cheap electronic relay circuit

[CampinGazz]

i need to build a number of circuits that can replace relays,

the voltage will be 12 volts, the load to be switched is a max of 1.5 amps,
the trigger viltage will be 12 volts, and a max current of 50Ma.

Basicaly i have a 8 way light switch (tough panel) that has latching ouputs,
that shoud power relays to switch the lights on and off, but the system
needs to be economical on the power, so having 8 relays drawing power must
be avoided,

it's all Direct current BTW.

i need something simple to build, that can replace a relay, i knwo you can
buy solid state relays, but they are too expensive for this project,

was told about SCR's, but they latch and need the power removing from the
load to turn off, i need something that will allow power to flow between 2
points when voltage is applied to another point, and interupt the power flow
when the voltage is removed from that point.. just like a relay, but
electronic.. mosfet circuit??

 

[Rolavine]

ubject: cheap electronic relay circuit

From: "CampinGazz" [email protected]
Date: 1/26/2004 5:02 PM Pacific Standard Time
Message-id: <[email protected]>

i need to build a number of circuits that can replace relays,

the voltage will be 12 volts, the load to be switched is a max of 1.5 amps,
the trigger viltage will be 12 volts, and a max current of 50Ma.

Basicaly i have a 8 way light switch (tough panel) that has latching ouputs,
that shoud power relays to switch the lights on and off, but the system
needs to be economical on the power, so having 8 relays drawing power must
be avoided,

it's all Direct current BTW.

i need something simple to build, that can replace a relay, i knwo you can
buy solid state relays, but they are too expensive for this project,

was told about SCR's, but they latch and need the power removing from the
load to turn off, i need something that will allow power to flow between 2
points when voltage is applied to another point, and interupt the power flow
when the voltage is removed from that point.. just like a relay, but
electronic.. mosfet circuit??

If you can stand the drop why not use PNP darlington transistor like a TIP125.
Connect the emitter to the +12, and the collector to your load to ground. With
an open to the base they are off, pull the base low via a resistor to about a
volt below the 12, and it turns on. I think this sounds like what you need.
TIP125's can be had in quantity for about 50 cents each.

Rocky

 

[Rheilly Phoull]

i need to build a number of circuits that can replace relays,

the voltage will be 12 volts, the load to be switched is a max of 1.5 amps,
the trigger viltage will be 12 volts, and a max current of 50Ma.

Basicaly i have a 8 way light switch (tough panel) that has latching ouputs,
that shoud power relays to switch the lights on and off, but the system
needs to be economical on the power, so having 8 relays drawing power must
be avoided,

it's all Direct current BTW.

i need something simple to build, that can replace a relay, i knwo you can
buy solid state relays, but they are too expensive for this project,

was told about SCR's, but they latch and need the power removing from the
load to turn off, i need something that will allow power to flow between 2
points when voltage is applied to another point, and interupt the power flow
when the voltage is removed from that point.. just like a relay, but
electronic.. mosfet circuit??

Yup, you need the Mosfet's !!

 

[Robert C Monsen]

i need to build a number of circuits that can replace relays,

the voltage will be 12 volts, the load to be switched is a max of 1.5 amps,
the trigger viltage will be 12 volts, and a max current of 50Ma.

Basicaly i have a 8 way light switch (tough panel) that has latching ouputs,
that shoud power relays to switch the lights on and off, but the system
needs to be economical on the power, so having 8 relays drawing power must
be avoided,

You can use latching relays. They need current only when changing their
state.

If you don't like that, use power mosfets. Just apply the 'trigger viltage'
to the gate. Most mosfets have three pins, a gate, a source, and a drain.

An N-channel MOSFET whose source pin is 0V will turn on (its resistance will
go to near 0) when the gate goes from 0 to 12V. You connect your load
between 12V and the drain pin. The gate doesn't need any current, just the
voltage, so its economical on power.

1.5A is not alot of power for a mosfet, but it might get hot, so a heatsink
would be required.

In the US, you can get them at Radio Shack, or online at places like
www.allcorp.com or www.goldmine-elec.com

Regards,
Bob Monsen

 

[Rolavine]

Subject: Re: cheap electronic relay circuit

From: "Robert C Monsen" [email protected]
Date: 1/27/2004 2:10 AM Pacific Standard Time
Message-id: <POqRb.124115$Rc4.975921@attbi_s54>




You can use latching relays. They need current only when changing their
state.

If you don't like that, use power mosfets. Just apply the 'trigger viltage'
to the gate. Most mosfets have three pins, a gate, a source, and a drain.

An N-channel MOSFET whose source pin is 0V will turn on (its resistance will
go to near 0) when the gate goes from 0 to 12V. You connect your load
between 12V and the drain pin. The gate doesn't need any current, just the
voltage, so its economical on power.

1.5A is not alot of power for a mosfet, but it might get hot, so a heatsink
would be required.

In the US, you can get them at Radio Shack, or online at places like
www.allcorp.com or www.goldmine-elec.com

Regards,
Bob Monsen

If you want to use an N channel logic level mosfet you will need about 17 volts
to apply to the gate to turn them on. This is because your switching the 'high
side'. Using a P channel mostet makes more sense for you.

