finally an electrostatic transportable

[Shawn]

4 hours ago, dingding123 said:

17k smd resistors are quite hard to find. Would you recommend 20k or 15k ohm resistors?

I didn`t check the schematic. However, the input resistors(Signal input) shouldn`t be a probably if you go with 15k or 20k.

[dingding123]

Has anyone tried building this? Not sure if it's just me - after power on, the board overheats within a minute and then comes the magic smoke. PSU voltages are correct but output balance (i.e. L+ vs L-) goes from -2 to -60v quite quickly then suddenly normalises to around 4v).

Pretty sure there are no shorts or issues with soldering on my board...

[kevin gilmore]

i am aware of the overheating issues.  i have an assembled board coming to me to look into this.  something clearly has changed from the original parts i used and there is some positive thermal runaway that did not happen with the prototype. there is a later version board now, 4 layers, still does not fix the problem.

putting 12v zeners in series with the 1.1meg resistors may introduce enough negative thermal compensation. otherwise the bias needs to be trimmed a lot.

hate the parts situation i do.

[kevin gilmore]

you are aware that you need a heatsink touching all the sot transistor.  still with heatsink it typically can take about 5 minutes to reach high temps.

[Michelag]

On 8/7/2024 at 5:16 PM, kevin gilmore said:

all three power supplies a total of $80.62 with shipping from china.

fully regulated 12v to 18v input.

i could not build these things for that price, no way.

I was thinking, Kevin, are those bricks rechargeable? Does it need a battery management board?

[judo]

These are DC-DC converters 

[Michelag]

2 hours ago, judo said:

These are DC-DC converters 

All right, thought they were batteries... So perhaps it's not really "portable", to the office yes, but around mmmm

[Michelag]

Perhaps a little early, compiled a BOM:

https://www.mouser.de/api/CrossDomain/GetContext?syncDomains=www&returnUrl=https%3a%2f%2fwww.mouser.com%2fTools%2fProject%2fShare%3fAccessID%3dd8cfd9e908&async=False&setPrefSub=False&clearPrefSub=False

[Michelag]

Tried a test build, with silicone thermal sheet, alu plate on some alu piece.

Yes, there's thermal runaway. Power consumption begins at 24V/0.8A and slowly takes up to almost 2A, when the board reached 80°C I shut it down.

The 400V B+ rail (so the B-) started around 9mA, up to 20mA before shutdown.

Offset was around 2V on one channel, 5V on the other one.

Amp was in idle condition.

@kevin gilmoreDr., you were saying putting a 12V sender in series to 1.1Meg resistor, newbie question, with what polarity?

What about trimming the bias ? Do you foresee adding a trimpot in the next release?

Thanks, hope it helps other fellow builders.

[psyduck]

Unfortunately the resistors drift with temperature as I've tried building it before. I'd recommend adding a fan to cool the top side.

[Michelag]

7 hours ago, psyduck said:

Unfortunately the resistors drift with temperature as I've tried building it before. I'd recommend adding a fan to cool the top side.

Yes, but it introduces noise 😁

[psyduck]

8 minutes ago, Michelag said:

Yes, but it introduces noise 😁

There's an updated board on google drive that has a 10k trimmer with dn2540 that you can adjust the current till it's stable.

https://drive.google.com/file/d/16cuirLgLsadI1xa9c6OoaIqW6KevyZPa/view?usp=drive_link

[Michelag]

2 hours ago, psyduck said:

There's an updated board on google drive that has a 10k trimmer with dn2540 that you can adjust the current till it's stable.

https://drive.google.com/file/d/16cuirLgLsadI1xa9c6OoaIqW6KevyZPa/view?usp=drive_link

You are one of the greatest, here on my side it's easier to open the mouth instead of go checking updates on the two repositories 😁😔

Hope it addresses also the thermal runaway... 

Edited 3 hours ago by Michelag

[psyduck]

When adjusting the trimmers, make sure they're balanced to minimize DC offset between outputs, recommend having another DMM for that.

To measure the current, you need to measure the voltage between the 100R resistors and divide the voltage by 100. The total current for one side is the sum of all 4 of them.