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Low 12V rail on the Sammich


schmidtc
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Hi All,

I was about to drop the SIDs in when I noticed the 12V rail is only 10.72V everywhere. I'm using an old Caere DC PSU with the center positive. It says it outputs 12VDC 400mA regulated OR 12VDC 500mA regulated, which seemed weird but I figured either was ok. My scope/DMM gives the following:

Between the 2 J1 terminals: 12.19V

Between base board ground and J1 1: -.73V

Between base board ground and J1 2: 11.45V

All 12V connections to base board ground: 10.72V

All 9V connections to base board ground: 8.98V

All 5V connections to base board ground: 4.99V

Seems like things are off by a diode drop or two here and there? Perhaps I need a new DC supply with more umph. Any Ideas are much appreciated. Thanks!

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Note that where I label "12V" is where it should be 12V when using a regulated 12V PSU and shunts in JBP, and where it will roughly be 12V for unregulated PSUs. The exact voltage will vary. With very little load (like no ICs installed), an unregulated 12V supply might be 15V or higher, but then drop down to closer to 12V or below under load.

So all that is expected behaviour, inc. the diode drop difference, because the DC input is going through a bridge rectifier.

That should be OK for 8580/6582A SIDs... I've been using a regulated 12V PSU still going through the bridge rectifier and the 9V supply is stable even with the input voltage being only ~1.5V above output.

If you notice the 9V or 5V connections drop after inserting SIDs, then that PSU isn't supplying enough current, but seems unlikely, because it's 400mA (your sammichSID should draw less, with a low-power LCD).

It's strange that it's 12.19V for a "regulated" 12V, which should be closer to 12V (see the voltages of 9V and 5V, only 0.01V-0.02V different). Watch what happens at J1 after you insert SIDs... does it stay at exactly 12.19V?

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Thanks Wilba,

I should've mentioned it's got the low level LCD and I was planning to use some 6581's. I tried a different power supply that outputs 12VDC regulated at 1A. Same problem. The new one puts out 11.92V so the DMM readings above are all the same with the new PSU, only a little less, that is:

Between the 2 J1 terminals: 11.92V

Between base board ground and J1 1: -.73V

Between base board ground and J1 2: 11.20V

All 12V connections to base board ground: 10.47V

All 9V connections to base board ground: 8.98V

All 5V connections to base board ground: 4.99V

I'm a novice at his stuff, and I don't understand the purpose of running a regulated PSU though a full wave rectifier. Is it to filter AC? Won't the output always be two diode drops below below the regulated input? i.e. isn't ~13.4 needed to give the 6581's the 12V they need? I can't thank you enough for putting together any awesome kit AND taking the time to help troubleshoot this kind of stuff.

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I'm a novice at his stuff, and I don't understand the purpose of running a regulated PSU though a full wave rectifier. Is it to filter AC? Won't the output always be two diode drops below below the regulated input? i.e. isn't ~13.4 needed to give the 6581's the 12V they need?

I think you answered your own question, but in case not... when you want to use a regulated 12V PSU, the shunts in JBP will short two diodes of the bridge rectifier so you will get exactly 12V on the 7809 inputs and also through the SIDs via JP. An interesting side effect though, if you stuff up the polarity, it will go through the two diodes that are not shorted by JBP and you'll get < 12V but still correct polarity.... so not harmful.

In theory you don't need a bridge rectifier for unregulated DC input too, but the bridge is also a neat way of polarity protection (i.e. doesn't matter if it's tip positive or tip negative).

You're one of the few people who will use 6581 in sammichSID... so let me know how it goes. Note all that noise you get is entirely the 6581 and not the PCB :wink:

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