Problem in design?

[audioguru]

The PNP transistors in your corrected circuit never turn off. Their base voltage needs to be the same as their emitter voltage which is 5V, not 3.3V.
You need to add level-shifting transistors so that the PNP transistors turn off when the input signal is 3.3V.

 

[alphadog]

The PNP transistors in your corrected circuit never turn off. Their base voltage needs to be the same as their emitter voltage which is 5V, not 3.3V.
You need to add level-shifting transistors so that the PNP transistors turn off when the input signal is 3.3V.

Thanks.
Do you have a scheme of the way you suggest to add the level-shifting transtitors?
I want to test tomorrow new circuit and i'm a bit new in it, so i'd very appreciate it.

Thank you.

 

[ccurtis]

1.Basically, my relay needs 1W comsumption to change state, and currently it consumes 0.89W. Is there anyway to increase the current so it would consume 1W?

Not with that circuit. The transistors are not perfect switches. Even FETs will not get you a full 1W. Your relay has to be connected directly across 5V to consume 1W. P=E²÷R= (5×5)÷25 = 1W

2. When both inputs are 0V, then the 5V regulator outputs 1.55A, and my 5V regulator can only push up to 300mA. Does it mean that the stand-by mode of this driver is when both inputs are 3.3V? The problem with that is i'll insert my MCU into sleep mode, then i'll have no 3.3V inputs.

Yes to your first question. Second, if the MCU outputs are floating, open circuit when in sleep mode, the bridge transistors are turned off, consuming little power. I don't know your MCU. As someone mentioned in an earlier post, you should check the MCU data about pulling up the MCU outputs to +5V through the 5.6K resistor. Just in case.

 

[alphadog]

Not with that circuit. The transistors are not perfect switches. Even FETs will not get you a full 1W. Your relay has to be connected directly across 5V to consume 1W. P=E²÷R= (5×5)÷25 = 1W

My relay can be switched with at least 3.75V @ 1W, so i though maybe we could decrease its voltage consumption but increase its current consumption.

Yes to your first question and you can never drive the relay with 1W if your 5V can deliver only 300 mA, even if the bridge transistors are perfect, unless a charged capacitor is used to deliver a high power surge for a brief period.

This would be the perfect solution, any my boss wanted to try it out but i dont know how to design it with capacitors.
Can you tell me please how to fit the capacitors into the design?

Second, if the MCU outputs are floating, open circuit when in sleep mode, the bridge transistors are turned off, consuming little power. I don't know your MCU. As someone mentioned in an earlier post, you should check the MCU data about pulling up the MCU outputs to +5V through the 5.6K resistor. Just in case.

My MCU (CC2430) can configurate its pins as inputs pull-up/pull-down/tri-state. You mean that generally MCUs can configurate their pins as open-circuit?

Thanks for everything.

 

[ccurtis]

My relay can be switched with at least 3.75V @ 1W, so i though maybe we could decrease its voltage consumption but increase its current consumption.

You coil resistance is 25 ohms. There is no way 3.75V will supply the relay coil with 1W. No need to get hung-up on supplying a full 1W if the relay will energize with 3.75V, which is 0.6W (not 1W).

My MCU (CC2430) can configurate its pins as inputs pull-up/pull-down/tri-state. You mean that generally MCUs can configurate their pins as open-circuit?

Tri-state is the same thing as open circuit, which results in all transistors turned-off (circuit I posted). Configuring as input would also turn all bridge transistors off. But you need to check what state the I/O pins are in, in sleep mode.

Note: I corrected an earlier mistake. 300 mA at 5V is sufficient to drive the relay with 1W.

 

[alphadog]

You coil resistance is 25 ohms. There is no way 3.75V will supply the relay coil with 1W. No need to get hung-up on supplying a full 1W if the relay will energize with 3.75V, which is 0.6W (not 1W).

I meant that the relay's voltage will be around 4V for exmample, at 250mA, which results in 1W consumption.

Tri-state is the same thing as open circuit, which results in all transistors turned-off (circuit I posted). Configuring as input would also turn all bridge transistors off. But you need to check what state the I/O pins are in, in sleep mode.

I thought that tri-state means that the external system determine the value of the input pin, so i was wrong?

I wanted to ask please, is it possible to include the capacitors in the design?
i'm not sure how they should be connected.

Thank you again for your help.
I appreciate it.

 

[ccurtis]

I meant that the relay's voltage will be around 4V for exmample, at 250mA, which results in 1W consumption.

I thought that tri-state means that the external system determine the value of the input pin, so i was wrong?

I wanted to ask please, is it possible to include the capacitors in the design? i'm not sure how they should be connected.

Thank you again for your help.
I appreciate it.

Forget the capacitors. That idea was based on your +5V supply not being able to deliver 200 mA, which it can deliver.

You need to scrap the circuit I provided. The CC2430 allows a maximum of 3.6 volts on its I/O pins. The +5V connected to the MCU through the 5.6K resistor exceeds that limit if the I/O pin is configured as input, or tri-state. Back to the drawing board. What are the limits on the parts you can use?

 

[alphadog]

Forget the capacitors. That idea was based on your +5V supply not being able to deliver 200 mA, which it can deliver.

