fit on my board and can easily dissipate 150 watts. The bottom > copper plate would need to be tapped to screw down a bunch of > (expensive, unavailable) TO-220 resistors or something. > > High thermal conductivity (ie, fairly pure) copper is gummy and a > nuisance to tap, but it can be done. That would roughly double the > cost of the cooler. At least TO-220 resistors don't need insulators > like mosfets would.how much inductance do you need? that could take up quite a bit of space too
Reply by ●March 19, 20232023-03-19
On Sun, 19 Mar 2023 17:24:32 +0000, piglet <erichpwagner@hotmail.com> wrote:>On 17/03/2023 17:22, John Larkin wrote: >> >> I was wondering if anyone had cute ideas. I considered PWM-ing a big >> resistor, but that has complications. >> > >Aha, yes I recall offering you that for a similar task in Aug 2016 > >pigletIf a customer might PWM his driver, with maybe flyback diodes or mosfet avalanche or something, the best dummy load is a resistor in series with an inductor. If he PWMs and we PWM it gets hard to think about. PWMing a resistor is fine for DC loads.
Reply by ●March 19, 20232023-03-19
On Sun, 19 Mar 2023 10:39:00 -0700 (PDT), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>s�ndag den 19. marts 2023 kl. 16.13.07 UTC+1 skrev John Larkin: >> On Sun, 19 Mar 2023 09:55:06 -0400, legg <le...@nospam.magma.ca> wrote: >> >> >On Sat, 18 Mar 2023 08:11:29 -0700, John Larkin >> ><jla...@highlandSNIPMEtechnology.com> wrote: >> > >> >>On Sat, 18 Mar 2023 07:13:33 -0400, legg <le...@nospam.magma.ca> wrote: >> >> >> >>>On Fri, 17 Mar 2023 14:41:51 -0700, John Larkin >> >>><jla...@highlandSNIPMEtechnology.com> wrote: >> >>> >> >>>>On Fri, 17 Mar 2023 17:14:59 -0400, legg <le...@nospam.magma.ca> wrote: >> >>>> >> >>>>>On Fri, 17 Mar 2023 08:21:23 -0700, John Larkin >> >>>>><jla...@highlandSNIPMEtechnology.com> wrote: >> >>>>> >> >>>>>> >> >>>>>>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. >> >>>>>> >> >>>>>>https://www.dropbox.com/s/x3e7xxi13n6wd1o/Load_Banks_1.jpg?raw=1 >> >>>>>> >> >>>>>>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. >> >>>>>> >> >>>>>> >> >>>>>> >> >>>>> >> >>>>>Active I=kV with a delay would give you an inductance. >> >>>> >> >>>> >> >>>>A real inductor stores energy, which a synthesized inductor usually >> >>>>doesn't. >> >>>> >> >>>>Stored energy pumps current into flyback diodes or equivalent. >> >>>> >> >>>>> >> >>>>>Does the driver really care? >> >>>> >> >>>>It certainly might. >> >>>> >> >>>>> >> >>>>>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. >> >>>> >> >>>>Oh, they are just a biggish aerospace company. >> >>>> >> >>>>No bid? Is that a good business model? >> >>>> >> >>>>> >> >>>>>RL >> >>>> >> >>>>https://en.wikipedia.org/wiki/Dummy_load >> >>>> >> >>> >> >>>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 >> >> >> >>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. >> >> >> > >> >Overv time you've talked about all sorts of low voltage loads >> >that swotted the heat in mosfets and a heatsink. >> > >> >Doesn't that have a placxe here? >> > >> >RL >> I'm planning a separate big single-channel electronic load board that >> uses mosfets on a CPU cooler and can do programmed constant-current >> and constant-resistance ac or dc over a huge range. That topology >> would be complex for a small 8-channel load that has onboard >> inductors. The mosfet thing needs isolated fast DACs and ADCs per >> channel and closed loop control in an FPGA. Resistors and SSR are >> sure easy. >> >> A CPU cooler is a possibility on the 8-channel board. A K199 type >> would fit on my board and can easily dissipate 150 watts. The bottom >> copper plate would need to be tapped to screw down a bunch of >> (expensive, unavailable) TO-220 resistors or something. >> >> High thermal conductivity (ie, fairly pure) copper is gummy and a >> nuisance to tap, but it can be done. That would roughly double the >> cost of the cooler. At least TO-220 resistors don't need insulators >> like mosfets would. > >how much inductance do you need? that could take up quite a bit of space tooI'm estimating that 5 or 10 mH would keep their PWM driver happy. A real solenoid or torque motor could be henries. In that case, they'd have to add it externally.
