# Massive power supply 80V 20A with SCR possible?

#### DieselPower

Joined Sep 24, 2009
2
Hi fellows, I am building a CNC controller, and need to build a power supply for the steppers. I need an unregulated supply of 80V max at 20 ampers DC. So I'm looking for cheap ways to regulate 120vac down to 80vdc. It has been years since I played with an SCR, but I have a couple 40A 600V SCR's in my box.

I plan to use the SCR's as switchable rectifiers to charge a cap bank up to 80V.

Is this even possible with SCR's?

What kind of trigger circuit would I need to control the scr based on voltage stored in a large cap? 80V is MAX, down to 50 under load is OK but not desirable. I **can** go to a microcontroller IF I have to...

Will just a simple pot from the cap+ & ground to the trigger work to regulate the voltage?

Many thanks for a solution.

BTW, I am quite familiar with electronics, so don't try to water it down too much!

Thanks!

LS

#### JDT

Joined Feb 12, 2009
658
The easiest way to get 80VDC is to use a transformer, bridge rectifier and smoothing capacitor combination. This is also safer, as the output is isolated from the mains supply.

Massive transformer required though. Fairly easy to get transformers wound to your specification. Rectifier will need cooling. See diagram.

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#### bluegreen

Joined Jul 22, 2009
15
<Diesel Power> Why not just use rectifiers(diodes). I assume you are using the capacitor bank to remove the AC ripple.

#### JDT

Joined Feb 12, 2009
658

Here is an SCR circuit.

You will need the inductor to minimise and smooth the SCR current. The gate signals have small pulse transformers for isolation. You will need a controller. You can get these ready made.

The DC output is not only not-isolated but is not connected to neutral either.

Full-wave rectification is essential otherwise your 20A DC current will be going around the mains supply wiring and equipment and will probably cause damage to switches and circuit breakers. As well as being VERY BAD!

The good thing about this circuit is that it will produce a regulated output. To do it properly it needs this circuit plus an isolating transformer! The isolating transformer and the inductor can be combined by using a leaky transformer like used in welders.

The other solution is by using a switch-mode approach with a high-frequency transformer. This would be an advanced electronics design and would be easier and cheaper to buy ready made!

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#### bluegreen

Joined Jul 22, 2009
15
The problem with using commercally available ICs for regulation is that they are rated for a much lower current than what you require.

#### BillB3857

Joined Feb 28, 2009
2,497
A company called Dynapower makes such units. They are used in the plating and wetcell battery industry. http://www.dynapower.com/

#### Papabravo

Joined Feb 24, 2006
12,407
I consider the chances of your being able to construct such a supply with appropriate performance at a reasonable cost in time and material to be exceedingly remote. The high cost item will be the transformer. It will be a massive piece of laminated iron with windings of 12AWG wire. I estimate the cost at $90-$150, but ask these guys they are the experts.

http://www.electromechinc.com/products/transformers.html

Beefy silicon rectifiers are no problem, and I don't see what using an SCR will buy you. The capacitors with a suitable working voltage will also be expensive long lead time items unless you can find some in stock somewhere. Be careful about applying high voltage to capacitors which may have been sitting around for a while. They have little plastic pressure relief plugs and have been know to react dramatically when hit with modest voltage for the first time.

To use it you will have to have it on a separate circuit since it will dra just under 20A from the mains and blow a standard breaker if ther is any other load on the circuit.