relay for photo enlarger

PGG wrote:
I'm looking for a recommendation on a mechanical relay for constructing a
digital timer for my photo enlarger. I'm not sure how much current it
draws, but typical timers sold for this purpose are rated at 600+ watts.

My plan is to control the relay either with an old Palm Pilot (via RS232),
or with a low-cost microcontroller board.

I have an education in electronics but I haven't practiced in years. Plus
I was never any good at analog stuff (I'm a 0s and 1s guy). Anything
else I should look out for?


600W / 100V = 6 amps. You'll need to find a relay that is rated for
that (or look at the current that your enlarger will pull, particularly
the inrush current when the lamp is cold).

I assume that you're planning on hacking some code so you can use the
RTS line from your palm? An RS-232 interface can't drive more than a
couple of mA (someone will say just how much), and you'll never find a
mechanical 6A relay that'll operate on that little current, so you
should plan on making a driver, probably powered from one of those
wall-warts you've been hoarding. It's a pretty simple circuit with only
a few kinks, so you should be able to dig one up from the web.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

In rec.photo.darkroom PGG wrote:
: I'm looking for a recommendation on a mechanical relay for constructing a
: digital timer for my photo enlarger. I'm not sure how much current it
: draws, but typical timers sold for this purpose are rated at 600+ watts.

: My plan is to control the relay either with an old Palm Pilot (via RS232),
: or with a low-cost microcontroller board.

: I have an education in electronics but I haven't practiced in years. Plus
: I was never any good at analog stuff (I'm a 0s and 1s guy). Anything
: else I should look out for?



The power draw is easy to determining. Simply add up the power drawn by everything that
the relay will supply power to. In most cases this will be the light bulb.

One of the things you need to be careful of is the amount of current drawn by the
relay coil. You also want to put a diode in parallel with the relay coil. Make sure
you install the cathode of the diode to the positive voltage connection of the relay.
If you can find a low voltage MOV (metal oxide varistor) you can use it. The advantage
of an MOV is that they don't have a polarity. The voltage should be just about the coil
voltage of the relay.

I'm assuming that you know that you have to make sure that the relay coil is the right
voltage and the driver can supply the required amount of current.



--




Keep working millions on welfare depend on you
-------------------

PGG wrote:

On Fri, 01 Oct 2004 11:00:40 -0700, Tim Wescott wrote:


PGG wrote:

I'm looking for a recommendation on a mechanical relay for constructing a
digital timer for my photo enlarger. I'm not sure how much current it
draws, but typical timers sold for this purpose are rated at 600+ watts.

My plan is to control the relay either with an old Palm Pilot (via RS232),
or with a low-cost microcontroller board.

I have an education in electronics but I haven't practiced in years. Plus
I was never any good at analog stuff (I'm a 0s and 1s guy). Anything
else I should look out for?



600W / 100V = 6 amps. You'll need to find a relay that is rated for
that (or look at the current that your enlarger will pull, particularly
the inrush current when the lamp is cold).

I assume that you're planning on hacking some code so you can use the
RTS line from your palm? An RS-232 interface can't drive more than a
couple of mA (someone will say just how much), and you'll never find a
mechanical 6A relay that'll operate on that little current, so you
should plan on making a driver, probably powered from one of those
wall-warts you've been hoarding. It's a pretty simple circuit with only
a few kinks, so you should be able to dig one up from the web.


Thanks. This is what I had in mind. Yes, I have a nice collection of
wall-warts

And you are right. A look at digi-key shows that for an 8-amp relay, the
coil resistence is 125 requires 40amps at 5 volts. I imagine a simple
circuit consisting of a single transistor would work...but I'll have to
Google it as it is beyond me.




Here's a circuit. D1 blocks the reverse voltage from RS-232 from your
transistor. R1 limits the base current, R2 keeps the transistor turned
off (and turns it off faster). D2 provides a path for the flyback
voltage on the coil. D2 can probably be a 1N4148 for your relay --
double check, though, I can't remember if it'll do 40mA. Q1 can be just
about anything that calls itself NPN, but it does need a fairly good
beta to work with RS-232.


VCC
+
|
.--o--.
| |
relay C| |
coil C| - D2
C| ^
| |
| |
R1 o-----'
D1 4.7k |
RTS ___ |/ Q1
-------|---|___|---o------|
| |
1N4148 .-. |
R2 | | |
4.7k | | |
'-' |
| |
| |
=== ===
GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"PGG" schreef in bericht
newsan.2004.10.01.17.50.13.90000@NO_SP_A_Myahoo. com...
I'm looking for a recommendation on a mechanical relay for constructing a
digital timer for my photo enlarger. I'm not sure how much current it
draws, but typical timers sold for this purpose are rated at 600+ watts.

My plan is to control the relay either with an old Palm Pilot (via RS232),
or with a low-cost microcontroller board.

I have an education in electronics but I haven't practiced in years. Plus
I was never any good at analog stuff (I'm a 0s and 1s guy). Anything
else I should look out for?



A quick and easy solution is a solid state relay. Conrad for instance sells
them. Only 8mAdc control current to switch up to 16A at 600Vac. (Type S 016
S02 order nr. 16 25 23-44 priced some 11 Euros.) I'm sure Conrad is not the
only one that sells them. A (real) RS232 line can provide the control
current. You may only need a resistor to limit it. Most important thing is a
save enclosure.

petrus bitbyter


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"PGG" a écrit dans le message de
newsan.2004.10.01.17.50.13.90000@NO_SP_A_Myahoo. com...
I'm looking for a recommendation on a mechanical relay for constructing a
digital timer for my photo enlarger. I'm not sure how much current it
draws, but typical timers sold for this purpose are rated at 600+ watts.

