Is there a way to block current when the load is too low?

Say we have a 60V source.

If the load is pulling 4A. Current should flow.
But if the load is pulling 0.01A, current should be blocked.
When the load increases, then it should be unblocked.

Does this question even make sense?

 

How do you determine that the load has increased when current is blocked?

 

Does this question even make sense?

Yes, but if the low-current is blocked, how would the current ever increase, as it has to start low before it goes high?

 

It would be more expedient if you told us what you are trying to do, and not "How to block low current?"
For example, can you show us the situation, device, circuit?

 

How do you determine that the load has increased when current is blocked?

Yes, that's what i thought.

It would be more expedient if you told us what you are trying to do, and not "How to block low current?"
For example, can you show us the situation, device, circuit?

It's about triac leakage current.

In my application, I'm passing 5A continuous DC @ 60V. So all devices must be able to handle that.
I need to block current bidirectionally.

Triacs are suited to this, except for one problem. When a triac is open, there's a small amount of leakage current.

When the triac is open, I want to block the leakage current with some component.
When the triac closes, i want to unblock the current.

I don't want to use a mosfet as the blocking device, because too large and expensive. If i use a mosfet as the blocking device, then i might as well discard the triac, and use two back-to-back mosfets. But again, too large and expensive.

 

It's about triac leakage current.

Why is this small leakage current a problem?

 

Why is this small leakage current a problem?

Can't stop thinking about it

 

The leakage through a standard triac (BTA16-600B for instance) is <5μA @ 25°C increasing to a maximum of 2mA @ 125°C.
So, is it leakage through the snubber? If so, place the snubber between the triac output and neutral, instead of across the triac.
But triacs are not suited to switching DC, because they can't switch it OFF.

 

Can't stop thinking about it

Sorry, but that's a non-sequitur for me.

 

You can set a limit on the maximum current.
You can set a limit on the minimum current at which point a lower current turns the system off. Now you need a method of turning the system back on when a certain load is detected.

 

triacs are not suited to switching DC, because they can't switch it OFF.

They can switch off if no current is flowing thru the triac. Which is my case.

Why is this small leakage current a problem?

I'm designing a cell switcher which, given two lithium cells, can assemble a battery using one or both cells. This is only for both charging and discharging, so i believe i need to block current in both directions when a switch is open. The connections are such that, if just one cell is switched into the circuit, then any leakage will flow from both terminals of both cells to both terminals of both cells. The dotted line shows the leakage path.

The leakage through a standard triac is 2mA @ 125°C.

Maybe that amount of leakage is ok for this application.

 

Not exactly an efficient circuit if you are charging a 3.7V lithium cell and losing 1V across the triac.

 

Triacs are suited to this

TRIACs only work with AC, not DC.

So what are you going to use for a switch?

 

Not exactly an efficient circuit if you are charging a 3.7V lithium cell and losing 1V across the triac.

For my purpose, efficiency doesn't matter. Low parts-count matters.
Triacs block in both directions, which i need. Mosfets only block current in one direction, so i'd have to use two back-to-back mosfets, doubling my parts count.

TRIACs only work with AC, not DC.

i believe you're incorrect.

 

For my purpose, efficiency doesn't matter. Low parts-count matters.

Sounds like it is commercial product, in which case it might have to meet efficiency standards in the ErP regulations.

 

i believe you're incorrect.

You are quit welcome to believe whatever you want, but a TRIAC will not turn off unless the current through it stops, as occurs twice every AC cycle.

 

TRIAC will not turn off unless the current through it stops

Which is my case.

might have to meet efficiency standards in the ErP regulations.

you mean, triacs aren't legal in Europe?

 

Which is my case.

Really, as you said it was DC.
If you have something that stops the current, then what does the TRIAC do?

 

Which is my case.

you mean, triacs aren't legal in Europe?

A triac switching 230V loses 1V. Efficiency = 229/230 = 99.6%
Efficiency switching 3.7V = 3.7V/4.7V = 78.7%
There may be regulations about the overall efficiency of the appliance. You will have to check.

 

If you have something that stops the current, then what does the TRIAC do?

1. Circuitry external to this question stops current going to triac.
2. Then the triac in this question will open. There's no current, so it can do that.
3. Then circuitry external to this question will turn current back on, allowing current to flow to other parts of the circuit. The triac in this question will block current in it's localized region of the circuit.