Coil driving

Are you planning to protect the semiconductors from inductive kickback?

On Oct 13, 10:58 am, Nuby wrote: > What's wrong with this picture? >

You're missing an interface driver. Perhaps a common ULN2003 would help. If you're running DC with SCRs, it will turn on one time and latch. Is that the goal? Lifting the ground as a master reset will complicate the TTL interface. Can you interrupt the positive instead?

What are you really trying to do?

GG

Maybe nothing. Assuming you have the correct polarity to gate the SCR's into conduction.

A snubber across the coils isn't a bad idea, but SRR's tend to be pretty rugged. I'd probably just go for it, and see if anything smokes.

You should be using opto-isolators that will drive the SCR etc. You must insure you use the proper relay coil for this. With SCR's, it would be a DC coil. with an TRIAC , it would be an AC coil.

This only assumes that you're using AC as the load source. if DC is going to be the source. The main relay you speak of must not be an SCR. it has to be something like a real relay, GTO or FET switch other wise, you'll get latch up until power is removed.

Actually, you can get SSR's (Solid State Relays) that can be driven from a TTL signal.

In overall concept it's ok but there are some practical considerations.

a. Do you really mean actual TTL ? TTL has annoying limits on output voltage swing and 20mA may also be an inadequate trigger current for the thyristors.

b. The common disable would make more sense in the 50V DC supply positive input.

c. As others have noted, you need protection from inductive kickback when the loads switch off.

Graham

Why ?

Ne never mentioned AC, why confuse the issue ?

Graham

Not the end goal, but an acceptable method.

I don't see the connection/problem. Can you explain?

I can, but having a common connection to Vcc and distinct/controlled connections to ground appears slightly perverse. I can afford to ground the frame, which simplifies the harness, but will not connect the frame to Vcc.

Control 100 miniature/high-density coils as cost-effectively as possible.

Um, have you priced such isolators recently? I'm looking for a circuit as simple as possible. .

Hunh? None of that makes sense to me. What relay?

What load are you talking about?

The only purpose of the big relay is to interrupt the latch effect.

Yes, but they are neither simple nor cheap. Why do I need such a complex component?

I think so. Typical outputs would drive 10-20 TTL loads. The actual control chip is a serial-to-parallel shift register.

Interesting point.

input.

OK.

How can I determine how much of a problem this would be? The energy in each active coils is going to be pretty small.

What's this chip's part number ?

Depends of course on the SCR you use. Did you have a specific one in mind ?

input.

the

A simple inverse parallel diode such as a 1N400x across each relay coil should be fine.

Graham

Never mind Jamie, he talks a lot of nonsense, including posting things that are

Graham

We have not chosen a specifc one yet. An example would be the MM74HC164 family.

thyristors.

Again we don't have a specific target yet. Any sensitive-gate SCR that can handle the (light) duty cycle would work for us.

the

be

OK, but I still don't know whether the protection is necessary.

Ah, fine. That's not TTL, it's HCMOS. Much better.

thyristors.

This would appear to do the job.

It's nice and cheap too.

when the

should be

It would be very silly not to fit it.

Graham

^^^^^^^^^^^^^^ One should practice what one preaches.

You're such an anus Mr. Ham...

When some one gives incomplete information. It can lead to to many roads. Until that poster makes it clear as to what they're trying to do, which apparently has taken place now. People that have been down many avenues can come up with so many scenarios that it would make some one like you get all confused. Sorry for your disablity in this area.

I guess that is what most would call "separating the men from the boys" when it comes to problem solving which you seem to be lacking.

Have a good day sucker.

the

be

yes, it's required on the coils, other wise, you'll take out the SCR at some point, and may also take out the TTL device.

The other problem is the (time on) gate signal. The coil is inductive which means you'll get low current in the SCR at the initial signal. If the Gate isn't kept on for the minimum time required that gets the SCR => it's holding current. It will not latch. This can be solved with a resistor across the coil if it becomes a problem.

To know that, you would have to know the inductive value of the coil and perform some calculations for the time frame your signal will be on.

Have a good day.