Solid State Relays for use on a 24V DC motor.

[John Crighton]

Hello All,
a friend of mine wishes to replace four heavy duty relays
that control a 24 V DC electric motor.
The motor is either off, runs forward or runs in reverse.
My friend tells me the normal running current is 10 amps
but at start up he thinks the current is around 50 amps.

He cant get the correct relays anymore, the equipment
is old. Suitable electrical replacement types are expensive
and physically don't fit. If modifications have to be made
he thought SSRs would be good.
Something like this.
http://www.crydom.com/pdf/crydom_D06D.pdf

Those particular SSRs look great.
The only problem is the price. 150 dollars locally
here in Australia. Too expensive for my friend.

As a hobbyist, I am even more of a cheapskate
than my friend is, so I am interested to know what
is in these things to make them so expensive.

We want to make something similar to the Crydom
relay, specification wise, with locally easy to obtain
parts and a lot cheaper than 600 dollars for four units.
All ideas welcome.

Regards,
John Crighton
Hornsby

 

[Phil Allison]

Hello All,
a friend of mine wishes to replace four heavy duty relays
that control a 24 V DC electric motor.
The motor is either off, runs forward or runs in reverse.
My friend tells me the normal running current is 10 amps
but at start up he thinks the current is around 50 amps.

He cant get the correct relays anymore, the equipment
is old. Suitable electrical replacement types are expensive
and physically don't fit.

** What drivel - the Farnell catalogue is FULL of 24 volt, 30 amp plus
SPCO relays that are compact and would do that job for as little as $8 each
plus gst.


Forget the other idea ( ridiculously complex) using mosfets.




............. Phil

 

[John Crighton]

** What drivel - the Farnell catalogue is FULL of 24 volt, 30 amp plus
SPCO relays that are compact and would do that job for as little as $8 each
plus gst.


Forget the other idea ( ridiculously complex) using mosfets.




............ Phil

Phil,
I can see that I didn't spell out my friends requirements very well.
He wants the contacts to handle direct current of 60 amps DC
The ratings of the original relay were 28V 60 amps.

The motor, when in use in a fishing boat auto pilot, continually
starts and stops and the contacts arc when the relay opens.
The relays don't last long, hence the solid state switch idea.

I registered at the Farnell website and now I can't get on it,
please choose a country, please choose a country, round and round.
Probably finger trouble with me.
I would like to see those relays that can handle 30 amps plus at 24V,
for $8. My friend may be able to parallel them. I think he prefers
not to parallel them but if the price is right, maybe.

Anyway, if you or anyone else knows of cheap relays for 24V
60 amp use, please point them out.
My old work mate would be delighted. A lot less trouble
than making a solid state switch but that would be better
in the long run.

Regards,
John Crighton
Hornsby

 

[Phil Allison]

Phil,
I can see that I didn't spell out my friends requirements very well.
He wants the contacts to handle direct current of 60 amps DC
The ratings of the original relay were 28V 60 amps.

** A very different requirement .

The motor, when in use in a fishing boat auto pilot, continually
starts and stops and the contacts arc when the relay opens.
The relays don't last long, hence the solid state switch idea.

** 28 volts DC at high current ( more than 10 amps ) poses a serious
arcing problem.

If you place a strong magnet near as possible to the contacts it helps
considerably to break any arcs.

I registered at the Farnell website and now I can't get on it,
please choose a country, please choose a country, round and round.
Probably finger trouble with me.
I would like to see those relays that can handle 30 amps plus at 24V,
for $8. My friend may be able to parallel them. I think he prefers
not to parallel them but if the price is right, maybe.

** There is one rated at 40 amps (resistive load) for $8.

But your problem is arcing - so the contacts need to open wide and fast.

Series operation of the contacts may be better than paralleling.





................ Phil

 

[Phil Allison]

Phil,
I can see that I didn't spell out my friends requirements very well.
He wants the contacts to handle direct current of 60 amps DC
The ratings of the original relay were 28V 60 amps.

