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Post Info TOPIC: Solid state relays


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Solid state relays


Hi there,

Old engel finally carked it, so purchased a brass monkey 40 litre upright. About same size as the old engel

Biggest issue was finding something that would fit existing hole without EXTENSIVE modifications, so 40 litres it is and was only $280 compared to new dometic at around $1,000 and a really strange internal shape .

 

Anyhow, I have been unhappy with the voltage drop so decided to run new bigger supply cables with separate 10A inline fuse). I wanted to use existing switch on a remote control panel.

So i have decided to use a solid state relay to turn on new power supply. Existing control cable more than adequate to switch SSR.

Issue is the ssr manufacturer recommends a back emf diode in parallel to the load (fridge).

Questions are:

1. Do i really need? Would the fridge already incorporate this? I don't want to void warranty by opening up electrical box.

2. How would i wire the diode?

3. Would a 36 volt 10A diode suffice?

4. The diode itself would normally introduce a 0.5v voltage drop but i believe would be less if wired in parallel?

 

SSR = 4-32VDC 100A@30VDC

Nominal 12V supply ( 2x 100AH Gel) house batts

Fridge max draw = 5A when running

 

Thanks

 



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 This may help:-

"Your DC SSR from Phidgets comes with a diode. This diode should be installed across your load, with the Cathode installed towards the positive terminal of the power supply (as shown in the diagram).

If the diode is installed backwards, as soon as the SSR is turned on, the load will be shorted out, likely destroying the diode, or the SSR, or your power supply. A fuse protecting your power supply is always a good idea. You can place the fuse in between the positive terminal of the power supply and the positive terminal of the load side of the SSR.

The diode protects the SSR from powerful residual currents after the SSR is turned off. While your load is being driven, inductance builds up magnetic fields around the wiring. Every load is inductive to some degree, and when the SSR turns off, the magnetic fields will ram current against the now open SSR, easily damaging it. The diode allows these currents to recirculate in the load until they have lost their energy."

 



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Here is a link  to look at, re diodes and relays. Some relays come with a diode already fitted  for DC switching purposes. It is in parallel and there are no voltage losses, select a high voltage rating say 24V 5a or bigger  and it will  last for ever.

https://electronicsarea.com/why-is-there-a-diode-connected-in-parallel-to-a-relay-coil/

Jaahn

Why is there a diode connected in parallel to a relay coil?



-- Edited by Jaahn on Saturday 1st of June 2024 12:39:38 PM

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The voltage of the spike will certainly exceed 24V and normally a 400V 1A diode is sufficient such as a 1N4004.



-- Edited by Linwood on Monday 3rd of June 2024 06:50:02 AM

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Linwood wrote:

In the second diagram above, the current from the collapsing magnetic field in the coil or motor in your case is flowing in the opposite direction to what is shown. In fact the diode is conducting and providing a shunt so no electronics is damaged. The voltage of the spike will certainly exceed 24V and normally a 400V 1A diode is sufficient such as a 1N4004.


There is nothing wrong with either diagram. After the circuit is interrupted, the current continues to flow through the inductor in the same direction. A 100V 1A snubber diode (1N4001) is sufficient to suppress any inductive spike from a relay coil. A snubber for a compressor would require a higher current rating.

 

 



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Dorian, 

You are correct.  I was thinking of the negative pulse emanating from the coil with the rapidly collapsing magnetic field and its directionality.



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Linwood wrote:

Dorian, 

You are correct.  I was thinking of the negative pulse emanating from the coil with the rapidly collapsing magnetic field and its directionality.


 Is that the left hand rule or the right hand rule? Too many years ago for me to remember.



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Linwood wrote:

I was thinking of the negative pulse emanating from the coil with the rapidly collapsing magnetic field and its directionality.


I think in terms of energy.

A charged capacitor stores an amount of energy given by E = 1/2 x C x V^2 (C= capacitance, V = voltage). If you short the terminals of this capacitor, you will be trying to dissipate all its energy in an instant. This generates a massive current spike.

In the case of an inductor, E = 1/2 x L x I^2 (L = inductance, I = current). When you interrupt the current in an inductor, you will be trying to dissipate all its energy in an instant, as before. This generates a massive voltage spike which throws an arc across the switch. The snubber diode prevents the arcing.



-- Edited by dorian on Monday 3rd of June 2024 08:24:11 AM

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I thought he was using a solid state relay operated by a switch.
If the switch is very often used a capacitor across the switch would suppress sparking and destruction of the switch. Remember what happened to the contacts in the engine spark plug distributer when the capacitor failed.
I would expect the fridge to suppress any switch off voltage spikes.
Easy enough to check with a multimeter.
Solid state relay circuit which incorporates its own load protection.

tried to add a circuit drawing of a solid state relay but it seems that *.png files are not supported.

