Use SCR as rectifier

[RogerN]

To use an SCR as a rectifier can you just tie the gate to the anode or
should you use a diode to prevent reverse current through the gate (in case
there is a gate resistor)? Also, would you need to use a resistor between
the gate and anode?

Thanks!

RogerN

 

[Lostgallifreyan]

"RogerN" <regor@midwest.net> wrote in
news:2M6dnTxsxcMwlzjRnZ2dnUVZ_g2dnZ2d@earthlink.com:

To use an SCR as a rectifier can you just tie the gate to the anode or
should you use a diode to prevent reverse current through the gate (in
case there is a gate resistor)? Also, would you need to use a resistor
between the gate and anode?

Thanks!

RogerN

I imagine nothing is needed, though a single diode ought to be a good
safeguard as it has a lower voltage drop so will guarantee the device turns
on in forward bias.

Why do you want to do this to an SCR anyway? Convenient high current device?

 

[Jamie]

RogerN wrote:

To use an SCR as a rectifier can you just tie the gate to the anode or
should you use a diode to prevent reverse current through the gate (in case
there is a gate resistor)? Also, would you need to use a resistor between
the gate and anode?

Thanks!

RogerN


I suppose you could, not sure as to why you would want this?

Hinksink mounted rectifier maybe?

 

[RogerN]

"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote in message
newsjupo.20395$vB4.11238@newsfe24.iad...

RogerN wrote:

To use an SCR as a rectifier can you just tie the gate to the anode or
should you use a diode to prevent reverse current through the gate (in
case there is a gate resistor)? Also, would you need to use a resistor
between the gate and anode?

Thanks!

RogerN


I suppose you could, not sure as to why you would want this?

Hinksink mounted rectifier maybe?

A guy needs to make a DC supply, about 50A, from 230VAC line. He has an old
drive that has the rectifier section but it uses SCR's so it can control
power to the DC bus. For the supply he's trying to make he just wants to
use the existing SCR's, already mounted to the heat sink, as rectifiers. He
found out it has SCR's instead of rectifiers and was about to start over, I
told him I thought he could wire the SCR's to act like rectifiers.

RogerN

 

[RogerN]

"Lostgallifreyan" <no-one@nowhere.net> wrote in message
news:Xns9E042DB809605zoodlewurdle@216.196.109.145...

"RogerN" <regor@midwest.net> wrote in
news:2M6dnTxsxcMwlzjRnZ2dnUVZ_g2dnZ2d@earthlink.com:

To use an SCR as a rectifier can you just tie the gate to the anode or
should you use a diode to prevent reverse current through the gate (in
case there is a gate resistor)? Also, would you need to use a resistor
between the gate and anode?

Thanks!

RogerN




I imagine nothing is needed, though a single diode ought to be a good
safeguard as it has a lower voltage drop so will guarantee the device
turns
on in forward bias.

Why do you want to do this to an SCR anyway? Convenient high current
device?

A guy I know of needs a 50A rectifier to rectify 230VAC. He has the
rectifier section to a motor drive but it has SCR's instead of rectifiers.
I think drives use SCR's so they can turn their bus voltage off, or perhaps
to soft start the power to the capacitors.

I also have a similar need. A guy gave me some small hockey puck SCR's and
I'm wanting to make a rectifier to convert my AC welder into a DC welder.
This project has been on back burner since I now have a DC welder.

RogerN

 

[Lostgallifreyan]

"RogerN" <regor@midwest.net> wrote in
news:07GdnYZaFcfbBTvRnZ2dnUVZ_t6dnZ2d@earthlink.com:

A guy needs to make a DC supply, about 50A, from 230VAC line. He has an
old drive that has the rectifier section but it uses SCR's so it can
control power to the DC bus. For the supply he's trying to make he just
wants to use the existing SCR's, already mounted to the heat sink, as
rectifiers. He found out it has SCR's instead of rectifiers and was
about to start over, I told him I thought he could wire the SCR's to act
like rectifiers.

I think you could do it. The diode as I said, but perhaps also a 220R
resistor in series with it. I remember a triac circuit from Maplin
Electronics, driven by an expensive Siemens touch dimmer IC, and that triac
often ended up doing things back through its own gate that burnt out IC after
IC. I managed to convince Maplin that their app circuit was wrong, and to
refund me for several of those IC's, but they never did edit the published
diagram with my series 220R fix in the triac gate! Got to feel sorry for all
those who assumed it was their error even when they followed the circuit
exactly as I did... The point being that if your circuit works with a bit of
series resistance in the gate line, add some. It will help protect both ends
of that line.

 

[Grant]

On Thu, 30 Sep 2010 22:29:41 -0500, Lostgallifreyan <no-one@nowhere.net> wrote:

"RogerN" <regor@midwest.net> wrote in
news:2M6dnTxsxcMwlzjRnZ2dnUVZ_g2dnZ2d@earthlink.com:

To use an SCR as a rectifier can you just tie the gate to the anode or
should you use a diode to prevent reverse current through the gate (in
case there is a gate resistor)? Also, would you need to use a resistor
between the gate and anode?

Thanks!

RogerN




I imagine nothing is needed, though a single diode ought to be a good
safeguard as it has a lower voltage drop so will guarantee the device turns
on in forward bias.

You need a resistor from anode to gate to turn SCR on, and either reverse
diode gate - cathode or series diode to protect gate from negative drive.

Why do you want to do this to an SCR anyway? Convenient high current device?

Okay if the SCRs already mounted on heatsinks, doubled forward loss
compared to straight diode.

Grant.

 

[Lostgallifreyan]

Grant <omg@grrr.id.au> wrote in
news:e7gsa61n44u9vqro5uvsgdu9l18ipkmuhs@4ax.com:

To use an SCR as a rectifier can you just tie the gate to the anode or
should you use a diode to prevent reverse current through the gate (in
case there is a gate resistor)? Also, would you need to use a
resistor between the gate and anode?

Thanks!

RogerN




I imagine nothing is needed, though a single diode ought to be a good
safeguard as it has a lower voltage drop so will guarantee the device
turns on in forward bias.

You need a resistor from anode to gate to turn SCR on, and either
reverse diode gate - cathode or series diode to protect gate from
negative drive.

You only need the resistor to limit the current. Some SCR's (or triacs) can
be damaged otherwise, or as I said in that other post, they can damage the
outputs of a device that controls them. If Roger connects a forward biased
(NOT reversed) diode between anode and gate it behaves as he expected, and
the whole circuit is protected against reverse bias too, so long as both
diode and SCR have a high enough peak inverse voltage value.

Why do you want to do this to an SCR anyway? Convenient high current
device?

Okay if the SCRs already mounted on heatsinks, doubled forward loss
compared to straight diode.

I think my first point made that distinction in voltage drops clear enough
already.