My plan is to control the relay either with an old Palm Pilot (via RS232),
or with a low-cost microcontroller board.

I have an education in electronics but I haven't practiced in years. Plus
I was never any good at analog stuff (I'm a 0s and 1s guy). Anything
else I should look out for?

You may use a triac instead of a relay:
http://www.bonavolta.ch/hobby/en/pho...30VTriacSwitch
The example (there is also the 115V version) uses an optocoupler as triac
driver which gives several advantages:
- separates the driving signal and the mains power.
- mains is switched on only when the sinewave crosses 0
- TTL level input signal

You can also find similar circuits integrated in one chip (Crydom, ...) also
know as solid-state relays, check their input power ratings.

On that page, you'll also find a relay schematic (not tested but should
work) an a DC switch for stablized supplies.

Good luck,
Claudio Bonavolta
http://www.bonavolta.ch

"PGG" wrote in message
newsan.2004.10.01.18.39.59.136000@NO_SP_A_Myahoo .com...
On Fri, 01 Oct 2004 11:00:40 -0700, Tim Wescott wrote:

PGG wrote:
I'm looking for a recommendation on a mechanical relay for
constructing a
digital timer for my photo enlarger. I'm not sure how much current
it
draws, but typical timers sold for this purpose are rated at 600+
watts.

My plan is to control the relay either with an old Palm Pilot (via
RS232),
or with a low-cost microcontroller board.

I have an education in electronics but I haven't practiced in
years. Plus
I was never any good at analog stuff (I'm a 0s and 1s guy).
Anything
else I should look out for?


600W / 100V = 6 amps. You'll need to find a relay that is rated for
that (or look at the current that your enlarger will pull,
particularly
the inrush current when the lamp is cold).

I assume that you're planning on hacking some code so you can use
the
RTS line from your palm? An RS-232 interface can't drive more than
a
couple of mA (someone will say just how much), and you'll never find
a
mechanical 6A relay that'll operate on that little current, so you
should plan on making a driver, probably powered from one of those
wall-warts you've been hoarding. It's a pretty simple circuit with
only
a few kinks, so you should be able to dig one up from the web.

Thanks. This is what I had in mind. Yes, I have a nice collection of
wall-warts

And you are right. A look at digi-key shows that for an 8-amp relay,
the
coil resistence is 125 requires 40amps at 5 volts. I imagine a simple
circuit consisting of a single transistor would work...but I'll have
to
Google it as it is beyond me.

Here are some ideas:
http://ourworld.compuserve.com/homep...age6.htm#PPRIC

"Tim Wescott" wrote in message
...
PGG wrote:

On Fri, 01 Oct 2004 11:00:40 -0700, Tim Wescott wrote:


PGG wrote:

I'm looking for a recommendation on a mechanical relay for
constructing a
digital timer for my photo enlarger. I'm not sure how much current
it
draws, but typical timers sold for this purpose are rated at 600+
watts.

My plan is to control the relay either with an old Palm Pilot (via
RS232),
or with a low-cost microcontroller board.

I have an education in electronics but I haven't practiced in
years. Plus
I was never any good at analog stuff (I'm a 0s and 1s guy).
Anything
else I should look out for?



600W / 100V = 6 amps. You'll need to find a relay that is rated for
that (or look at the current that your enlarger will pull,
particularly
the inrush current when the lamp is cold).

I assume that you're planning on hacking some code so you can use
the
RTS line from your palm? An RS-232 interface can't drive more than
a
couple of mA (someone will say just how much), and you'll never find
a
mechanical 6A relay that'll operate on that little current, so you
should plan on making a driver, probably powered from one of those
wall-warts you've been hoarding. It's a pretty simple circuit with
only
a few kinks, so you should be able to dig one up from the web.


Thanks. This is what I had in mind. Yes, I have a nice collection
of
wall-warts

And you are right. A look at digi-key shows that for an 8-amp
relay, the
coil resistence is 125 requires 40amps at 5 volts. I imagine a
simple
circuit consisting of a single transistor would work...but I'll have
to
Google it as it is beyond me.




Here's a circuit. D1 blocks the reverse voltage from RS-232 from your
transistor. R1 limits the base current, R2 keeps the transistor
turned
off (and turns it off faster). D2 provides a path for the flyback
voltage on the coil. D2 can probably be a 1N4148 for your relay --
double check, though, I can't remember if it'll do 40mA. Q1 can be
just
about anything that calls itself NPN, but it does need a fairly good
beta to work with RS-232.

It will need a extremely high beta to work with a 4.7k resistor! That
should be a lot lower to keep the transistor fully saturated.


VCC
+
|
.--o--.
| |
relay C| |
coil C| - D2
C| ^
| |
| |
R1 o-----'
D1 4.7k |
RTS ___ |/ Q1
-------|---|___|---o------|
| |
1N4148 .-. |
R2 | | |
4.7k | | |
'-' |
| |
| |
=== ===
GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Using a thyristor-based solid state relay is the easiest answer. I'd
use a Power-io solid state relay because it has a low,
current-limited, control input. That will make it more compatible
with the output fom your Palm. For example: some models have a 5mA
input requirement at any voltage from 4-32 vdc.

The solid state relays are online at www.power-io.com along with their
specifications.

Other solid state relays typically need a much stronger mA output,
especially as the voltage increases.