WA2LTD
12-23-2006, 07:15 PM
Linear Amplifier Inrush Current Control and Soft Starts
Thomas G. Siglin – WA2LTD
Part Two
Earlier I described how to construct a very basic unit that would work for your linear amplifier. ICL can be used on other radio equipment besides the linear. A Drake®, Heath ® or Collins® transmitter/transceiver can benefit from this technology and the concept is extendable to 240 -Volt applications as well.
The on/off switch in our first model actually functioned as the means to bypass the main resistor and required the operator to perform the switching. That extra step is a disadvantage in the long run, possible causing burned out resistors or other difficulties (although a fuse or circuit breaker in series with the resistor would prevent this problem).
Let’s improve the basic ICL described in part one through the use of a relay to obtain semi-automatic operation. Begin by removing the resistor lead from the screw terminal where the black AC line cord is connected. Remove the jumper wire between the second switch screw and Brass screw of the outlet. Attach the free end of the resistor to the brass screw on the outlet. Now add a 120 -Volt relay by wiring the two coil connections to the outlet, one to the brass screw side and the other to the silver screw side. Wire one set of Normally Open contacts to bypass or short out the resistor. (That means one wire of the NO contacts to the brass screw of the outlet and the other contact to the screw of the switch where the resistor is connected)
At this point we have duplicated, in function, the MFJ/Ameritron® ICL-120© inrush current limiter (with out the surge protection)
Test the ICL by turning the switch on the ICL OFF and plugging the amplifier into the ICL. Turn ON the amplifier power switch. Flip the ICL switch ON and the linear will power up and very shortly afterwards the relay will pull in bypassing the main series resistor. This action happens quickly but provides two favorable actions:
1) It removes a substantial amount of inrush current
2) It saves the equipment on/off switch from wear and arcing
What it does not provide much of is soft starting. For that we need a delay before the relay pulls in. We can provide a short delay by adding a resistor in series with the relay coil. The relay used in this ICL was an NTE Electronics Inc: R10-11A10-120, DPDT, 120 VAC (coil = 2,250 Ohms with two sets of 10 Amp SPDT contacts which could be paralleled to handle increased current or the second set can bypass the relay thermister to prepare it for the next cycle).
Using a Variac® variable transformer to test with, the relay pulled in at a voltage of 80 VAC. To increase the pull in voltage, a 1,000 Ohm 1 Watt resistor was added in series with the coil. This resistor value was selected to let the relay energize with about 10% more voltage than minimum pull-in when used with the linear amplifier plugged into the ICL. This value may have to be shifted downward if the amplifier draws more than 100 Watt during warm-up/standby and does not allow the relay to energize or use an NTC thermister described below or the main series resistor can be reduced to 10 Ohms at 20 Watts. That change will also give you less voltage drop across the resistor and increase inrush current but it’s still better than the alternative.
Alan, W4LGH (2) pointed out the use of a thermister in place of the coil resistor. The1000 Ohm type SL05-10201 thermister on hand provided a delay when used instead of the 1,000 Ohm dropping resistor. The thermister should be quite small in current handling as the relay draws less than 3 Watts or the relay swamped with additional load to improve the thermister action i.e. add a 2500 Ohm, 5 Watt power resistor across the relay coil. Alternately the thermister can be added in series with the 1000 Ohm resistor to provide additional delay.
If you actually build this model with the AC relay and a resistor or thermister, you will notice the relay buzzes and chatters as it attempts to pull-in. “Bzzzzzz Chatterrrr CLICK”… an annoying sound! We’ll address that in part three when a DC relay will be installed and we add a time delay.
Note that once we move away from “pure” inrush current limiter control, towards soft start, we enter a domain that only analog type of equipment works well in. Delays when powering on digital equipment must by necessity be short. Internal microprocessor circuits want stable quickly. If your linear amplifier is digital, it’s likely the manufacture already incorporated some form of soft start. Other digital equipment should be tested carefully with the ICL to ensure stable operation.
