Relay Driver Question

I have been thinking about building New York Daves Relay Driver Circuit to power some 4PCO and DPCO 24v relays
I plan to have multiple relays hanging off a single 24v DC line - no problem there...

I have been tearing my hair out trying to do the calculations for the resistors in the circuit
Here are my variables
2N4403
10mA - max 40v
100-300hfe
Info here
http://www.fairchildsemi.com/ds/2N/2N4403.pdf

DPCO 24v Relay
Resistance 1152R - current 20.8mA

4PCO 24v Relay
Resistance - 2000R - current 25mA

ANybody help me with the calculations - just a template will do fine - I can figure the rest out myself

just use an N channel mosfet and be done with it.

nothing but maybe a pulldown resistor (10k-47k) if your logic won't pull the gate down.

vcc to coil lead
drain to other coil lead
source to gnd
10k gate to source(pulldown)

12v switched to gate will turn it on and turn on relay.

MOSFETS are cheap and you can get one with the flywheel diode integral with the die, so no external parts!

you'll be much happier with it!

heck, if you need to do many you can get a multi-part device, or use a FET driver which is powered from one source and uses logic to switch outputs. you can get these is arrays of up to four in many logic styles.

take a look at microchip's TC4468: http://ww1.microchip.com/downloads/en/DeviceDoc/21425b.pdf

:thumb:

what calculations? You are thinking to hard about this circuit

It is just a switch circuit that should work with almost any PNP transistor hfe >= 200(most GP transistors are) I would say just looking at the circuit.

For more base drive you could scale down the R to 10k and 270ohms or less drive scale them up. but you should not have to what is drawn looks like it will be fine.

Tha good thing about switching relays with a circuit like this is that the switch currents in the remote wires to the circuit are lower than they would be if the relays were switched directly.

[quote author="Gus"]

Tha good thing about switching relays with a circuit like this is that the switch currents in the remote wires to the circuit are lower than they would be if the relays were switched directly.[/quote]

Is this the main reason for using a circuit like this for switching a relay?

[quote author="Gus"]what calculations? You are thinking to hard about this circuit
.[/quote]
I presumed that the resistors were set for a 12v circuit and I want to use it for a 24v circuit...

Change R1 to 150k to a 220K and build one on a breadboard and check it out.

jeeze.. talk about BJT love! one part is all you need.... :green:

come to the dark side.. you know you wanna..

the gate voltage calls you.. it beckons you come to the FET side!

[quote author="Svart"]jeeze.. talk about BJT love! one part is all you need.... :green:

come to the dark side.. you know you wanna..

the gate voltage calls you.. it beckons you come to the FET side![/quote]
I would have joined the dark side if that was the first thread that popped up
New York Dave beckoned me with his diagram - I can't help it - I 'm hooked

Thanks guys - of to design and build (soon)

There are many ways to wire relay circuits.

You can use an opto, a fet, a mosfet,NPN,PNP etc. Sometimes one way is better than another depends on the circuit and how close the relays are to sensitive parts of the circuit etc.

speaking of relays....

This ckt okay?:



So when the switch is pressed the signal is muted and the LED should light - right?

Ta :thumb:

Youse guys are all over-thinking the problem.

Junction, FET, resistors.... why the hell do you need a transistor or diode at ALL?

A cheap switch is rated 1A. The size of relay you need for Phantom or Phase or Assign is usually under 0.1A. Yeah, the inductive kick will arc the contacts, but at these power levels a switch can handle that.

It works in your car. The key-switch energizes a power relay ("Starter solenoid") that not only carries 200 Amps, it often also kicks the starter gear into the flywheel. The relays we use for switching little milliAmp or microAmp audio signals are a lot easier to drive.



Pick a relay that matches your available voltage. "5V" will work 4V to 6V, "12V" will work 10V to 14V, "24V" will work 20V to 28V.

If you must have an LED, you can pick an operating current and subtract the LED voltage from the supply voltage to arrive at a resistor value. Or just multiply Volts times 100Ω, tack in a resistor that size, and then fudge up or down to get less or more brightness.

:green: PRR is sadly right.. we just jumped to conclusions!

however do you need a relay at all? you can use analog switches (semiconductor parts) too, however I don't know what kind of noise or artifacts they might introduce to audio.. :thumb:

Yeah, I think the xsistor is overkill unless you are trying to switch LEDs/Relays/etc from something like the output of a uController. Then you would want to have the transistor controlling the large "work" load from the small crontrol load of the chip. I've used the same method that PRR posted a few times in mic pres with success so far.

Ian

Right. The driver circuit I posted is from a larger circuit I designed at my job. One contact closure from a dial-up DTMF remote control unit had to switch a whole mess o' relays.

PRR is right.

I did not know what the circuit was for so I stayed with the BJT.

Like I posted if circuits are sensitive to switching currents because the relay coil power wires run close to the signal wires solid state switching of relays can sometimes help. One good practice is to run wires that might cause problems at 90 degrees to each other.

Also remember the relay coil causes noise at power on and power off. There are ways to work around that. I learned alot when I built a relay 6 loop box for a friends effects petals I got it to switch without any relay coil noise and it was not the circuit you read in books they don't work like written up.

Uhhh ...

I've used that BJT circuit a millions times but with a capacitor added in. I thought the reason for the circuit was to avoid 'ticks' and 'pops' getting into the audio stream caused by contact bounce on the the switch. No????


Best Regards,
Carl Huff

I want to use the circuit for this reason...
I am a newbie and am just getting to grips with the requirements
I want to replace 2 and 4 pole switches on the calrec
I wanted to have a nice pushbutton (NO) with a 24v inside for cosmetic reasons
So i got a load of decent (v.cheap) 4PCO and DPCO relays
I will wire the switch to switch the relay and lamp on - the wire the contact side of the relay onto the holes on the Calrec board which house the 4 pole and 2 pole switches

I did a bit of background research and asked a few questions (thanks Jakob) and then found NY Daves circuit

I have built a 24v PSU to power the relays and lamps - and I was going to etch my own board for the relays