 



-- Edited by jegog on Wednesday 5th of June 2024 01:23:07 AM

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I don't think that the control current would pose any problem for a regular SPST switch.

https://www.phidgets.com/docs/Solid_State_Relay_Guide

SSR from Jaycar:

https://www.jaycar.com.au/solid-state-relay-4-32vdc-input-30vdc-100a-switching/p/SY4086

Input Circuit

- Control Voltage: 4-32VDC
- Control Current: 28mA max.
- Min Turn Off Voltage: 1.0VDC
- Input Resistance: 1.2kOhm

If you are using a 12V source for the input, then the control current would be 10mA.



-- Edited by dorian on Wednesday 5th of June 2024 09:18:39 AM

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Hi all,
Thanks for the comments and links.

The SSR i chose indeed comes from Jaycar

www.jaycar.com.au/solid-state-relay-4-32vdc-input-30vdc-100a-switching/p/SY4086

Jaycar spec sheet says
Note: Inductive loads must have diode suppression (Refer data sheet wiring diagram)

The supplier data sheet says:
1. Suppression circuit will be added when used for inductive load.

All very good but doesn't actually tell me much on ho to do.

My main point is wouldn't the fridge already include a suppresion circuit on the compressor motor? If so, do i need another diode paralleling the whole fridge load?

Thanks again

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Fridge65 wrote:


My main point is wouldn't the fridge already include a suppresion circuit on the compressor motor? If so, do i need another diode paralleling the whole fridge load?


Good question. I expect that the compressor is a 3-phase type where each phase is driven by a half-bridge consisting of two MOSFETs or IGBTs. Each of the semiconductors would have a body diode to account for inductive back-EMF.

Therefore, I suspect that no additional protection diode would be required.

The diagrams show an AC powered air-con or refrigeration system with a 3-phase DC motor powered by an inverter that is generated from the AC supply. In the RV case the AC supply would simply be discarded.

https://aseannow.com/topic/991333-hack-inverter-air-to-run-on-dc/

 

 



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Unfortunately not all inverters are "clean" so it is always a good idea to put a diode across the load to suppress back EMFs. In the Jaycar catalogue they list an ultrafast diode UF4003 (200v, 1A) at $1.15 each. So why wouldn't you use such a diode to protect a $50 solid state relay for peace of mind?

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Linwood wrote:

Unfortunately not all inverters are "clean" so it is always a good idea to put a diode across the load to suppress back EMFs. In the Jaycar catalogue they list an ultrafast diode UF4003 (200v, 1A) at $1.15 each. So why wouldn't you use such a diode to protect a $50 solid state relay for peace of mind?


 Which inverter are you talking about, and why would you choose a 1A diode when the load current is 5A?



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Some portable refrigerators use an inverter to drive the compressor motor, particularly those with a soft start.  The protection diode is not carrying the load current but the back EMF spike (higher voltage than supply but low current).



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Linwood wrote:

Some portable refrigerators use an inverter to drive the compressor motor, particularly those with a soft start.  The protection diode is not carrying the load current but the back EMF spike (higher voltage than supply but low current).


That's why I specifically chose an inverter driven compressor for each of my illustrations. AISI, an external diode would do nothing. All the dangerous spikes would be suppressed by the body diodes in the MOSFET/IGBT phase drivers.

Pick any two phases, follow the current path through the corresponding upper and lower phase driver, then switch off these two phase drivers and follow the path of the decaying current through the diodes in the phase drivers in the opposite side of each H-bridge.



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We don't even know if the Brass Monkey refrigerator has an inverter or not. Judging by its price, it probably does not have the most refined electronics. For all we know the compressor may be fed directly by 12V DC. It is pointless discussing this topic any further.

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Linwood wrote:

We don't even know if the Brass Monkey refrigerator has an inverter or not. Judging by its price, it probably does not have the most refined electronics. For all we know the compressor may be fed directly by 12V DC. It is pointless discussing this topic any further.


It's a variable speed inverter-based design. The motor is a BLDC type.

https://vanmarket.com.au/products/brass-monkey-40l-upright-fridge-with-freezer-zone

https://vanmarket.com.au/cdn/shop/products/2362556_600_500_460x.jpg?v=1676345603

https://www.lg.com/global/business/download/resources/CT00000308/LG_Catalogue_Linear+Reciprocating%20Compressor%5B20211026_124201%5D.pdf

https://www.lg.com/global/business/download/resources/CT00000308/LG_Leaflet_Reciprocating%20Compressor_BCA%5B20201204_165119%5D.pdf

https://www.lg.com/global/business/download/resources/CT00000308/LG%20BC%20Series%20Reciprocating%20Compressor%5B20230418_170201%5D.pdf

https://www.digikey.com/en/articles/how-to-power-and-control-brushless-dc-motors



-- Edited by dorian on Sunday 16th of June 2024 03:13:55 PM

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Perhaps a diagram explains it better.



-- Edited by dorian on Monday 17th of June 2024 06:02:31 AM

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