** A very different requirement .

The motor, when in use in a fishing boat auto pilot, continually
starts and stops and the contacts arc when the relay opens.
The relays don't last long, hence the solid state switch idea.

** 28 volts DC at high current ( more than 10 amps ) poses a serious
arcing problem.

If you place a strong magnet near as possible to the contacts it helps
considerably to break any arcs.

I registered at the Farnell website and now I can't get on it,
please choose a country, please choose a country, round and round.
Probably finger trouble with me.
I would like to see those relays that can handle 30 amps plus at 24V,
for $8. My friend may be able to parallel them. I think he prefers
not to parallel them but if the price is right, maybe.

** There is one rated at 40 amps (resistive load) for $8.

But your problem is arcing - so the contacts need to open wide and fast.

Series operation of the contacts may be better than paralleling.





................ Phil

 

[paul]

Hello All,
a friend of mine wishes to replace four heavy duty relays
that control a 24 V DC electric motor.
The motor is either off, runs forward or runs in reverse.
My friend tells me the normal running current is 10 amps
but at start up he thinks the current is around 50 amps.

He cant get the correct relays anymore, the equipment
is old. Suitable electrical replacement types are expensive
and physically don't fit. If modifications have to be made
he thought SSRs would be good.
Something like this.
http://www.crydom.com/pdf/crydom_D06D.pdf

Those particular SSRs look great.
The only problem is the price. 150 dollars locally
here in Australia. Too expensive for my friend.

As a hobbyist, I am even more of a cheapskate
than my friend is, so I am interested to know what
is in these things to make them so expensive.

We want to make something similar to the Crydom
relay, specification wise, with locally easy to obtain
parts and a lot cheaper than 600 dollars for four units.
All ideas welcome.

Regards,
John Crighton
Hornsby

Hello

The mosfet switching is not as easy as it sounds.

I built a mofet H-bridge rated at about 50A 24V some years ago. We also
tried 30A automotive relays which were cheap and easy to get.

The load was a 500 watt 24V DC motor. The run current was about 13 amps
but the peak start current was over 100 amps. Power came from two 12V
40Ah batteries in series and a charger.

Switching about 2000 times a day the relays lasted about about 6 weeks;
usualy failed in a meltdown.

The H-bridge had four 50A mosfets and a current driver for each leg. A
very low-R power resistance monitored the current and shut off the drive
if it exceeded a set point on each drive pulse (PWM drive). A bit of
logic ensured that both directions were not enabled at the same time. We
blew up some mosfets before getting it working.

Let me know if you want some more details.

It sounds like your application should go the mosfet way but I would
recommend an off-the-shelf H-bridge motor driver if you can find one.

Regards
Paul Bealing

 

[John Crighton]

Hello

The mosfet switching is not as easy as it sounds.

I built a mofet H-bridge rated at about 50A 24V some years ago. We also
tried 30A automotive relays which were cheap and easy to get.

The load was a 500 watt 24V DC motor. The run current was about 13 amps
but the peak start current was over 100 amps. Power came from two 12V
40Ah batteries in series and a charger.

Switching about 2000 times a day the relays lasted about about 6 weeks;
usualy failed in a meltdown.

The H-bridge had four 50A mosfets and a current driver for each leg. A
very low-R power resistance monitored the current and shut off the drive
if it exceeded a set point on each drive pulse (PWM drive). A bit of
logic ensured that both directions were not enabled at the same time. We
blew up some mosfets before getting it working.

Let me know if you want some more details.

It sounds like your application should go the mosfet way but I would
recommend an off-the-shelf H-bridge motor driver if you can find one.

Regards
Paul Bealing

Hello Paul,
Yes, I am very keen to learn more about your H bridge
motor driver. If one is available locally to do the job
my friend will definitely be interested in that also.
Regards,
John Crighton
Hornsby

 

[paul]

Hello Paul,
Yes, I am very keen to learn more about your H bridge
motor driver. If one is available locally to do the job
my friend will definitely be interested in that also.
Regards,
John Crighton
Hornsby

Hello

I should be able to dig out the circuit diagram and I've got an old
prototype unit here but I don't know if it works. It's missing a small
DC-DC converter that generates the high side drive voltage.
The files will be archived somewhere here. The prototype board has a
date on it of 1993. I'll do a search.