You need to scrap the circuit I provided. The CC2430 allows a maximum of 3.6 volts on its I/O pins. The +5V connected to the MCU through the 5.6K resistor exceeds that limit if the I/O pin is configured as input, or tri-state. Back to the drawing board. What are the limits on the parts you can use?

I see.
First i learned from you one more thing about that, so thanks!
The limits are to use transistors (BJTs or Mosfets), because currently we cant afford buying special components.
It is very desired that it wont be expensive.
But sometimes when you got something work you can after that look for how to get it cheaper.

 

[ccurtis]

If you want to get to a workable circuit without going through learning iterations, you have supply all information up front. So far we have:

Hardware:

MCU part number given
+5 Volt supply that can deliver 300 mA
Relay part number given

Requirements:

Birdirectional control of relay coil.
Very low power consumption when relay is not switched, but up to 250 mA from +5V supply pulses for brief periods (assume that MCU can supply the short pulses).
Very low cost using BJT and/or FETs and resistors, only.

Right?

 

[alphadog]

If you want to get to a workable circuit without going through learning iterations, you have supply all information up front. So far we have:

Hardware:

MCU part number given
+5 Volt supply that can deliver 300 mA
Relay part number given

Requirements:

Birdirectional control of relay coil.
Very low power consumption when relay is not switched, but up to 250 mA from +5V supply pulses for brief periods (assume that MCU can supply the short pulses).
Very low cost using BJT and/or FETs and resistors, only.

Right?

Yes.
I would add that i have 2 pins of MCU that can drive up to 20mA.
Hopefully i could test it tomorrow and celebrate

Thank you for everything

 

[colin55]

Why do you want to control a relay in either direction?

In other words, what do you want to do with the relay?

 

[alphadog]

Why do you want to control a relay in either direction?

In other words, what do you want to do with the relay?

I want to switch on and off relay.
I assume there is more behind it, thats the crew's call.

 

[colin55]

You don't need an "H-bridge" to power a relay. Just a buffer transistor from a microcontroller.


That's why you have to provide all the information at the beginning of a discussion, as I could see everyone was being directed down the wrong path.

Now, explain what you need:

 

[alphadog]

You don't need an "H-bridge" to power a relay. Just a buffer transistor from a microcontroller.


That's why you have to provide all the information at the beginning of a discussion, as I could see everyone was being directed down the wrong path.

Now, explain what you need:

- driver for 5V, 1W (can be also 3.75V, 1W but with higher current) latching relay (1 coil).
- 5V Vcc that can push up to 300mA.
- 3.3V GPIO inputs of MCU (CC2430) that can push up to 20mA.
- driver cosists of BJT, Mosfets and/or darlington and resistors.
- low cost and low power consumption on standby.

 

[KMoffett]

You don't need an "H-bridge" to power a relay. Just a buffer transistor from a microcontroller.

colin55,

If you had read this thread, you know that alphadog is determined to use a single-coil latching relay...which requires one polarity across the coil to latch it ON and the opposite polarity across the coil to latch it OFF.

A 2-coil latching relay, 2 transistors, 2 resistors and 2 diodes (necessitating twice your solution ) would be the simplest with an MCU...but that appears impossible for him.

Ken

 

[blueroomelectronics]

The particular relay requires twice the energy in its dual coil configuration.
Appears to be a bandaid soultion to a poorly engineered product. Worse yet on the cheap.

I'd be curious to know what mass produced product it is? We may never know.

What is known is it has a TI micro and a 10A latching relay but appears much of the design is set in stone.

Here's what I can't figure. If this is a current product and getting a field fix the cost of the parts will be far far less than the retrofit.
If it's in development then it should be re-designed properly before shipping to market.

Remember a bandaid is only a temporary fix.

Just my 2c

 

[KMoffett]

This whole thread seems to be "fix my solution".

Ken

 

[colin55]

It seems the whole project has been designed by someone without any knowledge of electronics at all and all the intelligence of helpers is not going to sort it out.
It reminds me of dozens of electronic gadgets designed by Taiwanese. They all had problems that made them fail after a few weeks or flatten the batteries after a month.

You can't "prop-up" a failure. It has to be designed correctly from the beginning.
Starting with a latching relay in this situation is an absurdity.

 

[Roff]

It seems the whole project has been designed by someone without any knowledge of electronics at all and all the intelligence of helpers is not going to sort it out.
It reminds me of dozens of electronic gadgets designed by Taiwanese. They all had problems that made them fail after a few weeks or flatten the batteries after a month.

You can't "prop-up" a failure. It has to be designed correctly from the beginning.
Starting with a latching relay in this situation is an absurdity.

OK, I'm confused. I thought the application is driving a latching relay. Did I miss a discussion of what he is using it for?

 

[colin55]

If you have a microcontroller, you have the feature of keeping the relay activated and you don’t need a latching relay.

Once you don't need a latching relay, the whole circuit is simplified.

Latching relays are only needed when you have just a push button.

This whole episode smacks of the customer who wanted a pressure-transmitter designed but did not want to give me any details.
I worked it out to be a transmitter for trucks to tell when the tyre became flat.
But how does anyone expect a design to be created when the parameters are not provided and actual operation of the circuit is not disclosed. He is obviously barking up the wrong tree and to be stuck on a latching relay as the centre-point is totally absurd.