Problem in design?

[blueroomelectronics]

Latching relays are also great for battery operated devices.

 

[colin55]

If you need a relay to operate a device, the current taken by the device will be considerable while the current taken by a relay is very small. Our CMOS relays require 5mA @5v.
The awkwardness of using a latching relay is not worth the effort.

 

[Roff]

If you need a relay to operate a device, the current taken by the device will be considerable while the current taken by a relay is very small. Our CMOS relays require 5mA @5v.
The awkwardness of using a latching relay is not worth the effort.

How much contact current can your CMOS relay switch?

 

[alphadog]

my relay switches up to 16Arms (we short both NO legs of relay).

 

[Roff]

my relay switches up to 16Arms (we short both NO legs of relay).

Paralleling relay contacts will not double the current-switching capability, because they will never switch at exactly the same time.
https://www.electro-tech-online.com/custompdfs/2009/02/13c3236.pdf

 

[colin55]

What are you powering?
How much current do you need?
What is the voltage of the device you are powering?

 

[colin55]

How much contact current can your CMOS relay switch?

The CMOS relay only handles 1Amp

 

[colin55]

I realise he wants 16 Arms. I have 2 arms, can anyone else help?

 

[Roff]

I realise he wants 16 Arms. I have 2 arms, can anyone else help?

He can use one of mine. I need the other one for various tasks.

 

[colin55]

If the current is over 15 amps, it would be better to use a MOSFET. Currents over 10 amp cause a lot of trouble with contacts. You get welding and pitting.

 

[ccurtis]

Alphadog, see the attached. It will not harm your I/O. In sleep mode your I/O retains the mode and state that is set before entering sleep mode. At power-up, your I/O is set to input mode with no pull-ups, so all bridge transistors will be off. You should put reversed biased diodes across each of the four transistors to protect the transistors against counter-EMF from the relay coil when it is turned off.

 

[electricity86]

Alphadog, see the attached. It will not harm your I/O. In sleep mode your I/O retains the mode and state that is set before entering sleep mode. At power-up, your I/O is set to input mode with no pull-ups, so all bridge transistors will be off. You should put reversed biased diodes across each of the four transistors to protect the transistors against counter-EMF from the relay coil when it is turned off.

Hey Curtis.
Thank you for your help.
I simulated your circuit with QBSR14 (NPN) and QBSR15 (PNP) and received I_RELAY = 5mA.

 

[ccurtis]

Hey Curtis.
Thank you for your help.
I simulated your circuit with QBSR14 (NPN) and QBSR15 (PNP) and received I_RELAY = 5mA.

Your simulation results are not the same as mine. Check that your bridge transistors have a gain (hfe) of at least 20 at collector curent of 200 mA, preferably more.

 

[ccurtis]

Alphadog, this circuit is simpler.

 

[blueroomelectronics]

Your simulator doesn't seem to be very accurate. At 500ma a typical 2N2222 can have a drop of up to 1V.

 

[ccurtis]

Yeah, but it's about .3 volts (max) at 200 mA, less typically. Still can't vouch for simulator's accuracy, though. Lots of pitfalls with simulators.

 

[Roff]

Your simulator doesn't seem to be very accurate. At 500ma a typical 2N2222 can have a drop of up to 1V.

The Fairchild datasheet for PN2222 says the maximum Vce(sat) at 500mA is 1.0V. Typical is less than 300mV (see Fig. 2).
I found a Philips datasheet that says max Vce(sat) for 2N2222 is 1.6V.

 

[colin55]

The simpler circuit (presented above) will need base resistors on the PNP transistors to keep them turned off as the bases are floating at the moment (say 4k7 from base to positive).
The 10uH choke will not be needed as this value will not have any effect in this situation. The 25R (27R) resistor will reduce the back emf from the (relay) coil.
The transistors may have to be up-rated to take the current.

 

[blueroomelectronics]

Either way a total drop of 0.15 v for the bridge is very optimistic.

 

[colin55]

One of the best transistors is ZTX851. (NPN) It is rated a 5A but at 2A the collector-emitter voltage is 150mV.

You will need to search the web for an equivalent PNP. This means the minimum loss will be 300mV.