Reply by ●March 19, 20232023-03-19
On Sun, 19 Mar 2023 17:28:35 +0000, piglet <erichpwagner@hotmail.com> wrote:>On 19/03/2023 15:24, John Larkin wrote: >> >> There are some very cool 2 and 3-channel isolated ADCs intended for >> use in electric meters. Cheap but not super fast; good for average >> measurements but not for closing fast control loops. >> > >So use the cool ADCs for metrology and do the control loop with op-amp? > >piglet > >Simulating a resistor needs multiplication. That could be done with an MDAC but gets complicated.
Reply by ●March 19, 20232023-03-19
On 19/03/2023 18:29, John Larkin wrote:> On Sun, 19 Mar 2023 17:28:35 +0000, piglet <erichpwagner@hotmail.com> > wrote: > >> On 19/03/2023 15:24, John Larkin wrote: >>> >>> There are some very cool 2 and 3-channel isolated ADCs intended for >>> use in electric meters. Cheap but not super fast; good for average >>> measurements but not for closing fast control loops. >>> >> >> So use the cool ADCs for metrology and do the control loop with op-amp? >> >> piglet >> >> > > Simulating a resistor needs multiplication. That could be done with an > MDAC but gets complicated. >Digital pot or PWM multiplier could do I guess, I have never used FPGAs so still go first to analog ways! piglet
Reply by ●March 19, 20232023-03-19
On Sun, 19 Mar 2023 19:22:18 +0000, piglet <erichpwagner@hotmail.com> wrote:>On 19/03/2023 18:29, John Larkin wrote: >> On Sun, 19 Mar 2023 17:28:35 +0000, piglet <erichpwagner@hotmail.com> >> wrote: >> >>> On 19/03/2023 15:24, John Larkin wrote: >>>> >>>> There are some very cool 2 and 3-channel isolated ADCs intended for >>>> use in electric meters. Cheap but not super fast; good for average >>>> measurements but not for closing fast control loops. >>>> >>> >>> So use the cool ADCs for metrology and do the control loop with op-amp? >>> >>> piglet >>> >>> >> >> Simulating a resistor needs multiplication. That could be done with an >> MDAC but gets complicated. >> > >Digital pot or PWM multiplier could do I guess, I have never used FPGAs >so still go first to analog ways! > >pigletA dpot could do the multiply well enough, at least over a moderate signal range. A dpot as the actual load resistor wouldn't work well. FPGAs are liberating in that you can do very complex signal processing and control loops basically for free. As long as you get someone else to grunt out the code.
Reply by ●March 20, 20232023-03-20
On 2023-03-19, John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:> On Sun, 19 Mar 2023 17:28:35 +0000, piglet <erichpwagner@hotmail.com> > wrote: > >>On 19/03/2023 15:24, John Larkin wrote: >>> >>> There are some very cool 2 and 3-channel isolated ADCs intended for >>> use in electric meters. Cheap but not super fast; good for average >>> measurements but not for closing fast control loops. >>> >> >>So use the cool ADCs for metrology and do the control loop with op-amp? >> >>piglet >> >> > > Simulating a resistor needs multiplication. That could be done with an > MDAC but gets complicated.Could be done with a digital pot, ah yeah, that's a type of MDAC I guess. -- Jasen. 🇺🇦 Слава Україні
