What would be the minumum PWM frequency to hold a relay? - Page 2 - Electronic Circuits Projects Diagrams Free

Boncuk · 14th February 2008, 02:15 PM

Here is an example of the circuit. The coil restance is 240Ohm and the current without limiting resistor is 56.1mA. With limiting resistor the initial current is high until the cap is discharged. Thereafter the current drops to 37.3mA.

Coils with higher restistance require a higher value limiting resistor for optimum power saving.

To hold the relay the capacitor has no influence.

blueroomelectronics · 14th February 2008, 02:37 PM

I could double the frequency to 150Hz or 300Hz.

**Nigel Goodwin** · 14th February 2008, 03:34 PM

Quote:

Originally Posted by blueroomelectronics

I could double the frequency to 150Hz or 300Hz.

I still really fail to see why you're wanting to complicate things by driving the relays using PWM? - relays are designed to be permanently powered - the only reason would be to reduce current consumption, but if it's battery powered you wouldn't be using relays anyway?.

Diver300 · 15th February 2008, 08:49 PM

Quote:

Originally Posted by k7elp60

On a 12V relay and a load I checked the voltage drop across the closed contacts with a 25A DC load the voltage across the contacts was 86.7mV with the normal 12V on the coil. With the coil voltage reduced to 6V the voltage across the contacts was 88.6mV. I figgured this was due to the decrease in pressure created by the magnetic field.

Most relays don't work like that. There is a spring so that the armature can move further after the contacts have closed. If the relay is holding in, the armature will usually be in contact with the core of the coil, so the pressure there will vary, but the spring will keep a constant pressure on the contacts.

blueroomelectronics · 15th February 2008, 09:23 PM

Quote:

Originally Posted by Nigel Goodwin

I still really fail to see why you're wanting to complicate things by driving the relays using PWM? - relays are designed to be permanently powered - the only reason would be to reduce current consumption, but if it's battery powered you wouldn't be using relays anyway?.

It's not mandatory, the design simply supports it. 8 relays drawing 100ma each is 800ma, a simple software toggle of the enable line would cut this in half. (heat) I thought the hardware PWM was overkill and wanted to use it elsewhere.

jpanhalt · 15th February 2008, 10:01 PM

Maybe you answered this before, but I missed it. Why not use a latching bi-stable relay. There is a choice of single- coil of two-coil, depending on how you want it to switch. I was surprised the DigiKey price for the Schrack is only $2.50.

http://relays.tycoelectronics.com/sc...bistable_d.pdf

John

Hero999 · 15th February 2008, 10:40 PM

I think it's a good idea.

The minimum frequency depends on the relay's resistance, inducatance and minimum holding current. You probably already know the resistance and possibly the minimum holding current, the only thing you need to measure is the inductance.

Also use a Shottky diode as it'll reduce the losses and alow the duty to be reduced even further.

blueroomelectronics · 15th February 2008, 11:41 PM

I'll be using a latching relay in the next kit after ladybug (PLC & Ethernect).
Cricket (RS485 Thermostat with 2.5 digit LCD)
Here are the relays I plan to use.

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14th February 2008, 02:15 PM	(permalink)
Boncuk	Here is an example of the circuit. The coil restance is 240Ohm and the current without limiting resistor is 56.1mA. With limiting resistor the initial current is high until the cap is discharged. Thereafter the current drops to 37.3mA. Coils with higher restistance require a higher value limiting resistor for optimum power saving. To hold the relay the capacitor has no influence. Last edited by Boncuk; 8th July 2008 at 12:49 AM.

14th February 2008, 02:37 PM	(permalink)
blueroomelectronics	I could double the frequency to 150Hz or 300Hz. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

15th February 2008, 10:01 PM	(permalink)
jpanhalt	Maybe you answered this before, but I missed it. Why not use a latching bi-stable relay. There is a choice of single- coil of two-coil, depending on how you want it to switch. I was surprised the DigiKey price for the Schrack is only $2.50. http://relays.tycoelectronics.com/sc...bistable_d.pdf John

15th February 2008, 10:40 PM	(permalink)
Hero999	I think it's a good idea. The minimum frequency depends on the relay's resistance, inducatance and minimum holding current. You probably already know the resistance and possibly the minimum holding current, the only thing you need to measure is the inductance. Also use a Shottky diode as it'll reduce the losses and alow the duty to be reduced even further. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez And http://www.silicontronics.com, same screen name as here.