Induction brass annealer redux

[BillK55]

They cost about $2 each.

That sounds like a reasonably inexpensive upgrade that would go a long way in making it a more robust design. Cut down on a lot of the heat. I hope the leads match up. BTW I don't get to say robust very often, so when I do I sound like the guy that used to do the old Dos Equis beer commercials. Ya know the "stay thirsty my friends" guy. Its not that I drink their beer, I just liked the commercial.

 

[BillK55]

MOSFET part number

I'm not necessarily saying put in a higher rated Zener, but that maybe we don't need as high a voltage PS. Won't the Zener just drain off the excess voltage as heat?

https://www.digchip.com/datasheets/parts/datasheet/139/1N4742-pdf.php

Looks like IN4742 from your attached drawing is rated at 12v. I don't have a board to look at. Anyone read that off their board to confirm? If that's right why are we using 48V PS's?

 

[GrocMax]

Skin depth, frequency, and material.

 

[SGK]

I'm not necessarily saying put in a higher rated Zener, but that maybe we don't need as high a voltage PS. Won't the Zener just drain off the excess voltage as heat?

https://www.digchip.com/datasheets/parts/datasheet/139/1N4742-pdf.php

Looks like IN4742 from your attached drawing is rated at 12v. I don't have a board to look at. Anyone read that off their board to confirm? If that's right why are we using 48V PS's?

We want power in the tank circuit. You need a pretty good voltage. Not to say you couldn't use a lower voltage, you'd just need more current or more time (or both) to get the job done.

You need to spend some time understanding MOSFETs, how they work and their key parameters. The gate of a MOSFET typically can only handle a much lower voltage than can be across the drain to source (Vdss). In this application the sources are held at GND (see schematic). From page 1 of the IRFP260 data sheet https://www.mouser.com/datasheet/2/196/irfp260mpbf-1228372.pdf we see the the absolute maximum Vgs for this device is 20V. Vdss on the other hand can be as high as 200V. Since the gates are being powered by our main supply (which is meant to be 48V) we need to lower the voltage at the gate. The easiest way to do that is with a Zener diode. 12V is well below the maximum and well above the minimum needed to fully turn the MOSFET on Vgs(th). If you wanted to, you could power the MOSFET gates from another supply e..g. the 12V supply and not need the Zener. But the board assumes people have only one supply available to them and that this feeds the MOSFET drains with much more than 20V for plenty of power through the tank.

How much current flows in the gate of a MOSFET? Make sure you know the right answer to that and then consider just how much the 12V zener needs to dissipate.

You're not going to find a part number written on that 3-4mm long part.

 

[SGK]

Skin depth, frequency, and material.

https://owenduffy.net/calc/SkinDepth.htm

Has the relevant variables for brass

How to apply it to annealing brass? No idea!

 

[Gina1]

Indeed!


One thing that is bugging me having spent a bomb for this Mean Well RSP750 48V PSU is the voltage sag under load. I mean, at 12A it can only produce c37V - a mere 450W and a far cry from its rated 750W!

Gina, when you are doing those five and a half second anneals what do you see on your volt/ammeter? Maybe the PSU takes awhile to react to the transient load.

I'm using the Mean Well SE-600-48 power supply. No remote controls (constant voltage, only) I have the output set to about 43.2 volts. This gives me my near max current (12-12.5 amps) for my 6BR dasher cases, in the correct depth, in the coil.
I did measure the voltages while annealing. The 48V (43.3) PS never changed voltage and stayed at 43.2 volts (full 12-12.5 amps). I did drop half a volt, through the wiring from the PS, through the inductor relay to the inductor PCB. (42.7 volts at the PCB terminals) although I'm using 12 gauge wire.
If your using the RSP750 you may what to rethink you constant current options, as it sounds like it's dropping the voltage to maintain the constant current. Just a thought.

 

[BillK55]

we need to lower the voltage at the gate.

Ok I see it now, its only the gate voltage we're dropping. Thanks

 

[SGK]

I'm using the Mean Well SE-600-48 power supply. No remote controls (constant voltage, only) I have the output set to about 43.2 volts. This gives me my near max current (12-12.5 amps) for my 6BR dasher cases, in the correct depth, in the coil.
I did measure the voltages while annealing. The 48V (43.3) PS never changed voltage and stayed at 43.2 volts (full 12-12.5 amps). I did drop half a volt, through the wiring from the PS, through the inductor relay to the inductor PCB. (42.7 volts at the PCB terminals) although I'm using 12 gauge wire.
If your using the RSP750 you may what to rethink you constant current options, as it sounds like it's dropping the voltage to maintain the constant current. Just a thought.

Yeah but the thing is 12 x 48V = 576W nowhere near the 750W rating. This thing is meant to be good for 15.7A at 48V. I'm getting about 12 x 38V = 456W. With no case in coil I get 48V for at least 13A (haven't tried loosening the limiter beyond that) 624W. With a case in the coil the voltage drops way off. There's something I'm not understanding and I have the feeling it is going to be one of those 'duh' moments.


