Looking to fit a resistor/relay combination in the supply line of a
3600VA 240v isolation transformer. I reckon the resistor should be 8
ohms - based on 200% loading, but anyone know what wattage is
required???

Thanks.

CS wrote:
Looking to fit a resistor/relay combination in the supply line of a
3600VA 240v isolation transformer. I reckon the resistor should be 8
ohms - based on 200% loading, but anyone know what wattage is
required???

Thanks.


It is difficult, as you realize, to calculate wattage for the resistor
without some knowledge of the transient it will see. If you go with a
resistor, you might start with several in parallel, so as to provide
improved heat dissipation. If the inrush duration is on the order of 10
mS, I guess I'd use maybe four 36 ohm, 50 watt resistors in parallel to
start with, and go from there.

For 3.6 kVA transformers, NTC thermistors are perhaps more popular these
days than simple resistors. Here's a link that describes the design process:

http://www.epcos.com/web/generator/W...hermistors.pdf
NTCThermistors.pdf

Good luck,

Chuck


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CS wrote:

Looking to fit a resistor/relay combination in the supply line of a
3600VA 240v isolation transformer. I reckon the resistor should be 8
ohms - based on 200% loading, but anyone know what wattage is
required???

Thanks.

Consider what happens if the relay fails to close. What is the maximum
wattage the resistor could dissipate with a short on the secondary of
the transformer? What will protect the circuit? If the protection
isn't onboard, you need to rate the resistor to stand 240V continuosly
without becoming a fire risk. That is difficult. I'd either use a much
higher value resistor and a second relay on the secondary side
controlled by spare contacts on the first relay to sequence the
application of the loads after the first relay has closed or an inline
NTC thermistor based solution (which still requires the relay to let it
cool again while the supply is on and needs adequate overcurrent
protection as well).

Google 'inrush current limiting' for some ideas.

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & 32K emails -- NUL:
'Stingo' Albacore #1554 - 15' Early 60's, Uffa Fox designed,
All varnished hot moulded wooden racing dinghy.

"Ian Malcolm" wrote in message
...
CS wrote:

Looking to fit a resistor/relay combination in the supply line of a
3600VA 240v isolation transformer. I reckon the resistor should be 8
ohms - based on 200% loading, but anyone know what wattage is
required???

Thanks.

Consider what happens if the relay fails to close. What is the maximum
wattage the resistor could dissipate with a short on the secondary of
the transformer?

What about using a light bulb?

Meindert

"Meindert Sprang" wrote in
:

What about using a light bulb?

Meindert




Shhh, Meindert! Too simple. I was watching to see what kind of massive
controller it was going to develop into. Now you've spoiled it....(c;

The other day someone asked me why that light was lit in my van so dim. I
told him I was charging my 9V electric screwdriver. I use light bulbs in
series all the time to charge Ni-Mh batteries in the truck.

Larry
--
Democracy is when two wolves and a sheep vote on who's for dinner.
Liberty is when the sheep has his own gun.

Larry wrote:
"Meindert Sprang" wrote in
:


What about using a light bulb?

Meindert





Shhh, Meindert! Too simple. I was watching to see what kind of massive
controller it was going to develop into. Now you've spoiled it....(c;

The other day someone asked me why that light was lit in my van so dim. I
told him I was charging my 9V electric screwdriver. I use light bulbs in
series all the time to charge Ni-Mh batteries in the truck.

Larry
The thought had crossed my mind, but I thought the possible outcome of a
*cheap* bulb in series with a large inductor could be *interesting*. I
have had several Bulgarian made supermarket own brand bulbs explode at
switch on. If you pick the right tipe of bulb, it *would* work, but
whats wrong with a PTC thermistor or wirewound resistor?

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & 32K emails -- NUL:
'Stingo' Albacore #1554 - 15' Early 60's, Uffa Fox designed,
All varnished hot moulded wooden racing dinghy.

In article , chuck wrote:
CS wrote:
Looking to fit a resistor/relay combination in the supply line of a
3600VA 240v isolation transformer. I reckon the resistor should be 8
ohms - based on 200% loading, but anyone know what wattage is
required???

Thanks.


It is difficult, as you realize, to calculate wattage for the resistor
without some knowledge of the transient it will see. If you go with a
resistor, you might start with several in parallel, so as to provide
improved heat dissipation. If the inrush duration is on the order of 10
mS, I guess I'd use maybe four 36 ohm, 50 watt resistors in parallel to
start with, and go from there.

For 3.6 kVA transformers, NTC thermistors are perhaps more popular these
days than simple resistors. Here's a link that describes the design process:

http://www.epcos.com/web/generator/W...nlinearResisto
rs/NTCThermistors/PDF/NTCThermistors,property=Data__en.pdf;/NTCThermistors.pdf
NTCThermistors.pdf

Beside NTC, a quick look turned up other possible devices to do this.

greg