# Transistor relay circuit

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#### more_power

I am looking to use a couple of transistors to replace a relay I am using in my automobile to turn on signal processors and amplifiers. The relay has a loud click when energized and I saw a fairly simple circuit which employed an NPN and PNP transistor that would accmplish this. Does anyone have a schematic for something like this?

Thanks,

Bill

#### palesha

It is a single NPN tr circuit. Just give ur + supply voltage to collector & take the output from emitter. To on the device put 1k resistance in series with base of the transistor. Give the + dc supply to one contact of switch and connect the other contact of the switch to 1k resistor on the other side of base. Select the to-3 transistor of 3 times the current required and mount on a heatsink. I have done this successfully. Hope u understand.

#### theChris

how would you make a circut to turn on and off an audio signal to an op amp. I assume you connect a +12v power supply to the collector, the AC audio signal to the base, and take output from the emitter. do i still use NPN? or can i do this with a regualr transistor? will i still use a 1kohm resistor?

#### Electro

A transistor is nothing more than a electronic switch. To switch on a transistor you need to know the gain of the transistor. Also you need to locate the base pin on the transistor and the voltage. After know those parameters, making a transistor to turn on is very simple.

I like using darlington transistors for switching motors or coils on and off because it uses very little current to turn on the transistor. A TIP120 NPN Darlington transistor has a gain of 1000 at 3 amperes.

To calculate the the base current:

beta = gain

Ib = Ic / beta

0.003 = 3 / 1000

After knowing base current, it is a little easier to find out what resistor to switch the transistor on.

Rb = (Vin - Vbe) / Ib

Rb = resistor connected to base pin to switch transistor on
Vin = supply voltage or 12 volts
Vbe = silicon turn on voltage (~0.6 volts)

Rb = (12 volts - 0.6 volts) / 0.003 amperes
Rb = 3800 ohms

When using any device that has coils, use a diode across the coiled device. If you don't, the transistor or MOSFET will fry from EMFs that are generated by the coiled device (relay, loudspeaker, motors, etc). A diode that has a PIV (peak inverse voltage) of 400 volts should last very long.

#### theChris

wow, that was a quick reply. ok, i think i understand. I will have to find some parts. I kinda want to make an audiophile type setup (kinda cheeply too). I saw that metal film resistors have 1% tolarances, and some capacitrors have 5% tolarances. is there any differance in tranistors and signal quality. are germainium any better than silicone? are J-FETs any better than BiPolar or Darlington?

#### Electro

I suggest you use a relay instead a transistor or MOSFET for switching off or on an audio path. Using relays doesn't saccfice sound quality. In your situation, a relay will be good and a transistor to energize the relay. Using an active solid-state device instead of relay to switch audio does saccfice sound quality. There are solid state relays but they should not be used for audio.

When selecting relays, try to find gold plated. This is because over time the connections will get oxidized. Reed relays might work but test before making a pernament discision.

Also you don't need very accurate resistors. You can pick a 4700 ohm resistor as a subsitute from the example that I gave in previous reply. In digital circuits most of the resistors are 5% tolerance.

If you really want a solid-state switch use a DIAC for DC circuits and TRIAC for AC. Don't forget to use a heatsink too.

#### palesha

I was not suggesting to use transistor in the signal path. It was to be used in the powersupply section. This has to be used as a transistor switch instead of mechanical switch which generates noise.

#### Electro

I was not suggesting to use transistor in the signal path. It was to be used in the powersupply section. This has to be used as a transistor switch instead of mechanical switch which generates noise.
This will be great if the op-amp uses a single power supply. These days op-amps uses dual power supply so you need two transistors.

In more_power first post he or she states:
...turn on signal processors and amplifiers...

Using transistors to switch on current hogging amplifiers will have to dissipate a lot of heat. Relays do dissipate heat when energized. This is only when the coil is consuming a lot of current. There are relays that have 5 volts coils and can handle several amperes through the switch of the relay. However, the a 5 volt relay will need a resister so the coil doesn't overheat.

#### JKLman

Using transistors to switch on current hogging amplifiers will have to dissipate a lot of heat....

more_power is using this for his car audio stuff. And to turn on car audio stuff you apply a small current to the "turn on led" of the unit that is going to be turned on and it will trigger a relay or something inside the unit.
When you have alot of units you dont want to take all the current from the main unit to turn on all the other units so you use a relay.
There isnt much current going to each unit (but you still dont want to bog down the main unit) so one could use a very small relay, probably with almost silent click or a transistor.
I dont think he want to run all current for every unit through a transistor, that would most sertainly make alot of heat as you say.

Use a smaller relay and you could probably put some padding around it to get it silent. Or build a transistor switch.

#### theChris

well, i never mentioned a car. a friend told me about a place where you can get old AT power supplies. they (or some) don't have a minimum turn on load, and have power switches on them to turn on. (oh well, there goes my 1 switch design i guess). these power supplies have +12 and -12 connectors on them, and are already regulated.

also, there is only 1 op amp in this cicut, how much current is going to be going through the transistors? assuming i use them only for a power supply. also, you said that transistor add distortion to the signal (which i do beleive), how much is added? I mean, is it a noticable amount to the untrained ear, or just a slight differance. I had assumed a relay would be worse on an audio signal because of the inductors.

as for the car stuff though, if the distortion isn't really noticable, i might make some neat stuff for my car too. I don't really feel like spending a lot of money on a lot of relays for my car, when a transitor (may be) is almost the same.

#### AudioFreak

the audio signal does not pass thru the coil (inductor) of the relay and so very little damage is done to the signal..... transistors do quite a bit of audible damage.

#### theChris

but then why doesn't an amplifier produce a huge amount of noise? i thought that most amplifiers used transistors in them. but ok. i think i get the idea, use relays if i can, transistors only for other signals.

#### AudioFreak

The idea is this...

If you use a remote soft start circuit, you can use a transistor to trigger this. Use relays for the power supply rails to minimize heat and relays for the signal input switching / output protection etc just use transistors (BJT or FET) when you need a buffer, gain stage etc where transistors can not be avoided.

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