Since you seem unsure of how to design this, you might want to look at "High
Side" Driver ICs, this will make life easier for you. These usually have some
protection built in which is a nice feature. On a switched power circuit,
unless the signal is isolated to the same circuit board I always put protection
on it (well unless it's got to be cheaper) I just designed some International
Rectifier high sides into some aircraft electronics, and they worked great.
Digi Key has them.

Rocky


Rocky

Good luck.

 

[Spehro Pefhany]

Since you seem unsure of how to design this, you might want to look at "High
Side" Driver ICs, this will make life easier for you. These usually have some
protection built in which is a nice feature. On a switched power circuit,
unless the signal is isolated to the same circuit board I always put protection
on it (well unless it's got to be cheaper) I just designed some International
Rectifier high sides into some aircraft electronics, and they worked great.
Digi Key has them.

Keep in mind that with most of these you can't have the "relay"
continously "on" because of the way the bootstrap supply works.

Best regards,
Spehro Pefhany

 

[Fred Bloggs]

i need to build a number of circuits that can replace relays,

the voltage will be 12 volts, the load to be switched is a max of 1.5 amps,
the trigger viltage will be 12 volts, and a max current of 50Ma.

Basicaly i have a 8 way light switch (tough panel) that has latching ouputs,
that shoud power relays to switch the lights on and off, but the system
needs to be economical on the power, so having 8 relays drawing power must
be avoided,

it's all Direct current BTW.

i need something simple to build, that can replace a relay, i knwo you can
buy solid state relays, but they are too expensive for this project,

was told about SCR's, but they latch and need the power removing from the
load to turn off, i need something that will allow power to flow between 2
points when voltage is applied to another point, and interupt the power flow
when the voltage is removed from that point.. just like a relay, but
electronic.. mosfet circuit??

If you are switching 1.5A loads, then it makes no sense to skimp over
20mA of relay drive- that is 0,020 Amps :

Please view in a fixed-width font such as Courier.

----relay------
| |
| / |
+--|----o o-----|---------> to load
| | |
12VDC>--+--|------+ |
| | | |
| | )|| |
| | )|| |
| | )|| |
| | )|| |
| | | |
| ---------------
| |
| |
| |
| 1N4001 |
+--|<|----+
|
|
c
3.3K |/
touch panel>--/\/\--| 2N4401
output |\
e
|
|
---

 

[John Woodgate]

I read in sci.electronics.design that Fred Bloggs <[email protected]>

If you are switching 1.5A loads, then it makes no sense to skimp over
20mA of relay drive- that is 0,020 Amps :

You could tell him how to reduce it, even so, with an RC parallel
network in series with the relay.

 

[CampinGazz]

I read in sci.electronics.design that Fred Bloggs <[email protected]>


You could tell him how to reduce it, even so, with an RC parallel
network in series with the relay.

OK, i'm lost now,

i guess i was assuming i'd use the standard automotive type relays, which
pull 180 Ma for the coil,
the largest load will be 1.2 amps for a standard 12 volt fluorescent tube,
smallest 50ma or so for a small led array, the rest 700ma for the ccfl's,

i'd also have a few other items i want to switch in the van where i could do
with loosing a relay to a solid state circuit,

but the only voltage i'll have onboard will be 12 volts (14.8 when the
charger is on line.. but most of the time it'll be 12.8 volts or lower as
the battery gets drained)

simple is what i'm after idealy, i'd found a simple circuit that just uses a
npn bipolar transistor and a 47k resistor to switch the negative side of a
load on when +12 volts is applied to one of the transistors legs through the
resistor,
this particular circuit is to switch a relay on.. but is done to present a
very low drain current to the power source turning the relay on.. in this
case a car stereo's power antenna output, that must be no more than 500ma,
apparantly it presents a 0.005 amp draw to the switch source,

that circuit handles 400ma, so i'd need something beefier for my circuit..
but i'd idealy like to switch the positive side rather than the negative
side of the circuits,
http://www.bcae1.com/circuits.htm is the page with that circuit on, half way
down.. titled "EXCESS JUNK"
that's where i got the origional idea from,

i was hoping to do something similar with a larger transistor and maybe a
few more components to switch the pos side,

on boats they have solid state split charge units.. sold as zero volt drop
diodes.. basicaly they need to connect 2 batteries together only when the
engine is running, the old way was to use a skottky diode or 2, and split
the alternators output, but the resulting volt drops affected the charge
rates,
but now they use a few mosfets back to back in the zero volt drop units, and
they are switched when the engine is started, basicaly a high power solid
state relay, but i'd imagine there's more to the circuit than a few mosfets
in a heatsink case.

 

[Robert C Monsen]

If you want to use an N channel logic level mosfet you will need about 17 volts
to apply to the gate to turn them on. This is because your switching the 'high
side'. Using a P channel mostet makes more sense for you.

Since you seem unsure of how to design this, you might want to look at "High
Side" Driver ICs, this will make life easier for you. These usually have some
protection built in which is a nice feature. On a switched power circuit,
unless the signal is isolated to the same circuit board I always put protection
on it (well unless it's got to be cheaper) I just designed some International
Rectifier high sides into some aircraft electronics, and they worked great.
Digi Key has them.

Rocky

Why does he need high side? Seems simpler with N-MOSFETs, given his
activation signal (high = light on). He is just turning on lights. They can
live between the mosfets and the 12V supply, right?

In order to do high side, he'll need a way to invert the signal (ie, your
high side driver IC)

Regards,
Bob Monsen