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Rugged PSU For Ham Radio Transceivers(LM338 )

2017-08-05 15:54  
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This article briefly describes the Rugged PSU For Ham Radio Transceivers (LM338). This principle is easy to understand, but also very practical. Depth understanding of circuit elements, you can better grasp this principle. In this circuit, you can learn about and purchase these components: LM338.

This rugged power supply is based on the popular LM338 3-pin voltage regulator. The LM338 is capable of supplying 5 A over an output voltage range of 1.2 V to 32 V with all standard protections like overload, thermal shutdown, over-current, internal limit, etc., built in. In this power supply, some extra protections have been added to make it particularly suitable for use with low to medium-power portable and mobileVHF/UHF (ham) and 27 MHz transceivers. Diodes D4 and D5 provide a discharge path for capacitors C1 and C2. Diode D8 protects the supply against reverse polarity being applied to the output terminals. Capacitor C1 assists in RF decoupling and also increases the ripple rejection from 60 dB to about 86 dB.

Figure:1 Circuit diagram

Figure 1 Circuit diagram

If junction R1-R2 is not grounded by switch S1A, transistor T2 starts to conduct, causing the regulator to switch to zener diode D7 for its reference voltage (13 V). ThePSUoutput voltage will then be 12.3 V. Normally, T2 will be off, however, and thePSUoutput voltage is then about 8.8 V. The high/low switch is useful to control the RF power level of modernVHF/UHF handhelds. Transistor T1, a p-n-p type BC557, acts as a blown-fuse sensor. When fuse F1 melts, T1 starts to conduct, causingLEDD6 to light. If, for whatever reason, thePSUoutput voltage exceeds about 15 V, thyristor THR1 is triggered (typically in less than a microsecond).

Such a high-speed ‘crowbar’ may look like a drastic measure, but remember that this kind of protection is required by digital ICs that will not stand much overvoltage. The crowbar, when actuated, will faithfully destroy fuse F1 rather than allow thePSUto destroy expensive ICs. The twoLEDs on the S1B contacts not only act as ‘high/low’ indicators but also as power-on indicators which are turned off when the mains voltage drops below about 160 V. If you envisage ‘heavy-duty’ use of thePSU, then voltage regulator IC1 should be mounted on as large a heatsink as you can get. The minimum we’d say is an SK129 heatsink from Fischer (Dau Components).

 

 


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