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Author Topic: first time homebrewer project  (Read 3075 times)
AK4YA
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Posts: 106




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« on: August 29, 2012, 11:04:08 AM »

So Ive never made anything before and and am considering trying to construct a Tx/Rx relay circuit.
My radio's PTT output is an open drain, its limitations are 48 VDC, 500 mA max.  My understanding of this type of circuit is that it presents a high impedance on the output when PTT is not set, and when it is set, it presents a ground to the output?
 
So, if I were to wire up one side of a 12vdc/160mA coax relay to +48vdc, and then use a 225 ohm resistor in between the other side of the relay and my radio, would this be a good way to handle the circuit, assuming my relay power source and the radio share the same ground?  This would have the relay dropping 12v and the resistor 36v/5.76watts.  Possibly could also use a .5Amp fuse inline as well?

Thanks for the help
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WX7G
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« Reply #1 on: August 29, 2012, 11:13:41 AM »

Yes that is correct. A 220 ohm resistor capable of dissipating 6 watts or more will work. I would use 10 watts.

When the relay is deenergized the relay coil inductance will continue to pump current and the transistor collector voltage will soar (and the transistor may be damaged). To prevent this connect a diode across the relay coil, cathode end to the 48 VDC supply. Diodes often used for this are the 1N914 or 1N4001 type.
« Last Edit: August 29, 2012, 11:16:21 AM by WX7G » Logged
WD8AJY
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Posts: 63




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« Reply #2 on: August 29, 2012, 11:46:41 AM »

why not use 12V then you will not need the resistor.

73 Bob.
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G3RZP
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« Reply #3 on: August 30, 2012, 01:06:03 AM »

Using a higher voltage and series resistor means the relay will pull in faster. The diode will slow the release, all of which has the advantage that the tx is less likely to see an open circuit while producing RF. I would use a bit less than 48 volts though.
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KA4POL
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« Reply #4 on: August 30, 2012, 07:34:49 AM »

I would certainly consider a safety margin and not use 48V for the relay circuit. The anti-parallel diode is a must else you'll kill the FET sooner or later. More critical is the TX power getting into the RX if its entrance is not grounded yet. So this got to be considered.
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AA4PB
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« Reply #5 on: August 30, 2012, 01:02:12 PM »

48 VDC, 500 mA max means that the MAXIMUM voltage is 48VDC and the MAXIMUM current is 500mA. You can use any voltage that is less than 48VDC so with a 12VDC relay I would just use a 12VDC source - which you probably already have available as the power supply powering the radio. Make sure your relay current at 12VDC is well below 500mA - certainly not over 250mA.

Definatly add the protection diode to protect the FET. It can be wired with the cathode to the relay-FET junction (PTT line) and the anode to ground.
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KA4POL
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« Reply #6 on: August 30, 2012, 10:30:25 PM »

Just to give you an idea: http://www.ifwtech.co.uk/g3sek/dx-book/sequencer/index.htm
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W8JI
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« Reply #7 on: August 31, 2012, 04:13:22 AM »

So Ive never made anything before and and am considering trying to construct a Tx/Rx relay circuit.
My radio's PTT output is an open drain, its limitations are 48 VDC, 500 mA max.  My understanding of this type of circuit is that it presents a high impedance on the output when PTT is not set, and when it is set, it presents a ground to the output?

They are giving the maximum open voltage with no current, and maximum current with no voltage.

This means you have two limits:

48 volts (probably positive) with relay off.

500 mA with near zero volts with relay active.
 
Quote
So, if I were to wire up one side of a 12vdc/160mA coax relay to +48vdc, and then use a 225 ohm resistor in between the other side of the relay and my radio, would this be a good way to handle the circuit, assuming my relay power source and the radio share the same ground?  This would have the relay dropping 12v and the resistor 36v/5.76watts.  Possibly could also use a .5Amp fuse inline as well?

You are switching an inductive load with a relay. Current starts slow and then reaches the dc level, but when the current is interrupted the inductance tries to keep current flowing. This means the relay coil starts adding to the voltage in an attempt to maintain current. The relay could add 50-100 volts or more back in series with the original 48 volts, so you could have over 150 volts back-pulse. Other than that, your calculations are good.

Without a back-pulse diode on the relay coil, you run a risk of back-pulse damage. Also, you are too close to the rating anyway. I'd use a lower voltage source.

By the way, many relay companies now have relays made in China as we close factories and send jobs overseas. The relays used to be very consistent in current when domestically manufactured, but now we are seeing relays vary over 50% in current or coil resistance. When the correct the current on a rejected sample, the screw up the pull-in voltage. You have to be careful with relays from the big C even from formerly reputable relay vendors, because they might not be what the ratings say.
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