HP3548 / HP3457 OLED display

[Kean]

I'm happy to dig into the firmware but it won't be for another week or so before I can try assembling any of my PCBs.

In the mean time, from a quick glance you can try to modify oled.c function oled_init() which initializes the display memory with "empty values".
https://github.com/xyphro/HP3457-OLED-display/blob/09a7be32dd252d96eb0008a0b00129694da555cc/software/source/HP3457Oled/oled.c#L141

I would try changing it to something like:

Code: [Select]

	// fill display memory with non-empty default values
	for (uint8_t digit=0; digit<12; digit++)
	{
		oled_chars[digit] = 58; // all digits lit
		oled_dots [digit] = 0xff; // annuciators / punctuation = all on
	}

You may also need to disconnect the serial input so the display does not update beyond that.

[TERRA Operative]

Turns out, the current draw is low enough on the OLED's, and the specs/size for power inductors is such that I'll have a huge margin for power.
I was looking to optimise the inductors to save space and cost, but the sweet spot is about 1Amp current rating if I want a reasonably low DC resistance, which is way above what the OLED's draw, so this issue is effectively resolved.

Last two steps is to design a simple jig to enable the OLED's to be easily aligned while soldering (I'll design it from FR4 so it can be made just like a normal PCB), and to see if the annunciaters can be made to illuminate on self-test (But that's firmware, so might take me a while....)

[Miti]

Last two steps is to design a simple jig to enable the OLED's to be easily aligned while soldering (I'll design it from FR4 so it can be made just like a normal PCB), and to see if the annunciaters can be made to illuminate on self-test (But that's firmware, so might take me a while....)

 As I said, I took a different approach. Single board and 3D printed back with places for the OLEDs. I've also simplified the schematic, linear regulator supplied from the unregulated 5V (13.5V on 3478A), no opto isolator but 5V tolerant buffers. This is for the 3478A only. All I have left is to select a linear regulator with EN for 8V.
Oh... and Kicad.
Can anyone suggest a jellybean linear regulator with enable for about 200-250mA up to 18V at the input?

Cheers,
Miti

[wkb]

old skool 7805? Cheap as chips. Probably overkill but hé...

[Miti]

old skool 7805? Cheap as chips. Probably overkill but hé...

With enable.

[eliocor]

I use those modules: https://www.aliexpress.com/item/1005007369519036.html they are cheap, have an enable pin and works rather well.
For more details:
https://protosupplies.com/product/mp2315-mini-adjustable-dc-dc-step-down-module/

[wkb]

I would go for a linear supply. No need add noise from a switcher into a precision measurement instrument!

[eliocor]

Right, he was asking for a LINEAR regulator....
take a look at those lists :

Fixed 8V regulators ordered by price

Adjustable regulators ordered by price

Rohm BDxxFC0 series? Available even on LCSC

[Miti]

Turns out, the current draw is low enough on the OLED's …

What is it?

[Miti]

Right, he was asking for a LINEAR regulator....
take a look at those lists :

Fixed 8V regulators ordered by price

Adjustable regulators ordered by price

Thanks, I think I found what I’m looking for.

[eliocor]

see my previous suggestions (message modified AFTER your last post)

[Miti]

see my previous suggestions (message modified AFTER your last post)

Thanks but I think an SOT23-5 would do it. Xyphro measured only 20mA supply current so by my calculation 13.5-8=5.5V x 50 mA = 275mW. So if it takes 50mA the regulator dissipates 275mW which should be low enough. I’m waiting for TERRA to confirm the supply current. Moreover, I found SOT23-5 with the same pin out from Digikey and from LCSC, under 50 cents each.

[TERRA Operative]

Turns out, the current draw is low enough on the OLED's …

What is it?

Under normal operation (idle in DC range after booting the meter) it draws about 24mA on the 8V rail.

I'm using approximately 1A rated inductors with a 4x4mm footprint (they were the best cost/availability/specification ratio) so I didn't do any further testing.

I'd say work on figuring at least double at about 50mA to be safe.

[Miti]

I'd say work on figuring at least double at about 50mA to be safe.

The way to calculate this is: PDmax = (Tjmax - Ta)/ ResJA for the ambient and ResJC for the copper.
For the AP2205 in SOT25 package for example that is: PDmax = (125 - 35)/160 = 560mW

For 13.5V in and 8V out that is 560/5.5 = 101.8 mA.

Any objection to this schematic?

[Miti]

Ok, les jeux sont faits, rien ne va plus, the boards are with JLC. I've also designed the plastic bracket, 3D printed it and looks ok. This is just the board side, once I get the OLED modules I will draw the OLED side with recess for each module for alignment. Unfortunately LCSC has exactly zero stock of 0.87" OLEDs  .