Schematics and construction pictures available via eMail from: tsiglin@SIGTOMICS.com
Notes:
® Indicate trademarks of the respective owners
(2) Alan, W4LGH comment posted: Dec. 14 2006,13:57 on QRZ.com
Additional help with drawings provided by Scott Duckworth – NA4IT,
[B][B]
Thomas G. Siglin – WA2LTD
Part Two
Earlier I described how to construct a very basic unit that would work for your linear amplifier. ICL can be used on other radio equipment besides the linear. A Drake®, Heath ® or Collins® transmitter/transceiver can benefit from this technology and the concept is extendable to 240 -Volt applications as well.
The on/off switch in our first model actually functioned as the means to bypass the main resistor and required the operator to perform the switching. That extra step is a disadvantage in the long run, possible causing burned out resistors or other difficulties (although a fuse or circuit breaker in series with the resistor would prevent this problem).
Let’s improve the basic ICL described in part one through the use of a relay to obtain semi-automatic operation. Begin by removing the resistor lead from the screw terminal where the black AC line cord is connected. Remove the jumper wire between the second switch screw and Brass screw of the outlet. Attach the free end of the resistor to the brass screw on the outlet. Now add a 120 -Volt relay by wiring the two coil connections to the outlet, one to the brass screw side and the other to the silver screw side. Wire one set of Normally Open contacts to bypass or short out the resistor. (That means one wire of the NO contacts to the brass screw of the outlet and the other contact to the screw of the switch where the resistor is connected)
At this point we have duplicated, in function, the MFJ/Ameritron® ICL-120© inrush current limiter (with out the surge protection)
Test the ICL by turning the switch on the ICL OFF and plugging the amplifier into the ICL. Turn ON the amplifier power switch. Flip the ICL switch ON and the linear will power up and very shortly afterwards the relay will pull in bypassing the main series resistor. This action happens quickly but provides two favorable actions:
1) It removes a substantial amount of inrush current
2) It saves the equipment on/off switch from wear and arcing
What it does not provide much of is soft starting. For that we need a delay before the relay pulls in. We can provide a short delay by adding a resistor in series with the relay coil. The relay used in this ICL was an NTE Electronics Inc: R10-11A10-120, DPDT, 120 VAC (coil = 2,250 Ohms with two sets of 10 Amp SPDT contacts which could be paralleled to handle increased current or the second set can bypass the relay thermister to prepare it for the next cycle).
Using a Variac® variable transformer to test with, the relay pulled in at a voltage of 80 VAC. To increase the pull in voltage, a 1,000 Ohm 1 Watt resistor was added in series with the coil. This resistor value was selected to let the relay energize with about 10% more voltage than minimum pull-in when used with the linear amplifier plugged into the ICL. This value may have to be shifted downward if the amplifier draws more than 100 Watt during warm-up/standby and does not allow the relay to energize or use an NTC thermister described below or the main series resistor can be reduced to 10 Ohms at 20 Watts. That change will also give you less voltage drop across the resistor and increase inrush current but it’s still better than the alternative.
Alan, W4LGH (2) pointed out the use of a thermister in place of the coil resistor. The1000 Ohm type SL05-10201 thermister on hand provided a delay when used instead of the 1,000 Ohm dropping resistor. The thermister should be quite small in current handling as the relay draws less than 3 Watts or the relay swamped with additional load to improve the thermister action i.e. add a 2500 Ohm, 5 Watt power resistor across the relay coil. Alternately the thermister can be added in series with the 1000 Ohm resistor to provide additional delay.
If you actually build this model with the AC relay and a resistor or thermister, you will notice the relay buzzes and chatters as it attempts to pull-in. “Bzzzzzz Chatterrrr CLICK”… an annoying sound! We’ll address that in part three when a DC relay will be installed and we add a time delay.
Note that once we move away from “pure” inrush current limiter control, towards soft start, we enter a domain that only analog type of equipment works well in. Delays when powering on digital equipment must by necessity be short. Internal microprocessor circuits want stable quickly. If your linear amplifier is digital, it’s likely the manufacture already incorporated some form of soft start. Other digital equipment should be tested carefully with the ICL to ensure stable operation.
Schematics and construction pictures available via eMail from: tsiglin@SIGTOMICS.com
Notes:
® Indicate trademarks of the respective owners
(2) Alan, W4LGH comment posted: Dec. 14 2006,13:57 on QRZ.com
Additional help with drawings provided by Scott Duckworth – NA4IT,
[B][B]