School me on relay latching, good articles welcome "how to make a relay latch"

[Andy Peters]

I've etched probably around 50 or so boards now.

Things I make sure I do are:

One other thing. One of our techs does our board etches for the simple test things we need, and his secret is to put the pyrex tray of ferric chloride etchant onto a hot plate and keep it warm. Not so hot that you burn your hands, but warm.

Also, wear nitrile or better protective gloves. That shit is nasty.

-a

 

[buildafriend]

Hot plate idea sounds like a good one! I'll have to take that tip and get one at goodwill. Give your tech a hi high five for me for that one.

 

[abbey road d enfer]

I've etched probably around 50 or so boards now.

Things I make sure I do are:

One other thing. One of our techs does our board etches for the simple test things we need, and his secret is to put the pyrex tray of ferric chloride etchant onto a hot plate and keep it warm. Not so hot that you burn your hands, but warm.

Also, wear nitrile or better protective gloves. That sh*t is nasty.

-a

that reminds me of the times we strated the company in the garden shed. The ferric chloride was conveniently placed on top the oil burner.

 

[JohnRoberts]

If I haven't repeated this story lately it seems worth the warning. Back in the '70s I was tech at a small company and we used the hot plate to heat the ferric chloride to etch faster. All good until that one time when the glass container holding the acid broke and the liquid ferric chloride vaporized on the hot heating element and coated all the metal in the room with caustic acid...

It was a very ugly mess and expensive to clean up. So make sure you are using containers that can stand the heat.

JR

 

[buildafriend]

haha noted noted and noted.  I really need to get some stuff to neutralize the chemical.

My etch turned out pretty good on this board. It's 4 regulated power supplies, 1 G9 HT rail, 1 48VDC rail, 1 24VDC rail, and 12.6VDC heaters. No voltage doublers or triplers, just transformers, rect bridges, smoothing caps, and regulator bias resistors. I went a little overkill all around because I'm hoping to daisy chain it one day to several tube preamp 19" rack boxes. So far I'm super proud, big steps for me here by adding my own tweaks to circuits and starting to actually understand what does what in regulated supplies.

I'm trying to figure what to do about the relay ground. What constitutes a good virtual ground for relays?

Im wondering what these 39V zener diodes are doing in the G9 supply.

 

[JohnRoberts]

I'm trying to figure what to do about the relay ground. What constitutes a good virtual ground for relays?

The relay grounds by definition will be dirty grounds. Give them their own wire or PCB trace back to the Power supply ground node (transformer CT or bottom of reservoir cap). Do not co-mingle this ground path with clean audio grounds.

JR

 

[buildafriend]

I'm trying to figure what to do about the relay ground. What constitutes a good virtual ground for relays?

Give them their own wire or PCB trace back to the Power supply ground node (transformer CT or bottom of reservoir cap).
JR

Tactful placement! Thanks

-JP

 

[buildafriend]

Oh i missed this by andy a while back.

"Then you actually switch the - terminal of the coil. So one switch terminal goes to the coil - and the other goes to a net we'll call "coil return." Draw a thick trace from the switch to a ground point near the power supply. Yeah, this is a star ground, which is something we don't do on circuit boards, but in this case you're trying to manage ugly current spikes."

 

[Chris_V]

Well you have to be carefull, but relay coils are inductors and so current are not so ugly (dI/dt is limited by the inductance value)
A led peak meter is potentially far more "noisy"

 

[JohnRoberts]

It depends on how much current the relay draws...

LEDs in meters may only draw mAs of current and I often put meter  LEDs in series so they may not even dump straight into the ground.

YMMV, I'd rather advise caution that isn't needed than not warn when it is...

JR

 

[Chris_V]

Yes ! An other point is the possible coupling between the coil and the contact pins.
I had a problem in a recent design where reed relays where use for gain selection in a non-inverting amp, switching a resistance to ground for gain=10 and leaving it open for gain=1. Gain 10 was less noisy than Gain 1 
Coil was connected to a "noisy" backplane 12V on one side and tied to ground by a fet in operation (gain10), so in gain 1 the coil was a good antenna injecting PS noise in the inverting pin of the opamp !! It was far beyond audio frequency so a simple LC filtering of the 12V solved the problem.

 

[abbey road d enfer]

Yes ! An other point is the possible coupling between the coil and the contact pins.
I had a problem in a recent design where reed relays where use for gain selection in a non-inverting amp, switching a resistance to ground for gain=10 and leaving it open for gain=1. Gain 10 was less noisy than Gain 1 
Coil was connected to a "noisy" backplane 12V on one side and tied to ground by a fet in operation (gain10), so in gain 1 the coil was a good antenna injecting PS noise in the inverting pin of the opamp !! It was far beyond audio frequency so a simple LC filtering of the 12V solved the problem.

Absolutely. Many years ago I had designed a product that had delayed turn-on bypass relays driven by a circuit that included a zener. The zener noise, amplified by the transistor driving the relay, propagated in the audio at a perceptible level. A mere capacitor across the transistor base fixed it. It shows that, although not critical, cleanness of relay power is to be considered.

 

[buildafriend]

The relays I chose have a polarity to the coils. That threw me off a bit.. I almost thought there was something wrong with the power supply.