Regards
Paul Bealing

 

[paul]

Hello Paul,
Yes, I am very keen to learn more about your H bridge
motor driver. If one is available locally to do the job
my friend will definitely be interested in that also.
Regards,
John Crighton
Hornsby

Hello

I've found the circuit diagram and turned it into a pdf.
It's about 1.5 MB.
You can download it from the bottom of this page:
http://www.pmb.co.nz/pub/index.htm

The layout and construction is quite important if you are driving with
PWM. For slower on/off relay replacement it shouldn't be a problem.

Regards
Paul Bealing

 

[John Crighton]

Hello

I've found the circuit diagram and turned it into a pdf.
It's about 1.5 MB.
You can download it from the bottom of this page:
http://www.pmb.co.nz/pub/index.htm

The layout and construction is quite important if you are driving with
PWM. For slower on/off relay replacement it shouldn't be a problem.

Regards
Paul Bealing

Hello Paul,
thanks very much for your time and effort there. I have told
my friend where to find your circuit. So he will be studying it.
Just one quick question.

Can you remember what you used, part number or
current rating, for the 15V isolated power supply?

Regards,
John Crighton
Hornsby

 

[Richard Freeman]

Hello Paul,
thanks very much for your time and effort there. I have told
my friend where to find your circuit. So he will be studying it.
Just one quick question.

Can you remember what you used, part number or
current rating, for the 15V isolated power supply?

John
One of the little 1Watt DC-DC converters from Farnell (NME2415 - cat no
253-285) should do the Trick they cost about $25 each but that may work out
cheaper than making one.
Actually I suspect that given the fact you are replacing relays and are
operating at (relatively ) low speed the driver circuit for the H bridge
could be simplified somewhat from pauls design.

Regards
Richard Freeman

 

[paul]

John
One of the little 1Watt DC-DC converters from Farnell (NME2415 - cat no
253-285) should do the Trick they cost about $25 each but that may work out
cheaper than making one.
Actually I suspect that given the fact you are replacing relays and are
operating at (relatively ) low speed the driver circuit for the H bridge
could be simplified somewhat from pauls design.

Regards
Richard Freeman

Hello

Used the 1Watt NME1212 from memory. I had a 7812 regulator off the 24V
to supply the logic etc. It acted as a pre-reg for the NME1212. The 24V
supply was very noisy.

Agreed, it could be simplified for low frequency switching.
I think I was running it at about 15 to 20 KHz.
The drive transistors deliver current to ensure that the mosfets switch
fast enough to reduce losses.

I'd leave the power-on-reset and logic. When you power up both
directions at the same time without the protection, mosfets explode
throwing bits of pastic around the room.
The fuse is ceramic HRC motor rated so it doesn't protect the circuitry.
It blows after the mosfets have failed preventing further damage.

The mosfets used are MTP50N06; 50A 60V.
The drive circuit shown will drive three or four mosfets in parallel on
each leg.

Regards
Paul Bealing

 

[John Crighton]

John
One of the little 1Watt DC-DC converters from Farnell (NME2415 - cat no
253-285) should do the Trick they cost about $25 each but that may work out
cheaper than making one.
Actually I suspect that given the fact you are replacing relays and are
operating at (relatively ) low speed the driver circuit for the H bridge
could be simplified somewhat from pauls design.

Regards
Richard Freeman

Hello Richard,
I will pass that part number information on to my old work mate
Les, up in Newcastle. He might want to try just an isolated supply
and an opto coupler just to make one stand alone relay unit.

If his SSR survives high side switching of the motor on and off all
day long, he will be pleased, then an H bridge will be made up.

Paul's H bridge circuit can then be used as a guide. There are
lots of good features there in Paul's unit, current limiting,
forward/reverse interlocking. All great stuff. Les will sort out
what features he wants. He will probably keep it as simple as
possible like you suggested Richard.

Regards,
John Crighton
Hornsby