Ugh...I must have cocked up the current limiting splice or something . I just smoked a piece of brass at 48V and 15.7A by turning the pot fully clockwise. (Now I see that annealing line on the brass and the change in colour of the Tempilaq!) That was after turning it fully anticlockwise and getting an even lower voltage. Something's wrong and I have a lot of disassembly to check it. Maybe I spliced fn pin 5 by mistake. Nah can't be that else surely it would show a lower voltage unloaded (rather than 48V). I hope I don't have a faulty PSU. Or maybe I do hope so - this thing is noisy as all get out.

 

[SGK]

At the moment BOTH current and voltage are being limited under load. Turn the limiter pot clockwise and voltage and current are allowed to rise. Weird.

I also find it odd, despite the note in @GrocMax 's wiring diagramme, that the control voltage is applied to the PSU pin without a reference. GND on the 48V PSU is floating versus all other components in the system (except the induction board). Surely there needs to be a GND connection between the 48V PSU and that which is providing the control voltage.

 

[SGK]

Current limiting demo, big 1/2" diameter steel rod and a screwdriver. Starts at Switch position 7 (15.7A) and drops down to position 1 (9.5A) in more or less 1A steps. Note current is limited by switch, voltage is capped by pot on the PS to like 48.3v. If the current limit is reached, voltage drops to maintain current. Voltage got low on last position so swapped over to a screwdriver.

Makes it easy to set up for different cases or operations like brazing tubing, and difficult to damage either the induction board or PS, regardless of what goes inside the coil while its operating.

I see @GrocMax 's video demonstrating the very same problem.

The current limiter should not drop voltage when the W are within spec. Towards the end when current is limited to a mere 9.5A, the voltage is under 10V. Less than 100W is being delivered. Voltage should only drop if the power of the supply is about to be exceeded (in fact the PSU limits at 105-125% of rated power by limiting current). A constant current implementation which dramatically starves the load of voltage as well is rather useless. Yes, we want to limit power but at the set voltage of 48V* x current limit.

I checked my wiring and am applying a voltage to pin 7. However, that voltage is referenced to the 5V supply on my control board which in turn is fed 12V from the 12V PSU. These latter two have a common ground. The 48V PSU ground is floating versus these.

Either this PSU current limiting feature is very weird or we haven't got the wiring right.

* Or however trimmed or remote programmed.

 

[GrocMax]

At the moment BOTH current and voltage are being limited under load. Turn the limiter pot clockwise and voltage and current are allowed to rise. Weird.

I also find it odd, despite the note in @GrocMax 's wiring diagramme, that the control voltage is applied to the PSU pin without a reference. GND on the 48V PSU is floating versus all other components in the system (except the induction board). Surely there needs to be a GND connection between the 48V PSU and that which is providing the control voltage.

No, makes it not work.

 

[SGK]

I see CN50 pins 10 and 11 are connected to the 48V PSU V- terminal (GND).

I have sent a note to Mean Well technical support. I can't believe this is their intended operation of current limiting. Who knows if they will respond however.

This smells of a grounding issue.

 

[SGK]

Ugh I think the penny just dropped. If a case in the coil (together with all the other 'leakage' loads in the system e.g. wiring losses, cap leakages, MOSFET RdsOn etc) represents a fixed load - think of it as a big resistor - then as we limit current more the voltage potential ahead of the load also necessarily falls since the drop over the load (I x RL = V) falls. (And the other side if the load is always at GND.) My turn to wear the dunce hat I think.

 

[kit-808]

Well spent 4 hours wiring up my build, connected everything, it all worked, the passive sensor worked, the timer worked, the volt and amp readout worked, thought ok, lets try it.
put a screw driver in the coil to load it up and wham, ssr sticks closed on the induction side and starts cooking everything

So no idea what i did wrong, tried another ssr and the same thing

anyone else had this happen??

on another note, anyone using the 1800w zvs unit ??

 

[dskogman]

What do you mean sticks, stays on after on signal is stopped ? How are you controlling the SSR, timer or PLC/Arduino? Is the SSR connected properly (pos/neg and signal and load posts)?

 

[SGK]

And which SSR (what’s it ratings)? Also, you current limiting? You can’t just stuff a screwdriver (a massive load) in the coil without a current limiter protecting your supply and switch.

 

[kit-808]

And which SSR (what’s it ratings)? Also, you current limiting? You can’t just stuff a screwdriver (a massive load) in the coil without a current limiter protecting your supply and switch.

yer, thinking about i did get a little carried away using 40amp ssr's so should get some sort of circuit protection

 

[SGK]

But for the SSR to latch you either have it wired incorrectly or you fused the switch with too much load.

It worked ok for a regular load? The SSR unlatches when power is disconnected or it is fried closed? Very easy for the load to demand 40A if it's a screwdriver.

 

[kit-808]

But for the SSR to latch you either have it wired incorrectly or you fused the switch with too much load.

It worked ok for a regular load? The SSR unlatches when power is disconnected or it is fried closed? Very easy for the load to demand 40A if it's a screwdriver.

Thanks for the kick in the butt guys

went out this morning and changed over the ssr (had a couple of 10amps) and it all works again, however have 17amps with an open coil, so having a read through 60 pages as i recall someone else had high amps. but thanks again guys, the cant see the forest for the trees run true today

 

[kit-808]

Thank you BillK55 you where right, i had put a 50amp instead of a 100 amp stunt in amps read normal now

Now only if i could find out why my coils get so hot