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Scr Voltage Regulator


bobmcree

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i want to design a simple inexpensive battery charger that will charge a NiMH string of 60-70 cells at 5A. i would like to power the system from 110 or 220vac. the termination voltage of 70 cells will be 100V, so what i want is a preregulator with an output of 105v. i have considered various switcher topologies, but it seems like an scr preregulator would be a good choice at this voltage and power level.

 

the end of charge detection is by temperature sensor, so great regulation of the charge voltage/current is not required. the major design limit is maximum output voltage of the pre-regulator, as with more than 110v output the rating of the secondary regulator stage will be exceeded if it is to have enough range to charge the battery at the required rate. the battery charge voltage will range from 70v-100v. i would clamp the output of the preregulator stage at 105v, but i don't want to have to clamp a lot of power.

 

i would like to use an lm338k as the secondary regulator, configured as a 5A current source. i intend to clamp its output at 70v to protect the regulator and to precharge a completely dead battery at a low rate up to 70v.

 

i do not have a lot of experience designing scr voltage regulator circuits, and i welcome any input or any suggested references. this is a small demand niche product but one i want enough for myself to build a few for others and bring the cost down.

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Hello bobmcree

 

Welcome to the forum.

 

You could use an SCR controlled bridge as a preregulator, but there are some issues to consider.

 

When the SCR is turned ON, it acts like a diode and there is minimal voltage drop across it.

In order to reduce the voltage down, you "phase" control the SCRs. That means you delay the turn ON of the SCR after the voltage zero crossing.

Hence the SCR is fully ON for part of the cycle.

If we delay the turn ON of the SCR by by 90 degrees, then the SCR will turn ON at the peak of the voltage waveform and the peak voltage will be the same. The average voltage will be halved. If you are feeding this into some form of controlled regulator, or into the batteries, you will need to apply some serious smoothing between the preregulator and the load in order to minimise damage due to the peak voltages and currents.

 

The SCR controller provides a reduction in average voltage and current, but not necessarily in the peak voltage and current.

 

Are you still interested??

 

Best regards,

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Hello bobmcree

 

Welcome to the forum.

 

You could use an SCR controlled bridge as a preregulator, but there are some issues to consider.

 

When the SCR is turned ON, it acts like a diode and there is minimal voltage drop across it.

In order to reduce the voltage down, you "phase" control the SCRs. That means you delay the turn ON of the SCR after the voltage zero crossing.

Hence the SCR is fully ON for part of the cycle.

If we delay the turn ON of the SCR by by 90 degrees, then the SCR will turn ON at the peak of the voltage waveform and the peak voltage will be the same. The average voltage will be halved. If you are feeding this into some form of controlled regulator, or into the batteries, you will need to apply some serious smoothing between the preregulator and the load in order to minimise damage due to the peak voltages and currents.

 

The SCR controller provides a reduction in average voltage and current, but not necessarily in the peak voltage and current.

 

Are you still interested??

 

thanks, that's about what i knew about scr's. i thought i could switch on at the zero crossing and switch off when the voltage reached my target, and regulate that way? Of course i would put in some filtering and can clamp short spikes, but i thought an scr regulator could be used this way; that if i adjust the gate voltage with feedback i can use it to produce a rectified output, and that in the 500w range this might be an efficient way to proceed..thanks for the advice and the welcome. there wasn't much on scr's in the switching power supply classes i took, and i'm trying to avoid a big inductor if i can.

 

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Hello Bobmcree

 

With SCRs, you can not turn them OFF, they turn OFF when the current passes through zero. To control them, you turn them ON late using the gate. This is known as phase control because you control them ON at a phase angle after the voltage zero crossing.

 

Using the SCRs for pre-regulation, you will need to have good capacitance somewhere to store the energy to fill the gaps where the SCRs are not conducting.

Certainly, you can use a controlled rectifier to give you an roughly regulated DC voltage, but you will need to be careful as you could easily end up with an overvoltage situation. If your input is low (or missing) the preregulator will phase the SCR firing forward for full conduction. If the voltage suddenly increases (or is applied) you will have full voltage which could damage your following regulator circuit.

If you have a long time constant in the charging circuit to the capacitors, you will have a usable reaction time and can phase back the next cycle to reduce the overvoltage situation.

The best way to achieve this is a "large" inductor before the capacitors.

 

Best regards,

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